Chapter XII
DISTRIBUTION OF MARINE MOLLUSCA--DEEP-SEA MOLLUSCA AND THEIR CHARACTERISTICS
Marine Mollusca may be divided roughly into Pelagic and non-Pelagic genera. To the former division belong all Pteropoda and Heteropoda, and a large number of Cephalopoda, together with a very few specialised forms of Gasteropoda (Ianthina, Litiopa, Phyllirrhoe, etc.). Pelagic Mollusca appear, as a rule, to live at varying depths below the surface during the day, and to rise to the top only at night. The majority inhabit warm or tropical seas, though some are exceedingly abundant in the Arctic regions; Clione and Limacina have been noticed as far north as 72°.[380]
The vertical range of Pelagic Mollusca has received attention from Dr. Murray of the Challenger, Professor Agassiz of the Blake and Albatross, and others. Agassiz appears to have established the fact that the surface fauna of the sea is limited to a comparatively narrow belt of depth, and that there is no intermediate belt of animal life between creatures which live on or near the bottom and the surface fauna. Pelagic forms sink to avoid disturbances of various kinds, to depths not much exceeding 150 to 200 fathoms, except in closed seas like the Gulf of California and the Mediterranean, where the bathymetrical range appears to be much greater.[381]
Non-Pelagic Mollusca are, from one point of view, conveniently classified according to the different zones of depth at which they occur. Thus we are enabled to distinguish Mollusca of (a) the littoral, (b) the laminarian, (c) the nullipore, or coralline, and (d) the abyssal zones. It must be borne in mind, however, that these zones cannot be exactly defined, and that while the littoral zone may be understood to imply the area between tide-marks, and the abyssal zone a depth of 500 fathoms and upwards, the limits between the laminarian and the coralline, and between the coralline and abyssal zones can only be fixed approximately.
The difficulty of assigning special genera or species to special ‘zones of depth’ is increased by two important facts in the phenomena of distribution. In the first place, it is found that species which occur in shallow water in northern seas often extend to very deep water in much lower latitudes. This interesting fact, which shows the importance of temperature in determining distribution, was first established by the dredgings of the Lightning and Porcupine off the western coasts of Europe. In the second place, a certain number of species seem equally at home in shallow and in abyssal waters, in cases where a great difference of latitude does not occur to equalise the temperature. Thus the Challenger found Venus mesodesma living on the beach (New Zealand) and at 1000 fath. (Tristan da Cunha); Lima multicostata in ‘shallow water’ (Tonga and Port Jackson) and at 1075 fath. (Bermuda); Scalaria acus from 49 to 1254 fath. (N. Atlantic); and S. hellenica from 40 to 1260 fath. (Canaries). The Lightning and Porcupine found, or record as found,[382] Anomia ephippium at 0 to 1450 fath., Pecten groenlandicus at 5 to 1785 fath., Lima subauriculata at 10 to 1785 fath., Modiolaria discors at 0 to 1785 fath., Crenella decussata at 0 to 1750 fath., Dacrydium vitreum at 30 to 2750 fath., Arca glacialis at 25 to 1620 fath., Astarte compressa at 3 to 2000 fath., and Scrobicularia longicallus at 20 to 2435 fath. Puncturella noachina has been found at 20 to 1095 fath., Natica groenlandica at 2 to 1290 fath., Rissoa tenuisculpta at 25 to 1095 fath. In many of these cases we are assured that no appreciable difference can be detected between specimens from the two extremes of depth.
In spite, however, of these remarkable vagaries on the part of certain species, we are enabled roughly to distinguish a large number of genera as ‘shallow-water’ and ‘deep-water’ respectively, while a still larger number occupy an intermediate position. Among shallow-water genera may be named Patella, Littorina, Nassa, Purpura, Strombus, Haliotis, Mytilus, Cardium, Solen; while among deep-water genera are Pleurotoma, Scissurella, Seguenzia, Dentalium, Cadulus, Limopsis, Nucula, Leda, Lima, and Axinus.
Theories on the geographical distribution of marine Mollusca have been revolutionised by the discoveries of recent exploring expeditions. The principal have been those of Torell (Swedish) (1859–61) on the coasts of Greenland and Spitzbergen; of the Lightning and Porcupine (British) in 1868–70, in the N.E. Atlantic, off the Scotch, Irish, French, and Portuguese coasts, and in the Mediterranean; of the Challenger (British), under Sir C. Wyville Thomson, in 1873–76, in which all the great ocean basins were dredged or sounded; of the Blake (American), under Alexander Agassiz, in 1877–80, in the West Atlantic, Gulf of Mexico, and Caribbean Seas; of the Travailleur (French) in 1880–83, off the west coasts of France, Portugal, and Morocco, Madeira, the Canaries, and the Golfe du Lion; of the Talisman (French) in 1882, off the west coast of Africa from Tangier to Senegal, the Atlantic Islands, and the Sargasso Sea; of the Albatross (American) in 1891, off the west coast of tropical America; of several other vessels belonging to the U.S. Fish Commission and Coast Survey, off east American shores; and of the Prince of Monaco in the Hirondelle and Princesse Alice at the present time, in the N. Atlantic and Mediterranean.
The general result of these explorations has been to show that the marine fauna of very deep water is much the same all the world over, and that identical species occur at points as far removed as possible from one another. The ocean floor, in fact, with its uniform similarity of temperature, food, station, and general conditions of life, contains no effectual barrier to the almost indefinite spread of species.[383] To give a few instances. The Challenger dredged Silenia Sarsii in 1950 fath., 1100 miles south-west of Australia, and also in 2650 fath. off the mouth of the Rio de la Plata; Semele profundorum in 1125 fath. near the Canaries, and in 2900 fath. mid N. Pacific; Verticordia deshayesiana in 155 fath. near Cape York, and in 350 fath. off Pernambuco; Arca pteroessa in 2050 fath. mid N. Pacific, in 1000–1675 fath. west of the Azores, and in 390 fath. off the West Indies; Arca corpulenta in 1400 fath. off N.E. Australia, in 2425 fath. mid-Pacific, and in 1375 fath. near Juan Fernandez; Lima goliath in 775 fath. off S. Japan, and in 245 fath. off S. Patagonia; Pleurotoma engonia in 700 fath. north-east of New Zealand, and in 345 fath. off Inoshima. A surprising range was occasionally found even in shallow-water species; thus Petricola lapicida was discovered by the same expedition in the West Indies and N. Australia, Cardita calyculata off Teneriffe and in Bass Strait, Arca imbricata off Cape York and in the West Indies, Modiolaria cuneata at Port Jackson and Cape of Good Hope, Lima squamosa at Teneriffe and the Philippines. In these latter cases it is not improbable that the species lives in deep water as well, from which it has not yet been dredged.
It follows from these considerations that any attempt to classify marine Mollusca under Regions and Provinces can only apply to Mollusca which occur at moderate depths. The most important factor in the environment, as determining distribution, is the temperature of the water, which is probably to be regarded as affecting not so much the adult Mollusca as their ova; for the adult might possibly support life under conditions in which the ova would perish. It appears that a sudden change of temperature is the most effective barrier to distribution,[384] and may bring the range of a species to an almost instantaneous stop, while a very gradual change will allow it to extend its range very widely.
It has been usual to classify marine Mollusca from moderate depths under the following regions and sub-regions:--
Regions Sub-regions
{1. Arctic. {2. Boreal. =A.= =Atlantic= and {3. Celtic. =Circumpolar= {4. Lusitanian. {5. West African. {6. South African.
=B.= =Indo-Pacific= {1. Indo-Pacific. {2. Japanese.
=C.= =Australian= {1. Australian. {2. Neozealanian.
{1. Aleutian. {2. Californian. {3. Panamic. =D.= =American= {4. Peruvian. {5. Magellanic. {6. Argentinian. {7. Caribbean. {8. Transatlantic.
=A. The Atlantic Region=
includes the whole to the eastern shores of the Atlantic, from the extreme north of the Cape of Good Hope, together with the circumpolar seas, which may be regarded as roughly bounded by the Aleutian Islands and the coast of Newfoundland.
(1) The Arctic Sub-region includes the circumpolar seas, and is bounded in the N. Pacific by a line drawn between Cape Avinoff in Alaska, and Cape Lopatka in Kamchatka, so as to exclude the Aleutian Islands. On the western shores of the Atlantic the cold Labrador current brings it as far south as the coast of Newfoundland, but on the eastern shores the influence of the Gulf Stream has the contrary effect, so that the North Cape may be taken as its southern limit.
The principal genera (many species of which are common to the whole sub-region) are Volutomitra, Buccinum, Buccinopsis, Neptunea, Trophon, Bela, Admete, Velutina, Trichotropis, Lacuna, Margarita, Philine, Pecten, Leda, Yoldia, Astarte, and Mya. The shells are generally unicoloured, and of a dead white or rather sombre tint.
(2) The Boreal Sub-region may be subdivided into two provinces, the European and the American. The former includes the entire coast-line of Norway, the Färoe Islands, and Iceland (except perhaps the northern coast), and possibly the Shetland Islands; the latter the American coasts from the Gulf of St. Lawrence to Cape Cod (lat. 42°). Thus the Boreal American province does not extend nearly so far south as the Boreal European, the reason being that on the American coasts the cold Labrador current, which hugs the land, bars back the advance of southern genera, but allows Boreal genera to spread southwards, while on the European side the warmer conditions produced by the Gulf Stream keep the Boreal species back, and allow more southern forms to spread northwards.
Many of the Boreal species occur on both sides of the Atlantic, and thus support the theory of a more continuous fringe of continental land once existing along the north of the Atlantic. Among the prominent genera, besides several of those mentioned under the Arctic Sub-region, are Purpura, Chenopus, Littorina, Gibbula, Natica, Patella, Tectura, Chiton, Doris, Aeolis, Tellina, Thracia.
(3) The Celtic Sub-region includes the British Islands (excepting perhaps the Shetland Islands), the coasts of the North Sea and the Baltic, with N. France to Cape Ushant. The absence of any cold or warm current exerting direct influence upon the coast-line of this sub-region causes a very gradual change in the conditions of life as we move either southward or northward. The fauna of the British seas contains a decided mixture of northern and southern forms. The following are among the common Boreal species which attain their southward range on our coasts: Tectura testudinalis Müll. (to Dublin Bay and Scarborough), Trichotropis borealis Brod. (to the Dogger Bank), Margarita helicina Fabr. (to Yorkshire and Dublin Bay), M. groenlandica Chem. (western Scotland), Natica montacuti Forb. (to Cornwall), Trophon truncatus Str. (to Tenby), Chiton marmoreus Fabr. (to Dublin Bay and Scarborough). Buccinum undatum and Littorina littorea become very scarce on our extreme south-western coasts. Among Lusitanian species which reach our coasts are Gibbula magus L. (to Orkney and Shetland Islands), Phasianella pullus L. (to Caithness), Galerus chinensis L. (to Milford Haven), Galeomma Turtoni Turt. (to Weymouth), Cardium aculeatum L. (to Isle of Man), Solen vagina L. (to north Ireland).
It appears from the Mollusca of our Crag formations that at the time of their deposition the temperature of our seas must have been considerably warmer than it is now. Thus in the Crag we find many species and even genera (e.g. Mitra, Fossarus, Triton, Vermetus, Ringicula, Chama) which now occur no farther north than the southern coasts of the Channel, the west of France, and the Mediterranean.
The Baltic, a sea specially liable to violent changes of temperature, with a large admixture of fresh water at its eastern end, appears to possess only about 65 species in all. More than 50 genera occurring on the western coasts of Denmark do not enter the Sound. In the eastern portion of the Baltic marine and fresh-water species live together (p. 12).
(4) The Lusitanian Sub-region extends from Cape Ushant in the north to Cape Juby (lat. 28°) in the south, and includes the whole of the Mediterranean, as well as the Azores, Canaries, and Madeira groups.
The English Channel acts as an effectual barrier to the northward extension of many species; as many as 81 species which occur in western France do not reach British coasts (P. Fischer). At the same time, the western coasts of France are rather intermediate between the two sub-regions than distinctly Lusitanian, for between 50 and 60 Mediterranean genera do not occur on those coasts.
The Mediterranean itself is exceedingly rich in species, about 1200 in all (including deep-water species) being known. A certain number of these belong to tropical genera which here find their northern limit, e.g. Fasciolaria, Cancellaria, Sigaretus, Siliquaria, Chama, Spondylus. Here too occur Carinaria, Lobiger, Oxynoe, Pedicularia, Cypraea, Marginella, Mitra, Dolium, Cassis, Cassidaria, Pisania, Euthria, Vermetus, Argonauta, and many others. A few Celtic and even Boreal species, which occur on the western coasts of Morocco, do not enter the Mediterranean. Among these are Purpura lapillus, Helcion pellucidum, and Tellina balthica. Halia, a rare West African genus akin to Pleurotoma, is found in Cadiz Bay, and the West African Cymbium occurs on the Spanish coasts as far as Malaga.
The Black Sea, whose northern and western coasts are exceedingly cold, is comparatively poor in species. The Sea of Azof is chiefly characterised by forms of Cardium.
(5) The West African Sub-region extends from Cape Juby to a point probably not very far south of lat. 30° S., the cold current which sweeps up from the Pole probably limiting the southward extension of tropical species on this side of Africa, while the warm Mozambique current on the eastern side permits the spread of many Indo-Pacific species almost as far south as the Cape. Owing to its extreme unhealthiness, and the absence of harbours, the sub-region is very little known.
The principal genera are Cymbium, Pleurotoma, Marginella, Terebra, Mitra, Agaronia, Murex, Cancellaria, Purpura, Pseudoliva, Natica, Tellina, Lucina, Tugonia, Schizodesma, and Arca. Studer has enumerated as many as 55 species common to West Africa and the opposite American shores. The north and south equatorial currents, which circulate in this part of the Atlantic, probably transport the larvae from one coast to the other. Purpura coronata Lam., a characteristic West African species, is represented by a well-marked variety in Demerara.
The Mollusca of St. Helena (178 known species) most resemble those of the West Indies, 50 per cent being common, while 30 per cent are common to the Mediterranean. From Ascension Island only 33 species are known, which in their general relations resemble those of St. Helena.[385]
(6) The South African Sub-region extends along the coast from about lat. 30° on the west, to about East London on the east. Mr. G. B. Sowerby enumerates 740 species from ‘South Africa,’ but includes in this list Natal species, which more properly belong to the Indo-Pacific fauna. Of these 740, 323 are peculiar, while 67 also occur in European seas, some being familiar on our own shores. It is remarkable to find in a sub-region separated from ourselves by the whole width of the tropics, such well-known forms as Mangilia costata Don., M. septangularis Mont., Cylichna cylindracea Penn., Pholas dactylus L., Solen marginatus Pult., Cultellus pellucidus Penn., Ceratisolen legumen L., Lutraria oblonga Chem., Tellina fabula Gmel., T. tenuis Da C., Modiolaria discors L., and many others.
The leading genera are Euthria, Triton, Cominella, Bullia, Nassa, Cypraeovula, Oxystele, Fissurella, Fissurellidaea, Patella, and Chiton.
The Mollusca of Kerguelen Island and the Marion and Crozets groups show relationship partly with South America, partly with the Cape, and partly with South Australia and New Zealand, thus showing some trace of a circumpolar antarctic fauna corresponding to, but not nearly so well marked as that of the circumpolar arctic sub-region. Among the remarkable forms discovered off Kerguelen are Neobuccinum and a sub-genus of Struthiolaria (Perissodonta).
=B. The Indo-Pacific Region=
includes the whole of the coast-line of the Indian and western Pacific oceans, from about East London in South Africa to the north of Niphon (lat. 42°) in Japan, with the Red Sea and Persian Gulf, the whole of the Indo-Malay Archipelago, Polynesia to the Sandwich Islands in the north-east, and Easter Island in the south-east, and Australia to Swan River in the west, and to Sandy Cape and Lord Howe’s Island in the east. It is especially the region of coral reefs, which furnish so favourite a home of the Mollusca, and which are entirely absent from the Atlantic Region.
(1) The Indo-Pacific Sub-region proper (which includes the whole of this region except that part defined below as the Japanese Sub-region) is by far the richest in the world. The marine Mollusca of the Philippines alone (in some respects the nucleus of the whole region) have been estimated at between 5000 and 6000 species, and Jousseaume estimates Red Sea species at about 1000. Some prominent genera are very rich in species. Garrett enumerates from Polynesia 81 species of Conus, 60 of which occur on the Viti Is., 21 on the Sandwich Is., and only 14 on the Marquesas, where coral reefs are almost absent; 82 species of Cypraea, Viti Is. 44, Sandwich Is. 31, Marquesas only 13; 167 species of Mitra (besides 29 recorded by others), Viti Is. 120, Sandwich Is. 36, Marquesas 7. Of 50 existing species of Strombus, 39 occur in this region, and 10 out of 11 Eburna.
The following important genera are quite peculiar to the region: Nautilus, several forms of Purpuridae, e.g. Rapana, Magilus, Rapa, Melapium, and Ricinula; Tudicla, several forms of Strombidae, e.g. Rostellaria, Terebellum, Pteroceras, and Rimella; Cithara, Melo, Neritopsis, Stomatia, Malleus, Vulsella, Cucullaea, Tridacna, Hippopus, Libitina, Glaucomya, Anatina, Aspergillum, and many others.
The number of species common to the Red Sea and Mediterranean is exceedingly small, some authorities even denying the existence of a single common species. The present author, from an examination of the shells dredged by MacAndrew at Suez, regarded 17 species as common, and Mr. E. A. Smith has confirmed this view with regard to 8 of the species in question.[386] The Mollusca occurring in Post-pliocene beds at Suez show that Mediterranean species lived there in comparatively recent geological times.
The opening of the Suez Canal appears to have already induced several species to start on their travels from the Mediterranean to the Red Sea and vice versâ. Two Red Sea species (Mactra olorina Phil., Mytilus variabilis Kr.) had in 1882 established themselves at Port Said, while two Mediterranean species (Pholas dactylus L., Solen vagina L.) had reached Ismailia.[387]
(2) The Japanese Sub-region consists of the Japanese Islands to Niphon, together with Corea and a stretch of adjacent mainland coast of unknown extent. The warm Kuro Siwo current, sweeping up between Luzon and Formosa, permits tropical species to extend much farther north than on the opposite shores of America, where a cold polar current keeps them back. A certain number of species, however, are common to the two shores of the Pacific, and a few circumpolar species occurring on our own coasts reach Japan, _e.g.__Trophon clathratus_, Puncturella noachina, Mya arenaria, Modiola modiolus, Lasaea rubra, and Nucula tenuis.
Among the characteristic genera are Fusus, Siphonalia, Columbarium, Hemifusus, Rapana, Chlorostoma, Pleurotomaria, Haliotis, and Cyclina.
=C. The Australian Region=
includes the Australian coast-line from about Swan R.[388] (lat. 32° S.) to Sandy Cape (lat. 25° S.), Tasmania, New Zealand, and the adjacent islands (except Lord Howe’s I.).
(1) The Australian Sub-region proper (which consists of the whole of the region excepting New Zealand and the adjacent islands) is determined by the influence of the Antarctic Drift, which washes the whole of the southern coasts of Australia, and runs strongly northward between Australia and New Zealand. The E. Australian warm current from the north is checked at Sandy Cape by this cold current, and flows off to New Zealand, the western shores of which island are consequently much warmer than the eastern. On the western coast of Australia the Antarctic Drift has less force, and tropical genera accordingly range some 7 degrees farther south on the western than on the eastern coasts.
The characteristic genera are Voluta (of which half the known species occur on Australian coasts[389]), Cominella, Siphonalia, Struthiolaria, Risella, Phasianella, a number of genera belonging to the Trochidae, e.g. Liotia, Clanculus, Euchelus, Thalotia, Elenchus, Trochocochlea, Zizyphinus, Bankivia, Trigonia, Myodora, Myochama, Solenomya, Ephippodonta, Anapa, Mylitta, Mesodesma, and Chamostrea. Trigonia, originally discovered as a recent form in Sydney Harbour (p. 65), is not peculiar to that locality, occurring also off Cape York, West Australia, and Tasmania.
(2) The Neozealanian Sub-region includes New Zealand, with the outlying islands (Chatham, Auckland, and Campbell Is.).
As many as 455 species (Cephalopoda, 8; Gasteropoda, 311; Scaphopoda, 2; Pelecypoda, 134) have been enumerated by Professor F. W. Hutton as occurring within the sub-region, of which only 64 are found elsewhere, the proportion of peculiar species being thus nearly 86 per cent. New Zealand therefore is, in its marine, no less than its land Mollusca, greatly isolated.
The characteristic genera are Anthora, Cryptoconchus, and Vanganella, which appear to be quite peculiar, Trophon, Cominella, Euthria, most of the Trochidae also characteristic of S. Australia, Haliotis, Patella, Taria, Mesodesma, Mylitta, Zenatia, Standella, and Myodora.
=D. The American Region=
includes the entire coasts of North and South America with the adjacent islands, south of Cape Avinoff on the western, and south of Cape Cod on the eastern coast, the portions north of these points belonging to the Arctic Sub-region.
(1) The Aleutian Sub-region consists of the islands of Yesso and Saghalien, with the adjacent shores of the Sea of Okhotsk to Cape Lopatka, the Aleutian Is., and the west American coast from about Cape Avinoff (lat. 60° N.) to St. Jean de Fuca Straits.
A certain number of species, probably of arctic origin, are common with British and also with East American shores, the former being the more numerous. Species as familiar to us as Lacuna divaricata Fabr., Trichotropis borealis Brod., Pholas crispata L., Mya truncata L., M. arenaria L., Mytilus edulis L., and Modiolaria nigra Gray, occur. The more characteristic genera are Chrysodomus, Volutharpa, Buccinum, Tectura, Scurria, Chiton, Cryptochiton (Cr. Stelleri Midd. is by far the largest known of the Chitonidae, 6 inches long), Tellina and Pecten.
(2) The Californian Sub-region extends from St. Jean de Fuca Straits (lat. 48° N.) to Cape St. Lucas, the Gulf of California belonging to the Panamic sub-region. The northern polar current, which washes the shores of this sub-region throughout their whole extent, prolongs the southward range of the more northern genera, and keeps back those more markedly tropical, the latter, however, creeping northward in the warmer waters of the Gulf of California. Some authorities subdivide this immense stretch of coast-line, as characterised by sub-temperate, temperate, and sub-tropical genera, into the Oregonian, Californian, and Lower Californian provinces.
The characteristic genera are--in the north, Argobuccinum, Zizyphinus, Chlorostoma, Tectura, Scurria, Chiton (Katharina, Mopalia, Tonicia), Cryptochiton, Placunanomia, and Mytilimeria; in the centre, Purpura, Monoceros, Amphissa, Norrisia, Platyodon, Tapes, and Macoma; and, towards the south, Olivella, Chorus, Macron, Pseudoliva, Trivia, and Haliotis.
(3) The Panamic Sub-region extends from the head of the Gulf of California to Payta in Peru (lat. 5° S.). It is exceedingly rich in species, about 1500 having been described. The Mollusca are entirely distinct from those of the Indo-Pacific Region, which, although extending from Natal to the Sandwich Islands, are unable to pass the enormous extent of sea which separates the nearest Polynesian island from the American coast.
On the two sides of the isthmus of Panama there occur certain pairs of species, which, while specifically distant, are evidently closely related to one another. Amongst these are, on the Panamic side, Purpura speciosa, Cypraea cervinetta, Cassis abbreviata, Natica Chemnitzii, and Strombus gracilior, corresponding to Purpura deltoidea, Cypraea exanthema, Cassis inflata, Natica maroccana, and Strombus pugilis, on the Caribbean. It is reasonable to conclude that these “analogous species” are descendants of a stock which was common to both seas when the isthmus was open (probably not later than Miocene times), and which have, since the closing of the isthmus, become modified, some species considerably more than others.
Among the characteristic genera (compare p. 3) are Conus, Pleurotoma, Terebra, Murex, Purpura, Oliva, Northia, Cantharus, Columbella, Anachis, Cypraea, Strombus, Cerithium, Coecum, Crepidula, Crucibulum, Vitrinella; Tellina, Semele, Tellidora; and Arca.
(4) The Peruvian Sub-region extends from Payta in Peru to about the latitude of Conception in S. Chili (37° S.), being checked from further extension southward by the cold Humboldt current, whose force is distinctly felt as far north as Callao. This cold current thus produces the same results as the similar current which impinges on S. Africa, but has even more effect in decisively separating the fauna on the two sides of the great peninsula, scarcely a single species being common to the western and eastern coasts of S. America. The characteristics of the coast-lines themselves contribute to this result. The Chilian coast is rocky, and descends abruptly to a great depth, while that of Patagonia and Argentina is sandy and very shallow to a great distance from land.
The characteristic genera are Cancellaria, Columbella, Monoceros, Concholepas, Trochita, Fissurella, Chiton; Ceronia, Malletia, and Cumingia. Some of the Californian genera, absent or poorly represented in the Panamic Sub-region, reappear in Chili, e.g. Scurria, Tectura, and Chlorostoma.
(5) The Magellanic Sub-region includes the coast-line and adjacent islands (with the Falklands) from Conception in S. Chili to about Port Melo in Eastern Patagonia (lat. 45° S.).
The principal genera (many of which find a habitat on the gigantic Macrocystis which grows on every rock at low water) are Euthria, Voluta (6 species, one, V. magellanica, the largest known) Monoceros, Photinula, Patella, Chiton; Modiolarca, Malletia, and Mulinia. Several genera characteristic of the Boreal and Arctic sub-regions recur, e.g. Trophon, Admete, Margarita, Puncturella, Cyamium, and Astarte.
(6) The Argentinian[390] Sub-region extends from about Cape Melo in Patagonia to the neighbourhood of S. Caterina I. in South Brazil (lat. 28° S.). The sub-region stands in the same relation to the Magellanic, on the east coast, as the Peruvian sub-region on the west, but, owing to the influence of the warm Brazil current, which overpowers the colder water of the Falkland branch of the Cape Horn current, it reaches a point much farther south.
The Mollusca are not well known. The prevailing genera appear to be Oliva, Olivancillaria, Voluta, Bullia, Crepidula; Periploma, and Lyonsia.
(7) The Caribbean Sub-region extends from S. Caterina I. in the south to Florida in the north, and includes the shores of the Gulf of Mexico and the whole of the West Indies. The influence of the warm Brazil current (a branch of the South Equatorial) carries the range of the purely tropical species to a point much farther south than is reached by the tropical species on the west coast.
The sub-region is very rich in species, especially on the coral reefs of the Bahamas and N. Cuba, but the exceedingly small tide-fall makes shore collecting somewhat difficult beyond a certain point. The leading genera are Murex, Purpura, Melongena, Latirus, Marginella, Strombus, Triton, Cerithium, Littorina, Nerita, Scalaria; Tellina, Strigilla, Lucina, and Venus. Pleurotomaria, a genus long regarded as extinct, has been dredged alive off Tobago.
As compared with the tropical fauna of the Old World, that of the New World is poor in peculiar genera (compare p. 368). The relations of this sub-region to the West African and the Panamic have been already dealt with (pp. 367 and 372).
(8) The Transatlantic Sub-region extends from Florida to Cape Cod (see p. 364). In the north the limits of the sub-region are distinctly marked, in the south Caribbean species intermingle. Gould and Binney, in their Invertebrata of Massachusetts, enumerate 275 species (Cephalopoda, 6; Gasteropoda, 159; Scaphopoda, 2; Pelecypoda, 108), of which 59 (Gasteropoda, 37; Pelecypoda, 22) are British.
Among the characteristic genera are Urosalpinx, Eupleura, Fulgur, Ptychatractus, Nassa, Crepidula; Solenomya, Mactra, Cypricia, Raëta, Astarte, and Yoldia. Our common Littorina littorea appears to have been introduced into Nova Scotian waters in about 1857, no previous trace of it occurring either in literature or shell-heaps. Since then it has spread rapidly into the Gulf of St. Lawrence, and also as far south as Newhaven, and is said to be driving out the indigenous L. palliata from New England shores.[391] The debt has been repaid by the introduction into British waters of the American clam (Venus mercenaria L.), which, according to the Manchester City News of 23rd March 1889, was first observed in the Humber in 1864, and has steadily increased up to the present time, when it bids fair to compete, in those waters, with the familiar Cardium edule.
=Characteristics of Abyssal Mollusca.=--Large shells appear to be rare in the great ocean depths, and are usually very fragile; even moderately-sized specimens are far from common. The only group in which species occur larger than the usual size is the Nudibranchs, which are represented by at least one form larger than an orange.
It would seem that abyssal molluscs are much less active and energetic than their brethren on the shores. This view is favoured by the looseness of their tissues, which seem ill adapted for prompt and vigorous action. The tenacious character of the mud on the ocean floor must make rapid motion very difficult. The shell itself is usually fragile and delicate, the upper layers of arragonite being thin as compared with shallow-water species, which makes the nacreous layer, when present, appear unusually conspicuous; in many cases the surface is characterised by a peculiar iridescence or sheen. The colour in the shell of deep-sea Mollusca is never very pronounced, and is often absent altogether. Light pink and salmon, pale yellow and brown, are not uncommon. If the colour is in pattern, it is usually in the form of necklaces of spots, which sometimes coalesce into bands. With regard to sculpture, stout knobs and powerfully buttressed varices, such as occur in the tropical Murex and Purpura, are not found in deep-sea species. But the ornamentation is frequently elaborate, and the sculpture rich and varied. There is an especial tendency towards strings of bead-like knobs, revolving striae, and delicate transverse waves, the sculpture being in many cases of a character which tends to strengthen the structure of the shell, like the ridges in corrugated iron.
A remarkable feature in some deep-sea Mollusca is their singular resemblance, in shape, and particularly in the possession of a strong green periostracum, to some of our common fresh-water species. According to Dr. Dall, the cause of this phenomenon is the same in both cases. The fresh-water Mollusca secrete a strong periostracum, in order to protect the shell against the corrosive influence of the carbonic acid gas with which the water is surcharged. The shells of deep-sea Mollusca, living, as they do, in water probably undisturbed by currents of any kind, have to protect themselves against the same eroding influence, and do so in the same way.[392]
Mollusca which live exclusively on algae and other forms of plant life are almost entirely wanting in the great depths, where vegetation is probably unknown. The struggle for existence must be much less keen than in the thickly populated shallows, where vicissitudes of every kind occur. The absence of rapid motion of water must obliterate many of those mechanical effects which tend to produce modifying influences upon the animals affected. In the absence of circumstances tending to cause variation, in the unbroken monotony of their surroundings, species must, one would think, preserve a marked uniformity over an exceedingly wide area of range.
Vegetable food being wanting, those genera which in shallower waters never taste flesh, are compelled to become carnivorous. Characteristic of the great depths are very remarkable forms of Trochidae, in whose stomachs have been found the remains of Corallines and Foraminifera. According to Dr. Dall, the results of this diet show themselves in the greatly increased space occupied by the intestine, in the diminution, as regards size, of the masticatory organs, the teeth and jaws, and also in the prolongation of the anal end of the intestine into a free tube, which carries away the excreta in such a way that they do not foul the water taken into the gills. The amount of nutriment contained in the bodies of dead Foraminifera is so small that a comparatively large quantity must be swallowed to keep the vital energies active, and therefore the amount evacuated must be proportionately larger also. The abyssal Trochidae, then, and many other genera, sustain themselves by feeding on the ‘rain’ of dead animal matter which falls upon the ocean floor, not so much hunting their prey as opening their mouths and eating whatever happens to fall into them. Genera which are normally carnivorous would appear to do the same. The Pleurotomidae, for instance, are a family markedly characteristic of very deep water. Representatives of the genus which occur in shallower water are known to secure their prey while in the living state. But, according to Dr. Dall, a singularly small proportion of deep-sea Mollusca, as compared with those from the littoral region, show signs of having been drilled or attacked by other Mollusca. This could hardly be the case if the Pleurotomidae retained their predatory habits, since they are more numerous in the great depths than any six other families taken together. It has already been mentioned (p. 186) that a large proportion of deep-sea Mollusca are perfectly blind.
Amongst other remarkable forms from the great depths may be mentioned Pleurotomaria, with its singular anal slit (Fig. 269, p. 407) extending in some cases half way round the last whorl. Three or four species of this genus, so characteristic of almost all fossiliferous strata down to the Cambrian, have been obtained in very limited numbers off the West Indies and Japan. Dentaliidae, especially the sub-genus Cadulus, find a congenial home in the slimy ocean mud. One of the greatest molluscan treasures procured by the Challenger was Guivillea alabastrina Wats., a magnificent Volute as white as alabaster, 6½ inches long, which was dredged from 1600 fath. in the South Atlantic, between Marion Island and the Crozets. Another very curious form, belonging to the same family, is Provocator pulcher Wats., a shell about half the size of Guivillea, of stouter proportions, and with an angulated and patulous mouth. This shell was dredged by the Challenger in comparatively shallow water (105–150 fath.) off Kerguelen Island. Among the Trochidae are the fine new genera Basilissa, Bembix, and Gaza. The exploring voyages of the American surveying steamer Blake, in the Gulf of Mexico and the Caribbean Sea, have given us the remarkable new forms Benthobia (possibly akin to Admete), Mesorhytis (a sub-genus of Fasciolaria hitherto only known from the Cretaceous of North America), and Benthodolium (possibly = Oocorys), a genus akin to Cassis.
In his report on the Pelecypoda obtained by the Challenger, Mr. E. A. Smith remarks that as a rule “very deep-water ‘benthal’ species certainly have a tendency to be without colour and of thin structure, facts no doubt resulting from the absence of light, the difficulty of secreting lime, the scarcity of food, and other unfavourable conditions of existence.” At the same time, he notices that most of the species obtained belong to genera which, even when occurring in shallow water, are thin and colourless, e.g. Neaera, Lima, Cryptodon, Abra, Verticordia, etc. Deep-water species of such genera as have a decided periostracum (Malletia, Limopsis, Leda, Nucula, Arca) retain it with little if any modification. The deep-water Pelecypoda of the Atlantic and Pacific Oceans present no special features of interest. The species are few in number, and the genera are not remarkable either for novelty or peculiarity of form.
The greatest depth at which Pelecypoda have been obtained is 2900 fath. mid North Pacific (Callocardia pacifica Sm., Abra profundorum Sm.); the greatest depth at which Gasteropoda have been obtained is 2650 fath. South Atlantic (Stylifer brychius Wats.), both by the Challenger. The deepest Challenger Nudibranch came from 2425 fath., and the deepest Chiton from 2300 fath. The greatest depth ever dredged is 4575 fath. off the east coast of Japan.
XIII. CLASS CEPHALOPODA
The Cephalopoda present a complete contrast to the majority of the Mollusca in habits and in many points of organisation. In their power of rapid movement and their means of progression, their extreme ferocity and carnivorous habits, their loss, in so many cases, of a shell, and in its constitution when present, in the general symmetry of their parts, in their reproductive and nervous system, they stand in a position of extreme isolation with nothing to connect them with the rest of the phylum.
Professor A. E. Verrill has collected many interesting details with regard to gigantic Cephalopoda occurring on the north-eastern coasts of America. From these it appears that the tentacular arms of some species of Architeuthis measure as much as 32, 33, 35, and 42 feet in length, while the total length, including the body, sometimes exceeds 50 feet. Even off the Irish coast a specimen was once captured whose tentacular arms were 30 feet long, the mandibles 4 inches across, and the eyes about 15 inches in diameter.[393] The strength of these giant Cephalopods, aided as they are by formidable rows of suckers and other means of securing a grip, is almost incredible. Cases are not uncommon, in which persons diving or bathing have been attacked, and have with difficulty made their escape.
Great damage is frequently inflicted by Cephalopoda upon shoals of fish on British coasts. Off Lybster (Caithness) Loligo and Ommastrephes devour the herring, large numbers of which are cut up and bitten on the back of the neck by these creatures. On the American coasts the mackerel fisheries are sometimes entirely spoiled by the immense schools of squid which infest the Bay of St. Lawrence.[394] When excited in the pursuit of fish Cephalopoda leap high out of the sea. Dr. W. H. Rush[395] relates that when about 300 miles off the coast of Brazil, a swarm of hundreds of decapods flew from the water and landed on the deck of the ship, which was 12 feet above the surface level, and they had to go over the hammock nettings to reach it.
The common Octopus vulgaris Lam., of British and south European coasts, inhabits some rocky hole, the approaches to which, like the den of a fabled giant, are strewn with the bones of his victims. Homer himself knew how hard it is to drag the polypus out of his hole, and how the stones cling fast to his suckers. The colour-changes, which flit across the skin of the Octopus, appear, to some extent, expressive of the different emotions of the animal. They are also undoubtedly protective, enabling it to assimilate itself in colour to its environment. Mr. J. Hornell[396] has noticed an Octopus, while crawling over the rock-work in his tank, suddenly change the colour of the whole right or left side of its body, and of the four arms on the same side, to a snowy whiteness. They have also been seen to change colour, as if involuntarily, according to the material on which they crawl. The nerve-centres which control the chromatophores or pigment cells, causing them to expand or contract, are found to connect with the optic ganglia; hence the changes of colour may be regarded as a reflex result of the creature’s visual perception of its surroundings.
=Order Dibranchiata.=
Cephalopoda with two symmetrical branchiae, funnel completely tubular, mouth surrounded by 8 or 10 arms furnished with suckers or hooks, ink-sac and fins usually present, eyes with a lens; shell internal or absent.
The Dibranchiata are not known from Palaeozoic strata, and first appear (Belemnites, Belemnoteuthis) in the Trias. Whether they are to be regarded as derived from some form of Tetrabranchiata, e.g. Orthoceras, or as possessing an independent origin from some common stock, cannot at present be decided. They attain their highest development at the present time. The earliest representatives of the Order (the Phragmophora) possessed a shell chambered like that of the Tetrabranchiata. These chambered Dibranchiates rapidly reached their maximum in the upper Lias and as rapidly declined, until at the close of the Cretaceous epoch they were comparatively scarce, only a few genera (Beloptera, Spirulirostra) surviving into Tertiary times.
The ordinary Dibranchiate Cephalopod may be regarded as consisting of two parts--(a) the head, in which are situated the organs of sense, and to which are appended the prehensile organs and the principal organs of locomotion; (b) a trunk or visceral sac, enclosed in a muscular mantle and containing the respiratory, generative, and digestive organs. The visceral sac is often strengthened, and the viscera protected, by an internal non-spiral shell. The ‘arms’ which surround the mouth are modifications of the molluscan foot (p. 200), and are either eight or ten in number. In the former case (Octopoda) the arms, which are termed ‘sessile,’ are all of similar formation, in the latter (Decapoda), besides the eight sessile arms there are two much longer ‘tentacular’ arms, which widen at their tips into ‘clubs’ covered with suckers.
Remarks have already been made on the generative organs of Cephalopoda (p. 136 f.), the branchiae (p. 170), the nervous system (p. 206), the eye (p. 182), the radula (p. 236), and the ink-sac (p. 241).
One of the most characteristic features of the Dibranchiata are the acetabula, or suckers, with which the arms are furnished. They are usually disposed on the sessile arms in rows (of which there are four in most Sepia, two in Octopus, and one in Eledone), and become more numerous and smaller at the tip of the arm. They are massed together in large numbers of unequal size on the ‘clubs’ in the Decapoda, particularly in Loligo. In most Octopoda their base is flush with the surface of the arm, but in Decapoda the acetabula are pedunculate, or raised on short stalks. In Octopoda again, the acetabula are fleshy throughout, but in the Decapoda they are strengthened by a corneous rim with a smooth or denticulate edge (Ommastrephes, Architeuthis). Many of the acetabula on the tentacular and sometimes on the sessile arms of the Onychoteuthidae enclose a powerful hook, which is retractile like the claws of a cat.
In mechanical structure the acetabula consist of a disc with a slightly swollen margin, from which a series of muscular folds converge towards the centre of the disc, where a round aperture leads to a gradually widening cavity. Within this cavity is a sort of button, the caruncle, which can be elevated or depressed like the piston of a syringe; thus when the sucker is applied the piston is withdrawn and a vacuum created (Owen).
In many Octopoda the arms are connected by a web (the umbrella), which sometimes extends up the greater part of the arms (Cirrhoteuthis, some Eledone), at others occurs only at the base. The use of the umbrella is perhaps to assist in locomotion, by alternate contraction and expansion.
A cartilaginous skeleton is well developed, especially in the Decapoda. In Sepia a cephalic cartilage forms a complete ring round the oesophagus, the eyes being situated in lateral prolongations of the same. In front of the cephalic cartilage occurs a piece like an inverted T, which supports the base of the anterior arms. The Decapoda have also a ‘nuchal’ cartilage, connecting the head with the anterior dorsal portion of the mantle, while cartilaginous knobs on the ventral mantle button into corresponding sockets on the funnel.
=Sub-order I.=--Octopoda.--Body round or bag-like, generally without fins, arms eight, suckers fleshy, usually sessile, oviducts paired, no nidamental glands, shell absent.
FAM. 1. Cirrhoteuthidae.--Body with two prominent fins; arms in great part united by a web; one row of small suckers, with cirrhi on each side.--Atlantic and Pacific Oceans, deep water (Fig. 241).
FAM. 2. Amphitretidae.--Body gelatinous, mantle fused with the funnel in the median line, forming two openings into the branchial cavity; arms with one row of suckers; umbrella extending more than two-thirds up the arms.--South Pacific (Fig. 242).
The two pocket-like openings into the branchial cavity are unique among Cephalopoda (Hoyle).
FAM. 3. Argonautidae.--Female furnished with a symmetrical, unilocular shell, spiral in one plane, secreted by thin terminal expansions (the vela) of the two dorsal arms, no attachment muscle; suckers in two rows, pedunculate; male very small, without veligerous arms or shell.--All warm seas (Fig. 243). Pliocene----.
The shell consists of three layers, the two external being prismatic, the middle fibrous. Its secretion by the arms and not by the mantle edge is unique, and shows that it is not homologous with the ordinary molluscan shell.
The great controversy on the Argonauta, which once raged with so much fierceness, is now matter of ancient history. It seems scarcely credible that between fifty and sixty years ago, two of the leading zoologists of the day, Mr. Gray and M. de Blainville maintained that the animal which inhabits the Argonaut shell is a parasite, without any means of depositing or forming a shell of its own, but which possesses itself of the Argonaut shell, either by expelling or succeeding the original inhabitant, a supposed nucleo-branchiate (Heteropod) mollusc akin to Carinaria. The final blow to this strange hypothesis--which was urged by the most ingenious series of arguments--was given by Professor Owen, who in 1839 brought before the Zoological Society of London the admirable observations of Madame Jeannette Power, who made a continuous study of a number of specimens of Argonauta in her vivarium at Messina. The result of these observations tended to show that the young Argonauta when first excluded from the egg is naked, but that in ten or twelve days the shell begins to form, that the principal agents in the deposition of shell are the two velated or web-like arms; and that portions of the shell, if broken away, are repaired by a deposition of calcareous matter.[397]
FAM. 4. Philonexidae.--Mantle supported by two ridges placed on the funnel; large ‘aquiferous’ pores (supposed to introduce water into the tissues) near the head or funnel; suckers in two rows, pedunculate.--Atlantic and Mediterranean.
Genera: Ocythoe, arms of unequal size, no intervening membrane, third arm on the right hectocotylised (see Fig. 51, p. 138), two aquiferous pores at the base of the siphon; male very small; Tremoctopus, two aquiferous pores between the eyes, two on the ventral side of the head.
FAM. 5. Alloposidae.--Mantle edge united to the head by three commissures; arms extensively webbed, acetabula sessile. Hectocotylised arm developed in a cavity in front of the right eye.--N. Atlantic.
FAM. 6. Octopodidae.--Head very large, arms elongated, similar, more or less webbed, acetabula usually in two rows, sessile; mantle supported by fleshy bands, no cephalic aquiferous pores.
In Octopus proper the web is usually confined to the lower part of the arms; Fischer separates off as Pteroctopus a form in which it reaches almost to their extremity. The third right arm (Fig. 52, p. 140) is hectocotylised, the modified extremity being, according to Hoyle, sometimes minute, sometimes spoon-shaped, with a tendency to transverse ridges, rarely slender and very long. The relative length of the pairs of arms varies in different species. Two cartilaginous stylets, imbedded in the dorsal mantle, are said by Owen to represent the shell.
Other genera; Pinnoctopus, body furnished with broad lateral wings which meet at the posterior end; Cistopus, a large web prolonged along the sides of the arms, fitted with oval aquiferous pouches, with pores at their base, between each pair of arms; Eledone (Fig. 244), one row of acetabula; Tritaxeopus, Iapetella.
=Sub-order II.=--Decapoda.--Body oblong, mouth surrounded by four pairs of sessile and one pair of tentacular arms, the latter terminated by a ‘club’; acetabula pedunculate and furnished with a corneous margin; mantle margin locked to the base of the funnel by a cartilaginous apparatus; head and anterior part of body furnished with aquiferous pores; fins present; mandibles corneous; oviduct single, large nidamental glands in the female; shell internal.
The tentacular arms, which are the principal external feature of the Decapoda, are not derived from the same muscular ring as the sessile arms, but arise from the cephalic cartilage, and emerge between the third and fourth arm on each side. In Sepia they can be entirely retracted into a kind of pocket behind the eyes, while in Loligo they are simply folded over one another. In Chiroteuthis the arms are six times as long as the body, and the clubs have four rows of denticulate suckers.
The anterior ventral[398] portion of the mantle is furnished with a singular contrivance for locking it to the funnel, and so rendering the whole animal more capable of resisting the impact of any force. This contrivance generally consists of a series of ridges or buttons which fit into grooves or button-holes, the ridges being on the interior face of the mantle and the corresponding grooves on the funnel, or vice versâ. The ‘resisting apparatus’ is most elaborate in the pelagic genera, and least so in the more sluggish littoral forms. A similar, but not so complex, arrangement occurs also in the Octopoda.
The different forms of the shell appear to indicate successive stages in a regular course of development. We have in Spirula (Fig. 247) a chambered shell of the Tetrabranchiate type, but of considerably diminished size, which has ceased to contain the animal in its last chamber, and has become almost entirely enveloped in reflected folds of the mantle. These folds gradually concresce to form a definite shell-sac, by the walls of which are secreted additional laminae of calcareous shell-substance. These laminae invest the original shell, which gradually (Spirulirostra, Belosepia) loses the spiral form and becomes straight, eventually disappearing, while the calcareous laminae alone remain (Sepia). These in their turn disappear, leaving only the plate or ‘pen’ upon which they were deposited (Loligo), which itself also, with the shell-sac, finally disappears, surviving only in the early stages of Octopus (Lankester).
The Decapoda are divided, according to the character of the shell, into Phragmophora, Sepiophora, and Chondrophora.[399]
A. PHRAGMOPHORA.--Arms furnished with hooks or acetabula; shell consisting of a phragmocone or chambered sac enclosed in a thin wall (the conotheca), septa pierced by a siphuncle near the ventral margin (in Spirula alone this chambered sac forms the whole of the shell). The apex of the cone lies towards the posterior end of the body, and is usually enveloped in a calcareous guard or rostrum. Beyond the anterior end of the rostrum the conotheca is extended forward dorsally by a pro-ostracum or anterior shell, which may be shelly or horny, and corresponds to the gladius of the Chondrophora. The rostrum consists of calcareous fibres arranged perpendicularly to the planes of the laminae of growth, and radiating from an axis, the so-called apical line, which extends from the extremity of the phragmocone to that of the rostrum. Distribution, see p. 380.
FAM. 1. Spirulidae.--Arms with acetabula, shell a loose spiral, without rostrum or pro-ostracum, partially external, enclosed in two lobes of the mantle (Figs. 247 and 248).
The single species of the single genus (S. Peronii Lam. = laevis Gray) has not yet been thoroughly investigated, although the shell occurs in thousands on many tropical beaches, and is sometimes drifted on our own shores. The animal appears to have the power of adhesion to the rocks by means of a terminal sucker or pore. The protoconch is present, and contains a prosiphon, which does not connect with the siphuncle. The septal necks are continuous, not broken as in Nautilus. The siphuncle is on the ventral margin of the shell, the last whorl of which projects slightly on the dorsal and ventral sides, but is even there covered by a thin fold of the mantle. The retractor muscles of the funnel and of the head find their point d’appui on the shell, the last chamber of which contains the posterior part of the liver, with which the membranous siphuncle is connected.
FAM. 2. Belemnitidae.--Arms hooked as in Onychoteuthis, fins large; phragmocone straight, initial chamber globular, larger than the second, rostrum often very long, investing the phragmocone, pro-ostracum sword- or leaf-shaped, rounded in front, seldom preserved, ink-sac present.--Lower Lias to Cretaceous.
The Belemnitidae are believed to have been gregarious, and to have lived in shallow water on a muddy bottom. Specimens are sometimes found in which even the ink-sac can be recognised in situ. The relative proportions of rostrum and phragmocone vary greatly in different groups, the rostrum being in some cases two feet long, in others only just enclosing the phragmocone. As a rule the rostrum is the only portion which has been preserved.
FAM. 3. Belosepiidae.--Phragmocone short, slightly curved, chambers small, placed at the posterior end of a sepion, rostrum solid, obtuse.--Eocene (Paris, Bracklesham, etc.).
Fam. 4. Belopteridae.--Sepion not known; phragmocone curved, siphuncle on the ventral margin, rostrum well developed, pointed. Principal genus, Spirulirostra.--Miocene of Turin.
These two families, with their small, curved phragmocone and (in the case of the Belosepiidae) large sepion, are clearly intermediate between the Phragmophora and Sepiophora. Some authorities place them with the latter group.
B. SEPIOPHORA.--Shell internal, consisting usually of (a) an anterior cancellated portion, (b) a posterior laminated portion, the laminae enclosing air. It terminates in a very rudimentary phragmocone and a rostrum, but there is no siphuncle.
FAM. Sepiidae.--Eyes with cornea complete, body oval, fins narrow, lateral, as long as the body, generally united behind; sessile arms short, tentacular arms long, acetabula generally in four rows, fourth left arm in the male hectocotylised near the base (Fig. 249).--World-wide.
The sepion or ‘cuttle-bone’ runs the whole length and width of the body. In Sepia it is very thick in front, while the posterior ventral end is concave and terminated by a prominent spine, the rostrum or mucro which points downwards. The whole shell is surrounded by a thin chitinous margin, which forms a lateral expansion. Other genera are Sepiella, Hemisepius, and Trachyteuthis (fossil only).
C. CHONDROPHORA.--Shell (gladius or pen) long, chitinous.
(a) Myopsidae:[400] cornea entire, species mostly sub-littoral.
FAM. 1. Sepiolidae.--Fins large, dorso-lateral; tentacular arms retractile; two first dorsal arms in the male hectocotylised; gladius narrow, half as long as the body.--World-wide.
Principal genera: Sepiola, dorsal mantle connected with the head by a broad cervical band, ventral mantle with the funnel by a ridge fitting into a groove; Rossia, dorsal mantle supported by a ridge, arms with never more than four rows of acetabula; Inioteuthis, Stoloteuthis, Nectoteuthis, and Promachoteuthis.
FAM. 2. Sepiadariidae.--Fins not as long as the body, mantle united to the head on the dorsal side, fourth left arm in the male hectocotylised; no gladius. Principal genera, Sepiadarium, Sepioloidea.--Chiefly Pacific Ocean.
FAM. 3. Idiosepiidae.--Fins very small, terminal; fourth pair of arms in the male hectocotylised, bare of suckers.
The only genus, Idiosepion, with a single species (I. pygmaeum Stp.) is from the Indian Ocean, and is the smallest known Cephalopod, measuring only about 15 mm. in length.
FAM. 4. Loliginidae.--Body rather long, fins varying in size, tentacular arms partially retractile, gladius as long as the back, pointed in front, shaft keeled on the ventral side.--World-wide.
Loligo proper has a pointed body with triangular posterior fins united behind; sessile arms with two rows of acetabula, tentacular arms with four; fourth left arm hectocotylised at the tip; funnel attached to the head. Other genera are Loliguncula, Sepioteuthis, and Loliolus. Belemnosepia, Beloteuthis, Leptoteuthis, and Phylloteuthis are fossil genera only, differing in the shape of the gladius.
(b) Oigopsidae: cornea more or less open; species pelagic.
FAM. 5. Ommastrephidae.--Body cylindrical, fins generally terminal, united together, regularly rhomboidal, sessile arms with varying number of rows of acetabula, mantle connexions elaborate; gladius horny, narrow lanceolate, with a hollow cone at the posterior end.--World-wide.
Ommastrephes proper has a natatory web on the sessile arms; the wrist of each club has a series of acetabula with corresponding cushions on the other wrist. In Thysanoteuthis (often made a separate family) the sessile arms have two rows of cirrhi, with lateral expansions of the skin; fins as long as the body. In Architeuthis, to which belong the largest Cephalopoda known, the fins together are shaped like a broad arrow-head; acetabula of sessile arms strongly denticulate; tentacular arms very long, with equidistant pairs of acetabula and fixing cushions throughout their entire length, and a group of the same at the base of the club. The acetabula and cushions correspond on the opposing tentacles, and enable them to pull together. Other genera are Dosidicus, Todarodes, Illex, Bathyteuthis and Mastigoteuthis.
FAM. 6. Onychoteuthidae.--Body cylindrical, fins terminal or lateral, mantle-locking apparatus elaborate, tentacular arms very long, sessile or tentacular arms furnished with retractile hooks, gladius lanceolate, with a terminal cone.--World-wide.
The prehensile apparatus of Cephalopoda reaches its maximum of power and singularity in this family. In Onychia, Onychoteuthis and Ancistroteuthis, the sessile arms have acetabula only, in Gonatus and Abralia they have hooks as well, while in Verania, Ancistrochirus and Enoploteuthis, the sessile arms have hooks only. The number of rows of hooks or acetabula varies with the different genera.
FAM. 7. Chiroteuthidae.--Head nearly as large as the body; fins terminal, tentacular arms very long, sessile arms slightly webbed, acetabula denticulated; mantle-supports consisting of cartilaginous ridges on the mantle, which fit into corresponding depressions on the funnel, gladius expanded at each end.--Atlantic Ocean.
The six dorsal arms in Histioteuthis are united by a broad web, while in Histiopsis the web only reaches half way up the arm. In Chiroteuthis the tentacular arms have scattered sessile suckers throughout their whole length, and four rows of very long pedunculate suckers on the clubs.
FAM. 8. Cranchiidae.--Head small, body rounded, barrel-shaped, fins terminal, eyes often very large, sessile arms short, tentacular arms long, thread-like.--World-wide.
Cranchia proper has the tentacular clubs finned, with eight rows of suckers, body sometimes covered with warty tubercles. Loligopsis has a very attenuated body, with fins terminally united; some species are spotted with colour, or have rows of tubercles on the ventral side. Taonius (Fig. 251) is doubtfully distinct from Loligopsis.
=Order Tetrabranchiata=
Cephalopoda with four branchiae and four kidneys; animal inhabiting the last chamber of an external multilocular shell; funnel consisting of two separate lobes; tentacles numerous, without suckers or hooks; no ink-sac.
The shell consists of two layers, the outer being porcellanous, and the inner, as well as the walls of the chambers or septa, nacreous. The septa vary greatly in shape. In most of the Nautiloidea they are regularly curved, as in Nautilus, or straight, as in Orthoceras, but in the Ammonoidea they are often exceedingly complex. The edge of the septum, where it unites with the shell-wall, is called the suture, and the sutural line, which is not seen until the porcellanous layer is removed, varies in shape with the septum.
The septa are traversed by a membranous tube known as the siphuncle, which in Nautilus is said by Owen to connect ultimately with the pericardium. The septal necks, or short tubular prolongations of the septa where they are perforated by the siphuncle, are in the great majority of the Nautiloidea directed backwards (Fig. 252), i.e., they project from the front wall of each chamber, while in nearly all Ammonoidea they are directed forwards. When the siphuncle is narrow, as in the Ammonoidea, it is simple, but when wide, as in many of the Nautiloidea, its walls are often thickened by the deposition of masses of calcareous matter, or by rings and radiating lamellae of the same material. In position, the siphuncle is sometimes central, sometimes sub-central, sometimes (Ammonoidea) marginal. In some cases its position is believed to change during the growth of the individual. The precise object served by the siphuncle is at present unknown. Some hold that it preserves the vitality of the unoccupied chambers, by connecting them with the soft parts of the animal; others have regarded it as a means for lightening the shell by the passage of some gas into the chambers.
The initial chamber in Nautiloidea consists of an obtuse incurved cone, marked on the outer surface of its posterior wall by a small scar known as the cicatrix, which may be slit-like, round, oval, or cruciform in shape. It has been held that the cicatrix originally communicated with the protoconch or larval shell, which probably dropped off as development proceeded. In the Ammonoidea, on the other hand, there is no cicatrix, and the initial chamber probably represents the protoconch, as seen in the nucleus of many Gasteropoda.
=Sub-order 1.= Nautiloidea.--Shell straight, bent, or coiled, aperture simple or contracted; siphuncle often narrowed by internal deposits, position variable; septal necks short, usually directed backwards; septa concave towards the aperture; initial chamber conical, with a cicatrix on the posterior wall.
The Nautiloidea, of which Nautilus is the sole living representative, date back to the Cambrian epoch, and attain their maximum in the Silurian and Devonian. At the close of the Palaeozoic era, every family, with the sole exceptions of the Orthoceratidae and Nautilidae, appears to have become extinct. The former disappear with the Trias, and after the lapse of the whole Secondary era, Aturia, a form closely related to Nautilus, makes its appearance.
(a) Retrosiphonata: septal necks directed backwards.
FAM. 1. Orthoceratidae.[401]--Shell straight or slightly curved, aperture simple, body-chamber large; siphuncle cylindrical, position variable. Single genus, Orthoceras (Fig. 254). Cambrian to Trias.
FAM. 2. Endoceratidae.--Shell straight, siphuncle wide, marginal, septal necks produced into tubes fitting into one another. Principal genera: Endoceras (specimens of which occur six feet long), and Piloceras--Ordovician.
FAM. 3. Actinoceratidae.--Shell straight or slightly curved, siphuncle wide, contracted at the septa by obstruction-rings. Principal genera: Actinoceras, Discosorus, Huronia, Sactoceras.--Ordovician to Carboniferous.
FAM. 4. Gomphoceratidae.--Shell globular, straight or considerably curved, aperture narrowed, T-shaped, body-chamber large, siphuncle variable in position. The aperture is in some cases so narrow that probably only the arms could be protruded. Principal genus, Gomphoceras (Fig. 255).--Silurian.
FAM. 5. Ascoceratidae.--Shell sac-like or flask-shaped, apex truncated, unknown, body-chamber occupying nearly the whole of the shell on the ventral side, contracting at the aperture, last few septa coalescing on the dorsal side and encroaching upon the body-chamber. The young form has a symmetrical shell like Orthoceras, attached to the sac-like shell above described; as growth proceeds the former portion is thrown off. Principal genera: Ascoceras, Glossoceras.--Ordovician and Silurian.
FAM. 6. Poterioceratidae.--Shell fusiform, contracted at both ends, aperture simple, siphuncle variable in position, inflated between the septa. The form generally resembles Gomphoceras, except for the simple aperture and fusiform shape.--Ordovician to Carboniferous.
FAM. 7. Cyrtoceratidae.--Shell conical or sub-cylindrical, slightly curved, body-chamber large, siphuncle variable in position. Single genus, Cyrtoceras.--Cambrian to Carboniferous.
FAM. 8. Lituitidae.--Shell coiled in a flat, sometimes loose spiral, last whorl straight, containing the body-chamber, often greatly prolonged. Principal genera: Lituites, Ophidioceras.--Ordovician and Silurian.
FAM. 9. Trochoceratidae.--Shell helicoid, with seldom more than two whorls, dextral or sinistral, last whorl sometimes partly uncoiled. Principal genera: Trochoceras, Adelphoceras.--Ordovician to Devonian.
FAM. 10. Nautilidae.--Shell with few whorls, more or less overlapping, septa simple, siphuncle central or sub-central, aperture not contracted.
The ‘tentacles’ are about 90 in number, and consist of four groups each of 12 or 13 labial tentacles surrounding the mouth, two groups each of 17 larger (brachial) tentacles on each side of the head, two thicker tentacles which combine to form the ‘hood,’ and two small tentacles on each side of the eye. When the animal swims, the tentacles are extended radially from the head, somewhat like those of a sea-anemone. The direction of the many pairs of tentacles at constant but different angles from the head, is the most striking feature in the living Nautilus, and accounts for its being described, when seen on the surface, as ‘a shell with something like a cauliflower sticking out of it.’[402] The funnel is not a complete tube, but is formed by the overlapping of the margins of two thin fleshy lobes (which are probably morphologically epipodia), so that when the two lobes are parted, a broad canal appears, leading to the branchial cavity. The head is conical, and the mouth and its appendages can be retracted into a sort of sheath, over which fits the ‘hood.’
Other genera are Trocholites, Gyroceras, Hercoceras, Discites, Aturia.--Ordovician to present time.
FAM. 11. Bactritidae.--Shell straight, conical, siphuncle small, marginal, septal necks long, funnel-shaped, sutures undulating, with a sinus corresponding to the siphuncle. This family, from the form of its sutures, appears to constitute a passage to the Ammonoidea. Single genus, Bactrites.--Silurian and Devonian.
(b) Prosiphonata.--Septal necks directed forwards.
The two genera are Bathmoceras (Ordovician), shell straight, conical always truncated, siphon marginal; and Nothoceras (Silurian), shell nautiloid with simple sutures.
=Sub-order 2.= Ammonoidea.--Shell multiform, straight, curved, flat spiral, or turreted, sutural line more or less complex, siphuncle simple.
Some authorities hold that the members of this great sub-order, now totally extinct, belong to the Dibranchiata, on the ground that the protoconch resembles that of Spirula rather than that of the Nautiloidea. Others again regard the Ammonoidea as a third, and distinct Order of Cephalopoda. Their distribution extends from the Silurian to (possibly) the early Tertiary. No trace has ever been found of an ink-sac, mandible, or hooks on the arms; the shell was undoubtedly external.
The sutural line, which indicates the septa, and is generally concealed beneath the outer layer of shell, consists of a number of lobes or depressions, the concave part of which is directed towards the aperture. Between these lobes lie corresponding elevations, or saddles, the convex part of which is directed towards the aperture. There are six principal lobes (Fig. 256): the siphonal or ventral, which is traversed by the siphuncle, the dorsal, and a superior and inferior lateral on each side; smaller auxiliary lobes may succeed these latter. The adjacent saddles have received corresponding names. As a rule the sutural line is very complex, but in some cases (Goniatites, Lobites) it is simple (Fig. 258, A). The first saddle of a large number of genera serves as a means of classification, according as it is broad or narrow. Some authorities reverse the terms ventral and dorsal, as applied above. It is probable, however, that the position of the animal of Ammonites in its shell resembled that of Nautilus. The siphuncle is dorsal (internal) in Clymenia only, ventral (external) in all other genera.
The aptychus of Ammonoidea is a corneous or calcareous valve-like body, generally formed of two symmetrical parts (Fig. 257). It has been regarded by some as the covering of the nidamental gland, and hence as occurring only in the female, by others, with more probability, as an operculum, covering or imbedded in a hood formed, as in Nautilus, of modified arms. Sometimes the Aptychus is in a single piece (Anaptychus), sometimes the two pieces are united on the median line (Synaptychus).
The Ammonoidea are thus classified by Dr. P. Fischer:--
(a) Retrosiphonata Goniatitidae.
{ First saddle, { Arcestidae, Tropitidae, { No Aptychus or { wide { Ceratitidae, Clydonitidae. { Anaptychus { { corneous, { { Pinacoceratidae, Amaltheidae, (b) Prosiphonata { single { First saddle, { Ammonitidae, { { narrow { Lytoceratidae. { { Aptychus calcareous, valves { Harpoceratidae, Stephanoceratidae. { double or united
(a) Retrosiphonata. FAM. 1. Goniatitidae.--Shell nautiloid, whorls sometimes disjoined, siphuncle ventral or dorsal, sutures simple. Principal genera: Clymenia, Goniatites (Fig. 258, A).--Devonian to Carboniferous.
(b) Prosiphonata. FAM. 2. Arcestidae.--Shell globular, smooth or striated and rayed, body-chamber very long, aperture often with a projecting hood, umbilicus closed by a callosity, lobes numerous, foliaceous, aptychus present. Principal genera: Arcestes, Lobites.--Principally Trias.
FAM. 3. Tropitidae.--Differs from Arcestidae mainly in the more highly ornamented surface, which is decorated with ribs which become granular at the periphery. Principal genus, Tropites.--Trias and Lias.
FAM. 4. Ceratitidae.--Shell ribbed and tuberculated, body-chamber short, lobes denticulated, saddles simple. Principal genera: Ceratites (Fig. 258, B), Trachyceras.--Principally Trias.
FAM. 5. Clydonitidae.--Shell variable in form, body-chamber short, sutural line undulated, simple. Principal genera: Clydonites, Choristoceras, Rhabdoceras, Cochloceras.--Trias.
FAM. 6. Pinacoceratidae.--Shell discoidal, usually smooth, body-chamber short, sutural line very complex, lobes numerous. Principal genera: Pinacoceras, Sageceras.--Carboniferous to Trias.
FAM. 7. Amaltheidae.--Shell broad, keeled, last whorl concealing most of the spire, sutures with auxiliary lobes, incised.--Principal genera: Amaltheus, Schloenbacia, Sphenodiscus.--Trias, Cretaceous.
FAM. 8. Ammonitidae.--Body-chamber long, whorls narrow, uncovered, more or less ribbed, aperture simple, sutural line normal, aptychus single, corneous. Principal genera: Ammonites, Aegoceras.--Principally Lias.
FAM. 9. Lytoceratidae.--Shell discoidal, body-chamber short, aperture simple, no aptychus. Principal genera: Lytoceras, Phylloceras (Fig. 258, D).--Trias to Cretaceous.
FAM. 10. Harpoceratidae.--Shell discoidal, compressed, margin keeled, surface with straight or arched ribs, aperture with lateral projections, suture with accessory lobes, aptychus in two pieces. Principal genera: Harpoceras, Oppelia, Lissoceras.--Jurassic to Cretaceous.
FAM. 11. Stephanoceratidae.--Shell discoidal, helicoid or straight, whorls sometimes disunited, surface often with bifurcating ribs, which are tubercled, aperture often with lateral projections, sutural line incised, aptychus in two pieces, sometimes united.
In the discoidal group, Stephanoceras is strongly ribbed, tubercled at the point of bifurcation, Cosmoceras has long lateral projections of the aperture when young, Perisphinctes has a large body-chamber and numerous smooth ribs. Other genera are Acanthoceras, Peltoceras, Aspidoceras, and Hoplites. Among the loosely whorled genera, Scaphites (Fig. 260, A) has the last whorl produced and bent back again in horse-shoe form, while the early whorls are concealed; Hamites, Hamulina, and Ptychoceras have a shell shaped like a single or double hook, the sides of which may or may not be united; Crioceras (Fig. 260, B) in form of whorls resembles a Spirula, Ancyloceras a Scaphites with the first whorls disunited. Macroscaphites (Fig. 259, B) is similar, but with the first whorls united and not concealed. Turrilites (Fig. 259, A) is turreted and sinistral, while Baculites is quite straight, with a long body-chamber.
XIV. CLASS GASTEROPODA--AMPHINEURA AND PROSOBRANCHIATA
=Order I. Amphineura=
Bilaterally symmetrical Mollusca, anus at the terminal end of the body, dorsal tegument more or less furnished with spicules.
=Sub-order 1.= =Polyplacophora= (Chitons).--Foot co-extensive with ventral surface of the body, dorsum with eight transverse plates, articulated (except in Chitonellus), a row of ctenidia on each side between the mantle and the foot. Silurian ----.
The Chitons are found in all parts of the world, ranging in size from a length of about half an inch to six inches or more in the giant Cryptochiton. Although in the main sub-littoral, they occur at very great depths; the Challenger dredged Leptochiton benthus Hadd. at 2300 fathoms. Chiton Polii exceptionally occurs at Malta--teste MacAndrew--above sea margin, but within reach of the ripple. As a rule, the Chitons live in concealment, on the under surface of stones or in deep and narrow fissures in the rocks. When the stone to which they are attached is turned over, they crawl slowly to the side which is not exposed, as if disliking the light. An undescribed species, however, which I took at Panama, crawled quite as fast as an ordinary snail. Chiton fulvus Wood, apparently is accustomed to crawl with some rapidity. MacAndrew took it in abundance on his anchor chain in Vigo Bay every time his yacht was got under weigh. He also found it crawling in sand on the shore, to which habit is no doubt due its extreme cleanness and freedom from the foreign growths which are so characteristic of many of the species. When detached a Chiton contracts the muscles of the whole body, and rolls up into a ball like a wood-louse.
The Polyplacophora are characterised, externally, by their usually articulated shell of eight plates or valves, which is surrounded and partly kept in position by a muscular girdle. These plates overlap like tiles on a roof in such a way that the posterior edge of the first, cephalic, or anterior valve projects over the anterior edge of the succeeding valve, which in its turn overlaps the next, and so on throughout. Seven-valved monstrosities very rarely occur.
A certain portion of each valve is covered either by the girdle or by the valve next anterior to it. This portion, which is whitish in colour and non-porous in structure, forms part of an inner layer which underlies the rest of the substance of the valve, and is called the articulamentum. The external portion of the valves, or tegmentum, is generally more or less sculptured, and is largely composed of chitin, impregnated with salts of lime, thus answering more to a cuticle than to a shell proper. It is very porous, being pierced by a quantity of minute holes of two sizes, known as megalopores and micropores, which are connected together by minute canals containing what is probably fibrous or nerve tissue, the mouths of the pores being occupied by sense organs connected with these nerves. The tegmentum of the six intermediate valves is generally divided into three triangular areas by two more or less prominent ribs, which diverge from the neighbourhood of the median beak or umbo. The space enclosed between these ribs is known as the median area or jugum, the other two spaces as the lateral areas or pleura. The ribs terminate with the edge of the tegmentum, and are not found on the articulamentum. In certain genera these areas are either non-existent, or are not distinctly marked. The sculpture of the lateral areas (which is, as a rule, much stronger than that of the median area) will generally be found to resemble that of the anterior valve, which has no proper median area. In the posterior valve the median area is very small, while the sculpture of the rest of the valve corresponds to that of the lateral areas generally (see Fig. 261).
The articulamentum of the intermediate valves is divided into two equal parts in the middle of the anterior edge, opposite to the beak, by a sinus. Each of the portions thus formed is again divided by a notch or suture into two unequal parts, the anterior of which is known as the sutural lamina, and is more or less concealed by the valve in front of it, while the lateral part, or lamina of insertion, is entirely concealed by the girdle. The articulamenta of the anterior and posterior valves are either simple or pierced by a series of notches (Fig. 263).
The girdle of the Chitonidae varies considerably in character. Sometimes its upper surface is simply corneous or cartilaginoid, with no other sculpture than fine striae, at others it is densely beset with spines or bristles, or tufted at intervals with bunches of deciduous hairs; again it is marbled like shagreen or mossy down, or covered with serpent-like scales. The width of the girdle varies greatly, being sometimes very narrow, sometimes entirely covering all the valves (Cryptochiton). As a rule, its outer edge is continuous, but in Schizochiton it is sharply notched over the anus.
A description has already been given of the dorsal eyes in Chiton (p. 187), the nervous system (p. 202), the branchiae (p. 154), the radula (p. 228), and the generative system (p. 126).
The recent Chitons are thus classified by Dr. W. H. Dall:--
SECTION I. CHITONES REGULARES.--Anterior and posterior valves of similar character.
A. Leptoidea.--Insertion plates obsolete, or, if present, unslit; Leptochiton, Hanleyia, Hemiarthrum, Microplax.
B. Ischnoidea.--Insertion plates sharp, smooth, fissured; with eaves; Trachydermon, Callochiton, Tonicella, Schizoplax, Leptoplax, Chaetopleura, Spongiochiton, Ischnochiton, Callistochiton.
C. Lophyroidea.--Insertion plates broad, pectinated, projecting backward; Chiton, Tonicia, Eudoxochiton, Craspedochiton.
D. Acanthoidea.--Insertion plates thrown forward; Sclerochiton, Acanthopleura, Dinoplax, Middendorffia, Nuttallina, Arthuria, Phacellopleura.
SECTION II. CHITONES IRREGULARES.--Posterior valve abnormal, or with a sinus behind.
E. Schizoidea.--Posterior valve fissured; Lorica, Schizochiton.
F. Placiphoroidea.--Posterior valve unslit, internally ridged, umbo nearly terminal; Enoplochiton, Ornithochiton, Plaxiphora.
G. Mopaloidea.--Posterior valve with posterior sinus and one slit on each side; Mopalia, Katherina, Acanthochiton, Notoplax.
H. Cryptoidea.--With double sutural laminae; Cryptoconchus, Amicula, Cryptochiton.
I. Chitonelloidea.--Posterior valve funnel shaped; laminae thrown forward; Chitonellus, Choneplax.
=Sub-order 2.= =Aplacophora.=--Animal vermiform, foot absent, or a mere groove, cuticle more or less covered with spicules.
According to Marion, one of the principal authorities on the group, the Aplacophora are perhaps Amphineura whose development has been arrested at an early stage, their worm-like exterior being due to adaptation to surroundings. They have hitherto been found chiefly in the N. Atlantic and Mediterranean, generally at considerable depths, and often associated with certain polyps in a way which suggests a kind of commensalism.
FAM. 1. Neomeniidae.--Foot a narrow groove, intestinal tube without differentiated liver, kidneys with common exterior orifice, sexes united, ctenidia present or absent. Genera: Neomenia (Fig. 266), Paramenia, Proneomenia, Ismenia, Lepidomenia, Dondersia.
FAM. 2. Chaetodermatidae.--Body cylindrical, no ventral groove, liver a single sac, kidneys with separate orifices into the branchial cloaca, two bipectinate ctenidia. Single genus, Chaetoderma (Fig. 267).
=Order II. Prosobranchiata=
Visceral loop twisted into a figure of 8 (streptoneurous), right half supra-intestinal, left half infra-intestinal; heart usually in front of the branchia (ctenidium), which is generally single; head with a single pair of tentacles; animal dioecious, usually marine, more or less contained within a shell, operculum generally present. Cambrian to present time.
=Sub-order 1.= =Diotocardia.=--Heart with two auricles (except in the Docoglossa and Helicinidae), branchiae bipectinate, front end free; two kidneys, the genital gland opening into the right (except in Neritidae); nervous system not concentrated; no proboscis or siphon, penis usually absent.
(a) DOCOGLOSSA (p. 227).--Heart with a single auricle, ventricle not traversed by the rectum, visceral sac not spiral, shell widely conical, non-spiral, no operculum; radula very long, with few hooked teeth in each row.
FAM. 1. Acmaeidae.--Left ctenidium alone occurring, free on a long stalk. Cretaceous----. Principal genera: Pectinodonta, front part of head much produced, radula 0 (1. 0. 1.) 0; Acmaea (= Tectura), with sub-genera Collisella and Collisellina, no accessory branchial ring, shell closely resembling that of Patella, but generally with a distinct internal border; Scurria, accessory branchial ring on the mantle.
FAM. 2. Lepetidae.--No ctenidia or accessory branchiae, animal generally blind. Pliocene----. Principal genera: Lepeta; Propilidium, apex with internal septum; Lepetella.
FAM. 3. Patellidae.--No ctenidia, the osphradial patch at the base of each alone surviving, a circlet of secondary branchiae between the mantle and sides of the foot. Ordovician----. (i.) Patellinae.--Three lateral teeth on each side, two of them anterior. Principal genera: Patella, branchial circlet complete; chief sections Patella proper, Scutellastra, Ancistromesus (A. mexicana Brod., measures 8–14 in. long); Helcion, branchial circlet interrupted in front; Tryblidium (Ordovician).--(ii.) Nacellinae.--Two developed laterals on each side, one anterior. Genera: Nacella, branchial circlet complete; Helcioniscus, branchial circlet interrupted in front.
(b) RHIPIDOGLOSSA (p. 225).--Ventricle of the heart traversed by the rectum (except in Helicinidae), one or two ctenidia; jaw in two pieces, radula long, marginals multiplied, rows curved.
Of all the Gasteropoda, this section of the Diotocardia approach nearest to the Pelecypoda, particularly in the least specialised forms. The auricle, the branchiae, and the kidneys are in many cases paired, and more or less symmetrical. The ventricle is generally traversed by the rectum, there is a long labial commissure between the cerebral ganglia, special copulative organs are usually absent, while the shell is often nacreous, like those of Pelecypoda of a primitive type.
SECTION I. ZYGOBRANCHIATA.--Two ctenidia, shell with apical or marginal slit or holes, corresponding to an anal tube in the mantle (p. 265).
FAM. 1. Fissurellidae.--Two symmetrical ctenidia and kidneys, visceral mass conical, shell conical, elevated or depressed, with a single anterior or apical slit or impression; no operculum. Jurassic----. (i.) Fissurellinae. Shell wholly external, apex entirely removed by perforation, apical callus not truncated posteriorly; central tooth narrow. Genera: Fissurella (Figs. 171, p. 261; 178, p. 265), Fissuridea, Clypidella. (ii.) Fissurellidinae. Shell partly internal, otherwise as in (i.); central tooth broad, mantle more or less reflected over the shell, apical hole very wide. Genera: Fissurellidaea, Pupillaea, Lucapina, Megatebennus, Macroschisma, Lucapinella. (iii.) Emarginulinae. Shell usually wholly external, apex usually not removed by perforation, sometimes with internal septum, anal tube in a narrow slit or sinus. Genera: Glyphis, externals of Fissurella, but holecallus truncated behind; Puncturella (sub-genera Cranopsis and Fissurisepta), slit just anterior to the apex, a small internal septum; Zeidora, large internal septum as in Crepidula: Emarginula, shell elevated, slit very narrow, on the anterior margin (in subg. Rimula, it is between the apex and the margin), radula bilaterally asymmetrical; Subemarginula, margin indented by a shallow groove; Scutus (= Parmophorus) shell oblong, depressed, nicked in front, largely covered by the mantle.
FAM. 2. Haliotidae.--Right ctenidium the smaller, epipodial line broad, profusely lobed; shell rather flattened, spire short, last whorl very large, with a row of perforations on the left side, which become successively obliterated; through these holes, the posterior of which is anal, pass tentacular appendages of the mantle; no operculum. Cretaceous----. Single genus, Haliotis; principal sub-genera Padollus, Teinotis.
FAM. 3. Pleurotomariidae.--Central tooth single, narrow, about 26 laterals, 60 to 70 uncini. Shell generally variously trochiform, nacreous, operculate, with a rather broad marginal sinus in the last whorl; as this sinus closes up it forms an “anal fasciole” or “sinus band.” Cambrian----. Principal genera: Scissurella, epipodial line with several long ciliated appendages at each side, shell very small, slit open, sinus band extending nearly to apex; Schismope, anal slit closed in the adult into an oblong perforation; Murchisonia (Palaeozoic only), shell long, turreted, whorls angulate or keeled with a sinus band; Odontomaria (Palaeozoic only), shell tubular, curved; Polytremaria (Carboniferous), shell turbinate, slit a series of small holes connected by a passage; Trochotoma, shell trochiform, perforation consisting of two narrow holes united by a slit; Pleurotomaria, branchiae almost symmetrical, radula as above, shell variously spiral.
In Pleurotomaria we have the case of a genus long supposed to be extinct. More than 1100 fossil species have been described, and within the last 38 years about 20 specimens, belonging to 5 species, have been discovered in a living state.
FAM. 4. Bellerophontidae.--Shell nautiloid, spire generally concealed, aperture large, sinus or perforations central (Fig. 179, p. 266). Ordovician--Trias. Genera: Bellerophon, Trematonotus, Cyrtolites.
SECTION II. AZYGOBRANCHIATA.--One ctenidium (the left) present.
FAM. 1. Cocculinidae.--A single cervical ctenidium, foot broad, no eyes, shell patelliform, with caducous spire. Single genus, Cocculina. Deep water.
FAM. 2. Stomatellidae.--A single (left) ctenidium, front third free, shell nacreous, spiral or patelliform, depressed, last whorl large. Jurassic----. Genera: Stomatella (subg. Synaptocochlea, Niphonia), shell depressed, spirally ribbed, spire short, operculum present; Phaneta, fluviatile only, shell trochiform, imperforate, last whorl keeled, sinuate in front; Stomatia, spire short, surface tubercled or keeled, no operculum; Gena, shell haliotis-shaped, surface smooth, aperture very large; Broderipia, shell patelliform, spiral apex often lost.
FAM. 3. Cyclostrematidae.--Tentacles ciliated, thread-like, snout bilobed, foot truncated in front, angles produced into a filament, shell depressed, umbilicated, not nacreous. Eocene----. Principal genera: Cyclostrema, Teinostoma, Vitrinella.
FAM. 4. Liotiidae.--Epipodial line with a lobe behind each eye-peduncle, shell solid, trochiform, longitudinally ribbed or trellised, aperture round, operculum multispiral, hispid, corneous, with a calcareous layer. Silurian----. Principal genera: Liotia, Craspedostoma (Silurian), Crossostoma (Jurassic).
FAM. 5. Trochidae.--Snout short, broad, frontal lobes often present, epipodial line furnished with cirrhi; shell nacreous, variously spiral, operculum corneous, multispiral, nucleus central (Fig. 182, p. 268). Silurian----. (i.) Trochinae.--Frontal lobes present, lateral teeth (= side centrals) 5 only, no jaws, peristome incomplete. Principal genera: Trochus (subg. Cardinalia, Tectus, Infundibulum, Clanculus), Monodonta (subg. Diloma), Cantharidus (subg. Bankivia, Thalotia), Gaza (subg. Microgaza), Callogaza, Bembix, Chlorostoma. (ii.) Gibbulinae.--Frontal lobes and jaws present, laterals often more than 5, peristome incomplete. Principal genera: Gibbula (subg. Monilia, Aphanotrochus, Enida), Minolia, Circulus, Trochiscus, Livona, Photinula, Margarita, Solariella, Calliostoma, Turcica, Basilissa, Euchelus (subg. Olivia, Perrinia). (iii.) Delphinulinae.--No frontal lobes, jaws present; shell solid, surface spirally lirate, scaly, spinose, umbilicate, peristome continuous. Single genus, Delphinula. (iv.) Umboniinae.--Eyes pedunculate, left tentacle attached to a frontal appendage, mantle reflected over edge of aperture, lateral teeth 6 on each side; shell polished, peristome incomplete, umbilicus generally closed by a callosity. Principal genera: Umbonium, Ethalia, Isanda, Camitia, Umbonella, Chrysostoma.
FAM. 6. Turbinidae.--Epipodial line with slender cirrhi, snout broad, short, eyes pedunculate at outer base of tentacles, a frontal veil between tentacles; shell turbinate, solid, aperture continuous, operculum solid, calcareous, usually paucispiral, convex exteriorly (Fig. 182, p. 268). Silurian----. (i.) Phasianellinae.--Shell bulimoid, polished, not nacreous, coloured in patterns, aperture oval. Single genus, Phasianella (Fig. 271). (ii.) Turbininae.--Shell very solid, nacreous within, aperture circular or long oval. Principal genera, Turbo, whorls rounded above and below, spines, if present, becoming more prominent with age, operculum smooth or granulose, nucleus sub-central; subg. Callopoma, Ninella, Marmorostoma, Sarmaticus, Prisogaster; Astralium, whorls flattened above and below, spines, if present, becoming less prominent with age, operculum oblong, often excavated at centre, last whorl large, nucleus marginal or sub-marginal; subg. Lithopoma, Imperator, Guildfordia, Bolma, Cyclocantha, Uvanilla, Cookia, Pomaulax, Pachypoma. (iii.) Cyclonematinae.--Shell nacreous, umbilicate, operculum conical outside, whorls scalariform. Principal genera: Cyclonema, Horiostoma (?), Amberleya (Silurian to Lias). (iv.) Leptothyrinae.--Shell small, solid, depressed, operculum nearly flat, nucleus sub-central. Genera: Leptothyra, Collonia (?).
FAM. 7. Neritopsidae.--Tentacles wide apart, long, eyes on short peduncles at the outer base; shell solid, neritiform or naticoid, aperture semi-lunar or oval; operculum (Fig. 183, p. 269) thick, calcareous, non-spiral, exterior face smooth, interior face divided into two unequal parts, with a broad median appendage. Devonian----. Principal genera: Neritopsis (one recent species), Naticopsis (Devonian to Miocene).
FAM. 8. Macluritidae.--Shell discoidal, whorls few, longitudinally grooved behind, right side convex, deeply umbilicated, left side flat; operculum very thick, nucleus excentrical, internal face with two apophyses, one very large. The general appearance is more that of an inequivalve bivalve, such as Requienia, than of a spiral gasteropod. Palaeozoic----. Single genus, Maclurea.
FAM. 9. Neritidae.--Snout short, tentacles long, eyes pedunculate at their outer base, branchia triangular, free at the front end, epipodium without cirrhi, penis near the right tentacle; shell solid, imperforate, turbinate to almost patelliform, spire short, internal partitions absorbed (p. 168), columellar region broad, edge simple or dentate, operculum calcareous, spiral or non-spiral, with prominent apophyses on the interior face, one of which locks behind the columellar lip. Jurassic----. Principal genera: Nerita (Fig. 13, p. 17); Neritina (chiefly brackish water and fluviatile), sub-genus Clithon, usually coronated with spines; Velates (Tertiary), Neritoma (Jurassic), Deianira (Cretaceous), Septaria (= Navicella), shell more or less narrowly patelliform, with terminal apex, aperture very large, with a broad columellar septum, operculum too small for the aperture, more or less covered by the integument of the foot; fluviatile only; Pileolus (Jurassic to Cretaceous).
FAM. 10. Hydrocenidae.--Branchia replaced by a pulmonary chamber, eyes at the outer base of the tentacles, marginals of the radula very oblique, centrals often wanting; shell small, conical, whorls convex, operculum calcareous, with a prominent apophysis. Recent. Principal genera: Hydrocena, Georissa.
FAM. 11. Helicinidae.--Branchia replaced by a pulmonary chamber, heart with one auricle; shell globular, with a short spire, internal partitions absorbed; operculum without apophysis. Carboniferous----. Principal genera: Helicina (Fig. 18B, p. 21; subg. Alcadia, Schasicheila, Heudeia, Calybium), Eutrochatella (subg. Lucidella), Stoastoma, Bourcieria, Dawsonella (Carboniferous).
FAM. 12. Proserpinidae.--Branchia replaced by a pulmonary chamber, mantle partly reflected over the shell, eyes sessile; shell depressed, discoidal, columella folded or truncated at the base, whorls with one or more internal plicae, internal partitions absorbed, no operculum. Eocene----. Single genus; Proserpina, subg. Proserpinella, Cyane, Dimorphoptychia (Eocene), and Ceres (Fig. 18C, p. 21).
=Sub-order II.= =Monotocardia.=--Heart with one auricle, one ctenidium (the left), monopectinate, fused with the mantle (except in Valvata), one kidney, not receiving the genital products, nervous system somewhat concentrated, proboscis and penis usually present.
(a) PTENOGLOSSA.--Radula with formula ∞. ᴑ. ∞, teeth similar throughout, outermost largest (p. 224).
FAM. 1. Ianthinidae.--Snout prominent, blunt, no eyes, shell helicoid, fragile, bluish, no operculum; eggs carried on a raft of vesicles attached to the foot (Fig. 42, p. 126). Pelagic only. Pliocene ----. Genera: Ianthina, Recluzia.
FAM. 2. Scalariidae.--Shell long, turriculate, whorls often partly uncoiled, with longitudinal ribs and prominent lamellae, aperture circular, operculum spiral, corneous, animal carnivorous. Ordovician ----. Principal genera: Scalaria, Eglisia, Elasmoneura (Silurian), Holopella (Silurian to Trias), Aclis.
(b) TAENIOGLOSSA.--Radula with normal formula 2.1.1.1.2, marginals sometimes multiplied (p. 223).
SECTION I. PLATYPODA.--Foot more or less flattened ventrally.
FAM. 1. Naticidae.--Foot very large, produced before and behind, propodium reflected upon the head, eyes absent or buried in the integument, central and lateral tooth of the radula tricuspid, middle cusp strong; shell globular or auriform, outer lip simple, operculum corneous or calcareous, nucleus excentrical. Carboniferous ----. Principal genera: Natica, with many sub-genera; Ampullina (Tertiary); Amaura; Deshayesia (Tertiary); Sigaretus (Fig. 91, p. 186), shell auriform, last whorl very large, operculum much too small for the aperture.
FAM. 2. Lamellariidae.--Mantle reflected over more or less of the shell, shell delicate, no operculum. Eocene----. Principal genera: Lamellaria, shell completely internal, transparent, auriform; some species deposit their eggs on compound Ascidians (p. 74); Velutina, shell almost entirely external, paucispiral, with a thick periostracum; Marsenina, shell auriform, partly internal; Onchidiopsis, shell a membranous plate, internal.
FAM. 3. Trichotropidae.--Branchial siphon short, eyes on the outer side of the tentacles; radula closely allied to that of Velutina; shell conical, last whorl rather large, periostracum thick and hairy, operculum blunt claw-shaped, nucleus terminal. Cretaceous----. Genera: Trichotropis, Torellia.
FAM. 4. Naricidae.--Tentacles broad in the middle, with sessile eyes at the exterior base, propodium narrow, quadrangular, a large epipodial veil on each side of the foot; shell naticoid, cancellated, with velvety periostracum. Jurassic----. Single genus: Narica.
FAM. 5. Xenophoridae.--Foot divided by a groove, anterior portion the larger; central tooth heart-shaped, with blunt cusps, lateral large, roughly triangular, marginals long, falciform; shell trochiform, somewhat flattened, attaching various fragments externally. Devonian----. Single genus, Xenophora (Figs. 25, 26, p. 64).
FAM. 6. Capulidae.--Ctenidium deeply and finely pectinate, visceral sac scarcely spiral, penis long, behind the right tentacle; shell roughly patelliform, with scarcely any spire, interior polished, usually with a septum or internal plate of variable form, no operculum. Devonian----. Principal genera (Fig. 155, p. 248); Capulus, shell cap-shaped, no internal plate; Platyceras (Palaeozoic, see p. 76), Diaphorostoma (Palaeozoic), Addisonia (?); Crucibulum, internal appendage funnel-shaped; Crepidula (including Crepipatella and Ergaea), shell slipper-shaped, with a large septum; Calyptraea (including Galerus and Trochita), internal lamina semi-spiral.
FAM. 7. Hipponycidae.--Foot aborted, animal sedentary, adductor-muscle shaped like a horse’s hoof, fastened on the ventral side to the region of attachment, or to a thin calcareous plate which closes the aperture like a valve; ventral side of the body surrounded by a mantle with papillose border, which corresponds morphologically to the epipodia, head emerging between the dorsal and ventral mantles. Shell thick, bluntly conical, surface rugose. Eocene----. Genera: Hipponyx; Mitrularia, a narrow half funnel-shaped appendage within the shell.
FAM. 8. Solariidae.--Foot large, eyes sessile, near the outer base of the tentacles, radula abnormal (p. 224); shell more or less depressed, lip simple, umbilicus wide, margins often crenulated, operculum variable. The proper position of the family is quite uncertain. Ordovician----. (i.) Solariinae. Genera: Solarium, shell depressed, highly finished, angular at periphery, operculum corneous, central tooth absent, laterals and marginals numerous, long, and narrow; Platyschisma (Silurian). (ii.) Toriniinae. Genera: Torinia, whorls usually rounded, operculum (Fig. 183) conically elevated, spiral externally, central tooth present, marginals few, edge pectinated; Omalaxis. (iii.) Euomphalinae, shell planorbiform, whorls rounded. Genera: Euomphalus, Ophileta, Schizostoma, Eccyliomphalus (all Palaeozoic).
FAM. 9. Homalogyridae.--Tentacles absent, eyes sessile, central tooth unicuspid on a quadrangular base, laterals and marginals replaced by an oblong plate; shell very small, planorbiform. Recent. Single genus: Homalogyra, whose true position is uncertain.
FAM. 10. Littorinidae.--Proboscis short, broad, tentacles long, eyes at their outer bases, penis behind the right tentacle; reproduction oviparous or ovoviviparous, radula very long; shell turbinate, solid, columella thickened, lip simple, operculum corneous, nucleus excentrical. Jurassic----. Principal genera: Littorina (radula, Fig. 16, p. 20), Cremnoconchus (p. 16), Fossarina; Tectarius, shell tubercled or spinose; Risella, base slightly concave; Lacuna, shell thin, grooved behind the columellar lip.
FAM. 11. Fossaridae.--Shell turbinate, solid, small, white, spirally ribbed, outer lip simple. Miocene----. Principal genus, Fossarus.
FAM. 12. Cyclophoridae.--Ctenidium replaced by a pulmonary sac, tentacles long, thread-like (radula, Fig. 17, p. 21); shell variously spiral, peristome round, often reflected, operculum circular. Terrestrial only. Cretaceous----. (i.) Pomatiasinae, shell high, conical, longitudinally striated, operculum consisting of two laminae united together. Single genus, Pomatias. (ii.) Diplommatininae, shell more or less pupiform, peristome thickened or reflected, often double. Genera: Diplommatina (subg., Nicida, Palaina, Paxillus, Arinia), shell dextral or sinistral, small, columella often denticulated; Opisthostoma (Fig. 208, p. 309), last whorl disconnected, often reflected back upon the spire. (iii.) Pupininae, shell more or less lustrous, bluntly conical, lip with a channel above or below. Genera: Pupina (subg., Registoma, Callia, Streptaulus, Pupinella, Anaulus), Hybocystis (Fig. 205, p. 305), Cataulus, Coptochilus, Megalomastoma. (iv.) Cyclophorinae, shell turbinate or depressed, operculum corneous or calcareous. Genera: Alycaeus, Craspedopoma, Leptopoma, Lagochilus, Cyclophorus (Fig. 206, p. 306); including Diadema, Aulopoma, Ditropis, and others), Aperostoma (including Cyrtotoma and others), Cyathopoma, Pterocyclus (subg., Myxostoma, Spiraculum, Opisthoporus, and Rhiostoma (Fig. 180, p. 266), Cyclotus, Cyclosurus, and Strophostoma.
FAM. 13. Cyclostomatidae.--Ctenidium replaced by a pulmonary sac, tentacles obtuse, foot with a deep longitudinal median groove; central tooth, lateral, and first marginal more or less bluntly cusped, second marginal large, edge pectinate; shell variously spiral, spire usually elevated, aperture not quite circular; operculum generally with an external calcareous and an internal cartilaginoid lamina, rarely corneous. Terrestrial only. Cretaceous----. Genera: Cyclostoma (subg., Leonia, Tropidophora, Rochebrunia, Georgia, Otopoma, Lithidion, Revoilia), Cyclotopsis, Choanopoma (subg., Licina, Jamaicia, Ctenopoma, Diplopoma, Adamsiella), Cistula (subg., Chondropoma, Tudora), Omphalotropis (subg., Realia, Cyclomorpha), Hainesia, Acroptychia.
FAM. 14. Aciculidae.--Ctenidium replaced by a pulmonary sac, tentacles cylindrical, pointed at the end, eyes behind their base, foot long and narrow; central tooth and lateral very similar, pinched in at the sides, external marginal broad, edge finely pectinate; shell small, acuminate, with a blunt spire, operculum corneous. Terrestrial only. Tertiary----. Genus, Acicula (= Acme).
FAM. 15. Truncatellidae.--Ctenidium replaced by a pulmonary sac, proboscis very long, eyes sessile, behind the base of the tentacles, shell small, evenly cylindrical, apex truncated in the adult. Eocene----. Genera: Truncatella (subg., Taheitia, Blanfordia, and Tomichia), Geomelania (subg., Chittya and Blandiella), Cecina (?).
FAM. 16. Rissoidae.--Eyes at the external base of the tentacles, epipodium with filaments, operculigerous lobe with appendages; central tooth pleated at the basal angles, lateral large, bluntly multicuspid, marginals long, narrow, denticulate at the edge; shell small, acuminate, often elaborately sculptured, mouth entire or with a shallow canal, operculum corneous. Marine or brackish water. Jurassic----. Principal genera: Rissoa (subg., Folinia, Onoba, Alvania, Cingula, Nodulus, Anabathron, Fenella, Iravadia, and others), Scaliola (shell agglutinating fragments of sand, etc.), Rissoina (lip thickened, operculum with an apophysis as in Nerita), Barleeia, Paryphostoma (Eocene).
FAM. 17. Hydrobiidae.--Eyes at the outer base of the tentacles, penis behind the right tentacle, prominent, operculigerous lobe without filaments; radula rissoidan, central tooth often with basal denticulations; shell more or less acuminate, small, aperture entire, operculum corneous or calcareous. Brackish or fresh water. Jurassic----. Principal genera: Baicalia, with its various sub-genera (p. 290); Pomatiopsis, Hydrobia, Bithynella, Micropyrgus (Tertiary), Pyrgula, Emmericia, Benedictia, Lithoglyphus, Tanganyicia, Limnotrochus (?), Jullienia, Pachydrobia, Potamopyrgus, Littorinida, Amnicola, Fluminicola (subg., Gillia, Somatogyrus), Bithynia, Fossarulus (Tertiary), Stenothyra.
FAM. 18. Assimineidae.--Ctenidium replaced by a pulmonary sac, no true tentacles, eye-peduncles long, retractile; radula that of Hydrobia; shell small, conoidal, operculum corneous, nucleus sub-lateral. Eocene----. Genera: Assiminea, Acmella.
FAM. 19. Skeneidae.--Radula resembling that of Hydrobia; shell very small, depressed, widely umbilicated, operculum corneous. Pleistocene----. Single genus, Skenea.
FAM. 20. Jeffreysiidae.--Mantle with two pointed ciliated appendages in front, tentacles ciliated, eyes sessile, far behind the base of the tentacles; marginal teeth sometimes absent; shell small, thin, pellucid, whorls rather swollen, operculum with marginal nucleus, divided by a rib on the inner face. Recent. Genera: Jeffreysia, Dardania. Marine, living on algae.
FAM. 21. Litiopidae.--Epipodium with cirrhi on each side, operculigerous lobe with appendages; radula rissoidan; shell small, conical, columella truncated, operculum corneous. Eocene----. Genera: Litiopa, living on the Sargasso weed, suspended by a long filament; Alaba, Diala.
FAM. 22. Adeorbidae.--Radula essentially rissoidan; shell depressed, circular or auriform, widely umbilicated, operculum corneous, paucispiral, nucleus excentrical. Pliocene ----. Principal genera: Adeorbis, Stenotis, Megalomphalus.
FAM. 23. Viviparidae.--Snout blunt, tentacles long, right tentacle in the male deformed, pierced with a hole corresponding to the aperture of the penis, two cervical lobes, the right being siphonal, foot with an anterior transverse groove; teeth broad, shallowly pectinate at the ends; shell turbinate, whorls more or less rounded, aperture continuous, operculum corneous, nucleus sub-lateral, with a false sub-central nucleus on the external face. Animal ovoviviparous. Fresh-water. Cretaceous ----. Genera: Vivipara (= Paludina), subg., Cleopatra, Melantho, Tulotoma; Tylopoma (Tertiary), and Lioplax.
FAM. 24. Valvatidae.--Branchia exserted, bipectinate, carried on the back of the neck, a filiform appendage (Fig. 66, p. 159) on the right of the neck, penis under the right tentacle, prominent, eyes sessile, behind the tentacles; radula like that of Vivipara; shell small, turbinate or flattened, operculum corneous, nucleus central. Fresh water. Jurassic ----. Single genus, Valvata.
FAM. 25. Ampullariidae.--Snout with two tentacles, tentacles proper very long, tapering, eyes prominently pedunculate, two cervical lobes, the left siphonal, respiratory cavity divided by a partition, a large branchia in the right chamber, the left functioning as a pulmonary sac (Fig. 65, p. 158); radula large, central tooth multicuspid, base broad, lateral and marginals falciform, simple or bicuspid; shell large, turbinate or flattened, spire small, whorls rounded; operculum generally corneous, nucleus sub-lateral, false nucleus as in Vivipara. Fresh water. Cretaceous ----. Single genus, Ampullaria (subg., Ceratodes, Pachylabra, Asolene, Lanistes, and Meladomus).
FAM. 26. Cerithiidae.--Branchial siphon present, short, eyes variable in position; central tooth small, evenly cusped, lateral hollowed at base, multicuspid, marginals narrow; shell long, turriculate, whorls many, generally tuberculate, varicose or spiny, aperture sometimes strongly channelled; operculum corneous, subcircular, nucleus nearly central. Marine or brackish water. Trias ----. Principal genera: Triforis, shell small, generally sinistral; Fastigiella, Cerithium (Fig. 12, p. 16), Bittium, Potamides (subg., Tympanotomus, Pyrazus, Pirenella, Telescopium, Cerithidea, Lampania, all brackish water), Diastoma (Eocene), Cerithiopsis; Ceritella (Jurassic), Brachytrema (Jurassic), and Planaxis (subg., Quoyia and Holcostoma).
FAM. 27. Modulidae.--No siphon, radula of Cerithium; shell with short spire, columella strongly toothed at the base, aperture nearly circular. Recent. Single genus, Modulus.
FAM. 28. Nerineidae.--Shell solid, long, sub-cylindrical, aperture channelled, columella and interior of whorls with continuous ridges, extending up the spire. Genera: Nerinea (Trias to Cretaceous), Aptyxiella (Jurassic).
FAM. 29. Melaniidae.--Border of mantle festooned, foot broad, with an anterior groove, penis present; radula closely resembling that of Cerithium; shell long, spiral, with a thick periostracum, surface with tubercles, ribs, or striae, suture shallow; operculum corneous, paucispiral, nucleus excentrical. Animal ovoviviparous. Fresh water. Cretaceous ----. Principal genera: Melania (with many sections or sub-genera), Pachychilus, Claviger (= Vibex), Hemisinus, Pirena, Melanopsis, Tiphobia, Paludomus (subg., Philopotamis, Tanalia, Stomatodon), Hantkenia (Eocene), Larina (?).
FAM. 30. Pleuroceridae.--Mantle edge not festooned, no copulatory organ, otherwise like Melaniidae; operculum with nucleus sub-marginal. Animal oviparous. Fresh-water. Cretaceous ----. Genera: Pleurocera (including Io, Fig. 12, p. 16, Angitrema, Lithasia, Strephobasis), Goniobasis, Anculotus, Gyrotoma.
FAM. 31. Pseudomelaniidae.--Shell resembling that of Melaniidae, but marine. Genera: Pseudomelania, Loxonema, Bourguetia, Macrochilus. Palaeozoic to Tertiary strata.
FAM. 32. Turritellidae.--Mantle with a siphonal fold on the right side; radula variable (p. 224); shell long, whorls many, slowly increasing in size, transversely ribbed or striated, aperture small; operculum corneous, nucleus central. Jurassic ----. Principal genera: Turritella, Mesalia, Protoma, Mathilda (?).
FAM. 33. Coecidae.--Tentacles long, eyes sessile at their base; shell small, spiral in the young form, spire generally lost in the adult, the shell becoming simply a straight or curved cylinder; operculum corneous, multispiral. Eocene ----. Single genus, Coecum.
FAM. 34. Vermetidae.--Visceral sac greatly produced, irregularly spiral, no copulatory organs (radula, Fig. 126, p. 223), shell tubular, irregularly coiled, last whorls often free, aperture circular; operculum corneous, circular, nucleus central. Carboniferous ----. Principal genera: Vermetus; Siliquaria (Fig 153, p. 248), a long fissure, or series of holes, runs along a considerable part of the shell, operculum with outer face spiral, elevated.
FAM. 35. Strombidae.--Foot narrow, arched, metapodium greatly produced, snout long, eye peduncles long, thick, eyes elaborate, siphon short, penis prominent, bifurcate; central tooth with strong median cusp, marginals falciform, slender, edge more or less denticulate; shell solid, spire conical, outer lip generally dilated into wings or digitations, channelled before and behind, a labial sinus at the base, distinct from the anterior canal; operculum small for the aperture, corneous, claw-shaped, edge notched. Lias ----. Genera: Strombus (Fig. 99, p. 200); Pereiraea (Miocene), Pteroceras (Fig. 277; digitations of the outer lip very strong), Rostellaria (spire produced, anterior canal very long), Rimella, Pterodonta, Terebellum (base of shell truncate, spire short).
FAM. 36. Chenopodidae (= Aporrhaidae).--Foot flat; lateral and marginal teeth not denticulate; shell resembling that of Strombus, outer lip dilated, wing-like, no labial sinus. Jurassic ----. Genera: Chenopus (= Aporrhais, Diastema, Malaptera, Harpagodes, Alaria) (last four from Secondary strata).
FAM. 37. Struthiolariidae.--Radula allied to that of Strombus, marginals occasionally multiplied; shell buccinoid, very solid, outer lip thickened, canal short, operculum claw-shaped, notched, nucleus terminal. Tertiary ----. Single genus, Struthiolaria (subg., Perissodonta, marginal teeth multiplied).
FAM. 38. Cypraeidae.--Mantle with two large lateral lobes reflected and meeting over the shell, siphon small; central and lateral teeth bluntly tricuspid or multicuspid, laterals fairly broad, edges cusped or finely pectinate; shell polished, solid, spire generally concealed in the adult or overlaid with enamel, aperture straight, narrow, nearly as long as the shell, toothed at the sides, channelled at each end, labium inflected; no operculum. Jurassic ----. Genera: Ovula (including Amphiperas, Transovula, Cyphoma, Radius, Simnia), Pedicularia, Cypraea (with subg., Cypraeovula, Cypraedia, and Trivia), and Erato.
FAM. 39. Doliidae.--Foot expanded, wider and longer than the shell, truncated and thickened in front, siphon very long and narrow; central tooth with very strong median and small lateral and basal cusps, lateral and marginals bluntly falciform; shell ventricose, without varices, spire short, outer lip generally simple, anterior canal rather wide, no operculum. Cretaceous ----. Genera: Dolium (subg., Malea, outer lip thickened, denticulate, reflected); Pirula, mantle with two lateral lobes reflected over part of the shell, shell fig-shaped (Fig. 278).
FAM. 40. Cassididae.--Foot broad, siphon long (radula, Fig. 125, p. 223); shell ventricose, with varices, spire short, outer lip reflected or thickened, anterior canal short, recurved narrow; operculum semi-lunar, with ribs radiating from a marginal nucleus. Cretaceous ----. Genera: Cassis (subg., Semicassis and Cypraecassis), Morio (= Cassidaria), Oniscia.
FAM. 41. Columbellinidae.--Shell solid, ribbed, usually cancellated, with an oblique posterior canal, columella callous, more or less reflected. Genera: Columbellina, Columbellaria, Zittelia, Petersia, Alariopsis (?). Secondary strata only.
FAM. 42. Tritonidae.--Foot short, narrow; siphon short, not prominent; radula allied to that of Cassididae; shell thick, varicose; outer lip inflected and thickened, canal long, periostracum often thick and hairy, operculum corneous, nucleus terminal or sub-marginal. Cretaceous ----. Genera: Triton (Fig. 191, p. 275; subg., Epidromus, Plesiotriton, Simpulum, Ranularia, Argobuccinum); Persona, aperture toothed, narrow; columella reflected upon the last whorl; Ranella, shell dorso-ventrally compressed, generally with two continuous lateral varices, posterior canal present.
The position of the following four families is doubtful:--
FAM. 43. Oocorythidae.--Siphon short, foot broad, eyes absent, radula taenioglossate; shell buccinoid or cassidiform, operculum corneous, spiral. Recent. Single genus, Oocorys.
FAM. 44. Subulitidae.--Shell elongate, fusiform, smooth; suture shallow, base truncate or rounded, aperture channelled or notched. Ordovician to Trias. Genera: Subulites, Fusispira, Euchrysallis.
FAM. 45. Seguenziidae.--Radula taenioglossate, shell trochiform, aperture channelled, columella twisted, operculum multispiral, nucleus central. Pliocene ----. Single genus, Seguenzia.
FAM. 46. Choristidae.--Anterior tentacles united by a frontal veil, posterior simple; eyes absent, foot with tentaculae before and behind; three central teeth, outer marginal with a basal plate; shell helicoid, suture deep, peristome continuous, operculum corneous, paucispiral. Pliocene ----. Single genus, Choristes.
SECTION II. HETEROPODA.--Foot fin-shaped, not flat.
The Heteropoda are free-swimming Mollusca, being, like the Pteropoda, Gasteropoda modified to suit their pelagic environment. Their nervous system is streptoneurous, and they are therefore probably derived from the Prosobranchiata, but they are highly specialised forms. Pelseneer considers them far more widely removed from the Streptoneura than the Pteropoda are from the Euthyneura. They swim on the surface “upside down,” i.e. with the ventral side uppermost.
The tissues and shell are transparent, permitting observation of the internal organs. In the Pterotrachaeidae the foot takes the form of a fan-shaped disc, usually furnished with a sucker. The body is compressed at the posterior end, often with a ventral “fin.” In Atlanta the foot consists of three very distinct parts: a propodium, a mesopodium, on which is a small sucker, and a metapodium, which carries the operculum. The branchiae are carried on the visceral sac, and are free in Pterotrachaea, slightly protected by the shell in Carinaria, and entirely covered in Atlanta; absent altogether in Firoloida.
The head carries two tentacles (except in Pterotrachaea), with large, highly organised eyes on short lobes at their outer base. The alimentary tract consists of a long protrusible proboscis, with a taenioglossate radula (Fig. 132, p. 227), a long oesophagus, and a slightly flexured intestine. In Atlanta the visceral sac is spiral and protected by a spiral planorbiform shell; in Carinaria the visceral sac is small, conical, protected by a very thin capuliform shell. There is no shell in Pterotrachaea or Firoloida.
The Heteropoda are dioecious. In the male there is a flagellum behind the penis, which is near the middle of the right side. Pterotrachaea lays long chains of granular eggs, and has been noticed to produce a metre’s length in a day. The eggs of Atlanta are isolated. The embryo has a deeply bilobed velum.
FAM. 1. Pterotrachaeidae.--Body long, with a caudal “fin;” branchiae dorsal, free or partly protected by a shell; foot consisting of a muscular disc, with or without a sucker.
Pterotrachaea proper has no mantle, shell, or tentacles. The branchiae are disposed round the visceral sac, at the upper part of which is the anus. In Firoloida the body is abruptly truncated behind, with a long filiform segmented caudal appendage; visceral sac at the posterior end: fin-sucker present or absent in both male and female. Cardiapoda resembles Carinaria, but the visceral sac is more posterior and is only slightly protected by a very small spiral shell. Carinaria (Fig. 279) has a rugose translucent skin, visceral sac sub-median, apparently pedunculated, covered by a capuliform shell. The larval shell, which persists in the adult, is helicoid.
FAM. 2. Atlantidae.--Shell spiral, operculate, covering the animal. Branchiae in a dorsal cavity of the mantle; foot trilobed, with a small sucker on the mesopodium.
The shell of Atlanta is discoidal and sharply keeled, while that of Oxygyrus is nautiloid, with the spire concealed, no keel, aperture dilated.
(c) GYMNOGLOSSA.--Radula and jaws absent; proboscis prominent, sexes probably separate, penis present. The section is probably artificial and unnecessary, the families composing it being, in all probability, Taenioglossa which have lost their radula in consequence of changed conditions of life (pp. 79, 225).
FAM. 1. Eulimidae.--Proboscis very long, retractile, mantle forming a siphonal fold; shell small, long, subulate, polished; suture shallow, aperture continuous, operculum present or absent. Animal often parasitic, sucking the juices of its host by its long proboscis. Trias----. Genera: Eulima (subg., Subularia, Arcuella, Apicalia, Mucronalia, Stiliferina, and others), Stilifer, Scalenostoma, Niso, and Hoplopteron.
FAM. 2. Pyramidellidae.--Tentacles auriform, proboscis as in Eulimidae, a prominent mentum or flap under the buccal orifice; shell usually small, conical; suture shallow, apical whorls (the embryonic shell) sinistral (p. 250), operculum corneous, paucispiral; nucleus excentrical. Trias----. Genera: Pyramidella (subg., Syrnola, Otopleura, Chrysallida, Mumiola), Odostomia, Eulimella, Murchisoniella, Turbonilla (subg., Dunkeria and Cingulina).
(d) RACHIGLOSSA (p. 220).--Proboscis long, retractile; siphon distinct, radula without uncini, sometimes without laterals; teeth strongly cusped; shell generally wholly external.
FAM. 1. Muricidae.--Eyes sessile at the outer base of the tentacles, penis large, behind the right tentacle, radula within the retractile proboscis, central tooth (Fig. 119, p. 220) with at least three strong cusps, laterals plain; shell solid, more or less tuberculate, spiny and varicose, anterior canal varying from a mere notch to a long channel. Cretaceous----. Principal genera: (i.) Muricinae, nucleus of operculum sub-terminal; Trophon, Typhis, Murex (with many subdivisions), Ocinebra (including Cerastoma, Vitularia, and Hadriania), Urosalpinx, Eupleura, Pseudomurea. (ii.) Purpurinae, nucleus of operculum lateral; Rapana (including Latiaxis), Purpura (with subg., Cuma, Iopas, Vexilla, and Pinaxia), Monoceros (including Chorus), Purpuroidea (Secondary strata), Pentadactylus, Sistrum, Concholepas.
FAM. 2. Coralliophilidae.--Animal living in Madrepores, resembling Purpura, radula absent; shell variously shaped, often deformed or tubular, operculum that of Purpura, if present. Miocene----. Principal genera: Rhizochilus, Coralliophila, Leptoconchus, Magilus (Fig. 29, p. 75), Rapa.
FAM. 3. Columbellidae.--(Radula, Fig. 123, p. 222.) Shell small, solid, fusiform, aperture narrow, canal short, outer lip thickened. Miocene----. Single genus, Columbella (subg., Nitidella, Anachis, Meta, Strombina, Atilia, Conidea, Amphissa, Mitrella, and others).
FAM. 4. Nassidae.--Foot long and broad, often with terminal appendages; siphon long, eyes on outer base of tentacles, central tooth of radula arched, multicuspid, lateral strongly bicuspid, with small denticles between the cusps; shell rather small, buccinoid, columella more or less callous, outer lip thickened, often toothed; operculum corneous, edges often toothed. Miocene----. Principal genera: Nassa (with many sections), Amycla, Desmoulea, Cyclonassa, Canidia (subg., Clea and Nassodonta), Dorsanum, Bullia (= Buccinanops, Fig. 62, p. 185), Truncaria.
FAM. 5. Buccinidae.--Siphon rather long, eyes at outer base of tentacles; central tooth of radula with 5 to 7 cusps, laterals bicuspid or tricuspid (Fig. 118, p. 220); shell more or less fusiform, thick, covered with a periostracum, canal of varying length, outer lip simple or thickened; operculum corneous, nucleus variable in position. Cretaceous----. Principal genera: Group i. Chrysodomus (with sections Neptunea, Volutopsis, Pyrolofusus, Jumala), subg., Sipho; Siphonalia (subg., Kelletia). Group ii. Liomesus (= Buccinopsis). Group iii. Buccinum (Fig. 1 B, p. 6; subg., Volutharpa, Neobuccinum). Group iv. Cominella, Tritonidea, Pisania, Euthria; Anura (Miocene), Genea (Pliocene), Metula, Engina. Group v. Phos, Hindsia. Group vi. Dipsaccus (= Eburna), Macron. Group vii. Pseudoliva.
FAM. 6. Turbinellidae.--Central tooth of radula tricuspid, median cusp strong, lateral bicuspid, cusps unequal (Fig. 117, p. 220); shell fusiform or pear-shaped, heavy, canal often long, operculum corneous, claw-shaped, nucleus terminal. Miocene----. Principal genera: Turbinella, Cynodonta, Tudicla (subg., Streptosiphon); Piropsis (Cretaceous), Perissolax (Cretaceous), Strepsidura (Eocene, subg., Whitneya), Melapium, Fulgur (= Busycon, Fig. 150, p. 249, including Sycotypus), Melongena (subg., Pugilina, Myristica); Liostoma (Eocene), Hemifusus (subg., Megalatractus), Ptychatractus, Meyeria.
FAM. 7. Fasciolariidae.--Eyes at the outer base of the tentacles (radula, Fig. 121, p. 221); shell fusiform, spire long, canal often very long, columella often with a fold at the base; operculum corneous, nucleus terminal. Cretaceous----. Principal genera: Fusus (including Sinistralia, Aptyxis, Troschelia), with subg., Serrifusus (Cretaceous), Clavella (subg. Thersites), Fasciolaria, Latirus (subg. Polygona, Peristernia, Leucozonia, Lagena; Mazzalina (Eocene), Chascax).
FAM. 8. Mitridae.--Siphon rather long, with anterior appendages, eyes on the side of the tentacles, proboscis very long; radula variable, laterals sometimes lost (Fig. 120, p. 221); shell fusiform, solid, spire more or less pointed, columella with several prominent folds, the posterior the largest, aperture rather narrow, no operculum. Cretaceous----. Principal genera: Mitra (with many sections), subg., Strigatella, Mitreola, Mutyca, Dibaphus; Plochelaea (Tertiary), Thala; Turricula (with several sections), Cylindromitra, and Imbricaria.
FAM. 9. Volutidae.--Foot broad in front, head laterally dilated into lobes, on which are placed the sessile eyes; siphon prominent, with appendages at the base (radula, Fig. 122, p. 221); shell thick, often shining, fusiform, globular or cylindrical, columella projecting anteriorly, with several folds, the anterior of which is the largest, aperture notched, canal not produced, operculum generally absent. Cretaceous----. Principal genera: Cryptochorda (Eocene), Zidona, Provocator, Guivillea, Yetus (= Cymbium), Voluta (with many sections), Volutolithes (chiefly Eocene), Volutolyria, Lyria, Enaeta, Volutomitra.
FAM. 10. Marginellidae.--Foot broad, siphon without appendages, mantle largely reflected over the shell; radula without laterals, central tooth comb-like, cusps rather blunt; shell oval or conoidal, polished, aperture narrow, outer lip thickened, columella with many folds; no operculum. Eocene----. Principal genera: Marginella, with many sections and so-called sub-genera; Persicula, Pachybathron (?), Cystiscus, Microvoluta.
FAM. 11. Harpidae.--Foot large, with a transverse groove, separating off a semi-lunar propodium; mantle partly reflected over the shell; shell ventricose, polished; spire short, strongly longitudinally ribbed, ribs prolonged over the suture, columella callous; no operculum. Eocene----. Single genus, Harpa (subg., Silia).
FAM. 12. Olividae.--Propodium semi-lunar, with a longitudinal groove above, mesopodium reflected laterally over the shell; central tooth of radula tricuspid on a very broad base, lateral simple, hooked; shell sub-cylindrical or fusiform, polished; aperture narrow, operculum present or absent. Cretaceous----. Principal genera: Oliva (Figs. 283, and 98, p. 199), Olivancillaria (including Lintricula and Agaronia), Olivella, Ancilla (subg., Ancillina).
(e) TOXOGLOSSA (p. 218).--Radula with normal formula 1·0·1, teeth large; oesophagus with a large poison gland; animal carnivorous, exclusively marine.
FAM. 1. Terebridae.--Eyes at the end of the tentacles, shell subulate, many whorled, operculum with terminal nucleus. Eocene----. Single genus, Terebra, with several sections.
FAM. 2. Conidae.--Eyes on outer side of tentacles, siphon prominent; shell conical or fusiform, aperture narrow. Cretaceous----. Principal genera: Conus, shell solid, spire short, aperture narrow, straight, internal partitions partly absorbed; Conorbis, Genotia (with several sections, chiefly Tertiary), Pusionella, Columbarium, Clavatula, Surcula, Pleurotoma; Borsonia (Eocene), Drillia (subg., Spirotropis), Bela, Mangilia (including Daphnella, Clathurella, and others), Halia.
FAM. 3. Cancellariidae.--Proboscis short, usually no radula, shell oval, columella strongly plicate; no operculum. Cretaceous----. Single genus, Cancellaria (subg., Merica, Trigonostoma, Admete).
Chapter XV
CLASS GASTEROPODA (continued): OPISTHOBRANCHIATA AND PULMONATA
=Order III. Opisthobranchiata=
Visceral loop not twisted (except in Actaeon) in a figure of 8 (Euthyneurous type, p. 203), auricle usually behind the ventricle, ctenidium often replaced by secondary branchiae, pallial cavity, if existing, more or less open, shell present or absent, operculum absent (except in Actaeon), animal hermaphrodite, with separate sexual openings, marine only.--Carboniferous to present time.
The character of their nervous system decisively removes the Opisthobranchiata from the Prosobranchiata, and approximates them to the Pulmonata. Actaeon, however, which is streptoneurous, as well as possessing an operculate shell with prominent spire, forms an interesting link with the Prosobranchiata. At the opposite extreme to Actaeon stand forms like Siphonaria and Gadinia, which are probably close links with the Pulmonata (p. 19). The generative system of the whole group, which is, as in the Basommatophora, of the hermaphrodite type, without mutual fecundation, is another link of connexion with the Pulmonata. The respiratory organs present the most varied forms, sometimes consisting of one ctenidium (never two), sometimes of secondary branchiae, variously placed, while sometimes no special organ exists.
The prolongation of the foot into lateral epipodia or parapodia (possibly to aid in swimming), and the effect of the epipodia upon the shell, according as they involve it completely or partially, are among the most instructive features of the Opisthobranchiata. If the epipodia are developed on the anterior portion of the body, and do not become reflected, they may, as in most Pteropoda Thecosomata, not directly affect the shell. But when, as in the Tectibranchiata, the epipodia are medio-lateral, and tend to envelope the shell, their effect may be traced by a series of forms varying in proportion to the amount of shell-surface covered by the epipodia. The two principal lines along which modification takes place are the gradual reduction of the spiral nature of the shell, and the gradual lessening of its solidity. Both these changes are the direct result of the additional protection afforded to the visceral mass by the reflected epipodia, which renders the existence of a shell less and less necessary. A precisely similar line of change is seen in the Pulmonata, culminating in forms like Arion (p. 174).
The habits of life of the Opisthobranchiata are very varied. Some, especially the heavier types, burrow in sand, and are then usually furnished with a broad cephalic disc, as a digging apparatus; some (certain Bulla) flit about in shallow pools on mud flats; others (Phyllirrhoe and the Pteropoda) swim freely in the open sea; others (most Nudibranchiata) crawl slug-like on sea-weeds or corallines, and in colour singularly harmonise with their environment (p. 71 f.); others again (Siphonaria, Gadinia), stick limpet-like to rocks between tide marks. As a rule, they occur only in clean salt water, but Embletonia has been found in the Victoria Docks at Rotherhithe, as well as in parts of the Baltic, where the water has only 7 parts of salt in 1000, while Limapontia occurs in nearly fresh water at Bornholm and Gothland.
Their food varies greatly. As a rule, they are frugivorous, but many cases of carnivorous habit occur. Scaphander has been seen to swallow Dentalium six at a time, and in six hours the shells of all were reduced to tiny fragments. Glaucus devours the soft portions of the pelagic Porpita and Velella; Idalia elegans eats its way into the test of Ascidians, and completely buries itself in the body of its prey.[403]
The Opisthobranchiata may be classified as follows:--
{ Bulloidea { 1. TECTIBRANCHIATA { Aplysioidea { { Pleurobranchoidea { { Siphonarioidea { =Opisthobranchiata= { 2. ASCOGLOSSA { { 3. NUDIBRANCHIATA { Cladohepatica { { Holohepatica { { 4. PTEROPODA { Thecosomata { { Gymnosomata
=Sub-order I.= =Tectibranchiata.=--Right ctenidium usually present, more or less concealed by the mantle fold, visceral ganglia united by a very long commissure, shell variable in form, more or less enveloped in folds of the mantle and foot, often becoming rudimentary.
SECTION I. BULLOIDEA.--Shell more or less spiral, internal or external, epipodia more or less developed, a broad cephalic disc, distinct from the dorsal region, usually no tentacles, eyes sessile.
FAM. 1. Actaeonidae.--Shell spiral, solid, entirely covering the animal; spire generally prominent, operculum corneous, visceral loop streptoneurous, no epipodia, radula multiseriate, teeth numerous, very small. Carboniferous ----. Genera: Actaeon (Fig. 286a); Volvaria (Tertiary), Fortisia (Eocene) Actaeonina (Carboniferous), Cylindrites (Secondary strata), Actaeonella (Cretaceous).
FAM. 2. Tornatinidae.--Shell spiral, cylindrical, entirely covering the animal; spire concealed, cephalic disc with two large tentaculiform appendages behind, no radula. Genera: Tornatina (= Utriculus), Volvula.
FAM. 3. Scaphandridae.--Shell more or less external, covering all or nearly all the animal, spire concealed, cephalic disc simple or notched behind, epipodia well developed, radula with first lateral very large, stomach sometimes with powerful gizzard. Genera: Scaphander (Fig. 287 B); Sabatia (Pliocene), Smaragdinella, Atys (Fig. 286 D), Cylichna (Fig. 286 C), Amphisphyra.
FAM. 4. Bullidae.--Shell external or partly internal, spire quite or nearly hidden, cephalic disc broad, without appendages, epipodia often large; radula usually multiseriate. Genera: Bulla (subg. Haminea), Acera, mantle with long filiform appendage, epipodia touching over the shell; Cylindrobulla, Volvatella.
FAM. 5. Aplustridae.--Shell partly internal, overlaid by the posterior part of the cephalic disc, spire not prominent, epipodia reflected, tentacles auriform. Single genus, Aplustrum (Fig. 286 B; subg. Hydatina).
FAM. 6. Ringiculidae.--Shell small, solid, covering all the animal; spire somewhat prominent, aperture narrow, plicated; peristome thick, sometimes channelled, cephalic disc with a kind of posterior siphon. Genera: Ringicula; Avellana (Cretaceous).
FAM. 7. Gastropteridae.--Shell completely internal, nautiloid, small; epipodia very large, rounded, united behind; cephalic disc simple. Single genus, Gastropteron.
FAM. 8. Philinidae.--Shell completely internal, thin, slightly spiral; epipodia thick, cephalic disc large, thick, simple; stomach usually with powerful gizzard. Genera: Philine (Fig. 287 E), Colpodaspis, Colobocephalus, Chelinodura, Phanerophthalmus, Cryptophthalmus.
FAM. 9. Doridiidae.--Shell completely internal, a mere pellicle with a small spiral nucleus, mantle with two posterior lobes and a caudal filament, epipodia reflected. Single genus, Doridium.
SECTION II. APLYSIOIDEA.--Shell small, usually not spiral, sometimes absent, no cephalic disc, head prominent, with two pairs of tentacles, epipodia large, more or less reflected.
FAM. Aplysiidae.--Characters those of the section. Genera: Aplysia (Fig. 287 D), shell arched, flattened, animal large (the “sea hare”); Dolabella, shell subtriangular (Fig. 286 F); Dolabrifer, shell sub-quadrangular, not spiral; Notarchus, shell microscopic, spiral; Phyllaplysia, body very depressed, oval, no shell.
SECTION III. PLEAUROBRANCHOIDEA.--Dorsal region protected by a wide notaeum or dorsal covering, or by a shell; no epipodia, ctenidium large, external, between the right under surface of the notaeum or shell and the foot; head short, shell present or absent.
FAM. 1. Pleurobranchidae.--Shell internal or absent, notaeum with spicules, radula multiseriate. Genera: Pleurobranchus (Fig. 286 H), (?) Haliotinella, Pleurobranchaea, (?) Neda.
FAM. 2. Runcinidae.--Branchial lamellae few, under the posterior right notaeum, no shell. Single genus, Runcina.
FAM. 3. Umbrellidae.--Shell external, depressed patelliform, not covering all the animal; foot very thick, ctenidium large, head depressed, small; radula multiseriate, teeth innumerable, very small. Genera: Umbrella (Fig. 5A, p. 10), Tylodina.
SECTION IV. SIPHONARIOIDEA.--Shell patelliform, branchia replaced wholly or in part by a pulmonary sac, pulmonary orifice closed by a small lobe, radula multiseriate, teeth very small.
FAM. Siphonariidae.--Characters those of the section. Genera: Siphonaria (branchia as well as pulmonary sac), Gadinia (no branchia). These genera, hitherto placed among the Pulmonata, have been recently shown (see p. 19) to be modified Opisthobranchiata.
=Sub-order II.= =Ascoglossa.=[404]--Branchia, mantle cavity, and shell generally wanting, liver ramified, rami enclosed in external papillae (cerata) or beneath the dorsal surface, kidney not compact, branched; radula with one series of strong teeth (Fig. 288), worn out teeth at the front end not dropping off, but preserved in a special sac (ἀσκός).
According to Bergh, the Ascoglossa form a link between the Tectibranchiata,--especially the Aplysiidae and Bullidae--and the Cladohepatic Nudibranchs, while the Pleurobranchidae form a somewhat similar link between the Holohepatic Nudibranchs and the other Tectibranchiata.
FAM. 1. Oxynoeidae.[405]--Animal long, tentacles auriform, epipodia large, simple, or wing-like, a ctenidium and branchial chamber on right side, shell small, thin, slightly spiral, not covering much of the body. Genera: Oxynoe (= Lophocercus), Lobiger.
FAM. 2. Hermaeidae.--Body depressed, cerata in several rows, no branchiae, no shell. Genera: Hermaea, Phyllobranchus, Stiliger, Alderia.
FAM. 3. Elysiidae.--Body depressed, head rather elevated, tentacles auriform, sides of body dilated into two large wings, which enclose branches of the liver and sometimes fold over the dorsal surface, no branchiae, no shell. Genera: Elysia, Thridachia, Placobranchus.
FAM. 4. Limapontiidae.--Body slug-like, liver scarcely ramified, no branchiae, shell, or appendages. Genera: Limapontia, Actaeonia, Cenia.
=Sub-order III.= =Nudibranchiata.=--Shell absent in the adult, no ctenidium proper, or osphradium, cerata dorsal or dorso-lateral, nervous system concentrated, kidney not compact, ramified, penis retractile, jaws and radula usually present.
SECTION I. CLADOHEPATICA.--Cerata usually latero-dorsal, elongated, or arborescent, buccal mass strong, jaws present, liver generally ramified, rami generally entering the cerata.
FAM. 1. Aeolidiidae.--Body slug-like, head with tentacles and rhinophores, dorsal area with rows of cerata, which usually contain sting-cells, radula variable. Genera: Aeolis, Cratena, Tergipes, Coryphella, Favorinus, Facelina, Flabellina, Fiona, Glaucus, Janus, Hero, with many sub-genera.
FAM. 2. Tethymelibidae.--Body slug-like, large, cerata very large, no sting-cells, head large, cowl-shaped, no tentacles, rhinophores much foliated, no radula. Genera: Tethys, Melibe. The cerata of Tethys, which are capable of independent movement when severed, have been described as parasitic worms. Tethys feeds on molluscs and Crustacea.
FAM. 3. Lomanotidae.--Body slug-like, dorsum prominent, undulating or lobed, with one row of small cerata, no tentacles, rhinophores much foliated, radula with uncinated dentate laterals. Single genus, Lomanotus.
FAM. 4. Dotonidae.--Body slug-like, small, two rows of cerata, each ceras surrounded by a ring of tubercles, rhinophores simple, radula uniseriate. Single genus, Doto.
FAM. 5. Dendronotidae.--Body slug-like, somewhat compressed, two rows of arborescent cerata, no tentacles, frontal margin with arborescent papillae, rhinophores arborescent, radula multiseriate. Genera: Campaspe, Dendronotus.
FAM. 6. Bornellidae.--Two rows of dorsal papillae, with branchiform appendages at the base, rhinophores foliate, radula multiseriate. Single genus, Bornella.
FAM. 7. Scyllaeidae.--Body oblong, compressed, two large foliated cerata with branchial appendages on the inner side, no tentacles, rhinophores large, radula multiseriate. Single genus, Scyllaea.
FAM. 8. Phyllirrhoidae.--Body much compressed, with bovine head and neck, tail tapering, no tentacles, rhinophores simple, teeth few, no marginals. Single genus, Phyllirrhoe.
FAM. 9. Pleurophyllidiidae.--Body elongate-oval, snout broad, covered by an arched shield with lateral angles prolonged, branchiae consisting of two rows of lamellae placed between the notaeum and the foot, no tentacles, rhinophores short, hidden, radula multiseriate. Single genus, Pleurophyllidia.
FAM. 10. Pleuroleuridae.--Animal resembling Pleurophyllidia, but without the branchial lamellae. Single genus, Pleuroleura.
FAM. 11. Tritoniidae.--Body long, two rows of unequal arborescent cerata, rhinophores with ramose appendages, liver not prolonged into the cerata. Genera: Tritonia, Marionia.
SECTION 2. HOLOHEPATICA.--Cerata medio-dorsal, retractile or not, usually paucifoliate, liver never ramified, usually no jaws.
FAM. 1. Dorididae.--Branchia consisting of a circle or semicircle of pinnate leaves united at the base, surrounding the anus, almost always retractile into a cavity, rhinophores foliate, no suctorial proboscis, radula multiseriate. Genera: Bathydoris, Hexabranchus, Archidoris (Fig. 289), Discodoris, Diaulula, Cadlina, Centrodoris, Platydoris, Chromodoris, Miamira, with many sub-genera.
FAM. 2. Doriopsidae.--Branchia and rhinophores as in Dorididae, oral aperture pore-shaped, suctorial, no radula. Single genus, Doriopsis.
FAM. 3. Phyllidiidae.--Body oval, depressed, leathery, a ring of branchial lamellae, only interrupted by the head and genital papilla, under the pallial edge, oral aperture pore-shaped, suctorial, no radula. Genera: Phyllidia, Fryeria. Bergh unites this and the preceding family in the group Porostomata, which, with Fam. 1, form the group Dorididae cryptobranchiatae.
FAM. 4. Polyceridae.--Body slug-like, branchiae not retractile, usually surrounding the anus, rhinophores foliate, tentacles simple, radula variable, central tooth generally wanting. Genera: Notodoris, Triopella, Aegires, Triopa, Issa, Triopha, Crimora, Thecacera, Polycerella, Palio, Polycera, Ohola, Trevelyana, Nembrotha, Euplocamus, Plocamopherus, Kalinga.
FAM. 5. Goniodoridae.--Body oval, depressed, branchia multifoliate, usually disposed in shape of a horse-shoe, rhinophores foliate, retractile or not, mouth with a large suctorial proboscis, radula variable. Genera: Akiodoris, Doridunculus, Acanthodoris, Adalaria, Lamellidoris, Calycidoris, Goniodoris, Idalia, Ancula, Drepania.
FAM. 6. Corambidae.--Body otherwise Doris-like, but with two posterior branchiae under the mantle edge, jaws present, no central tooth, about five laterals. Single genus, Corambe (= Hypobranchiaea). Bergh unites this and the two preceding families in the group Dorididae phanerobranchiatae.
=Sub-order IV.= =Pteropoda.=--The Pteropoda are pelagic animals in which the lateral portions of the foot are modified into fins, which are innervated by the pedal ganglia. Their systematic position has undergone recent revision. It has been the custom to regard them as an Order of equivalent value to the other four, while some have held them to be a subdivision of Cephalopoda. Modern authorities, chief among whom is Pelseneer, regard the Pteropoda not as a primitive, but as a derived and recent group. They are “Gasteropoda in which the adaptation to pelagic life has so modified their external characters as to give them an apparent symmetry.”
The principal point which relates the Pteropoda to the Gasteropoda is the asymmetry of the visceral organs, intestine, heart, kidney, and genital gland, which results from their development on one side only of the body. Their hermaphroditism and the structure of their nervous system relate them to the Euthyneura rather than to the Streptoneura. Resemblances in the organs of circulation and generation approximate them to the Opisthobranchiata rather than to the Pulmonata, while of the two groups of the former, they tend to closer relationship with the Tectibranchiata than with the Nudibranchiata. The two sections of Pteropoda have been considered of distinct origin, the Thecosomata being derived from the Bulloidea, the Gymnosomata from the Aplysioidea.[406]
Thus the Pteropoda are a group whose true relations are masked by the special conditions of their existence, which have tended towards the development of certain organs, the so-called “wings” and the shell, which give them an apparent symmetry; this symmetry disappears on a closer investigation of the internal organs. They are hermaphrodite; the genital gland has a single efferent duct (except in some Cavolinia), a seminal groove leading to the copulatory organ, which in the Thecosomata is on the right side of the head, in the Gymnosomata on the right side of the foot. The genital system resembles that of the Opisthobranchiata and of the “digonoporous” Pulmonata.
SECTION 1. THECOSOMATA.--Shell or cartilaginoid test always present, fins united by an intermediate lobe, ctenidia as a rule absent, replaced by secondary branchiae, no very distinct head or eyes, one pair of tentacles; cerebral ganglia on the sides of and under the oesophagus; radula with three rather large teeth in a row, generally unicuspid, jaw in two pieces, stomach with horny plates, anus generally on the left side.
The Thecosomata feed on Protozoa and the lower Algae; they have no proboscis, and the intestine is flexured. The fins are always closely connected with the head, or what answers to it. About 42 species are known, belonging to 8 genera.
FAM. 1. Limacinidae.--Fins very large, branchial chamber dorsal, anus on right side; shell spiral, sinistral (ultra-dextral, see p. 249), operculate. Genera: Limacina, shell helicoid, deeply umbilicated (L. helicina swarms in Arctic seas and furnishes food for many Cetacea); Peraclis, spire turreted, aperture large, elongated, produced anteriorly, no umbilicus; operculum sinistral, in spite of the shell being ultra-dextral.
FAM. 2. Cavoliniidae.--Fins large, branchial chamber ventral, shell a non-spiral cone, angular or round, very thin, embryonic portion distinct, or formed of two separate plates.
In Cavolinia (= Hyalaea, Fig. 5 B, p. 10) the shell consists of two plates, the ventral being convex, with one to three sharp spines at the posterior end, the dorsal flatter, without spines. The aperture is broad, contracted dorso-ventrally. Two long pointed prolongations of the mantle project from the lateral slits of the shell, and probably serve to balance the bulky body when swimming. Fins trilobed at the margin. Cleodora has only rudimentary lateral prolongations, fins bilobed, shell triangular, angles greatly produced, aperture very wide, dorsal side keeled. In Cuvierina the shell is straight, sub-cylindrical, with a median partition, slightly expanding towards the apex, which is truncated in the adult. The principal sub-genera of Clio are Creseis, which has an elongated sub-cylindrical shell, sometimes slightly curved, smooth or grooved; and Clio proper, in which the shell is long, angular, with a dorsal rib, apex (= embryonic shell) rounded, constricted. Styliola and Hyalocylix also belong to this group.
FAM. 3. Cymbuliidae.--Test (which is not homologous with the shell of other Thecosomata) slipper-shaped, cartilaginoid, simply a thickening of the mantle; embryo with a calcareous, spiral, operculate shell. Genera: Cymbulia, Cymbuliopsis, Gleba.
Three other families, Hyalithidae, Pterothecidae, and Conulariidae, from Palaeozoic strata, are generally added to the Thecosomata. All are fossil only, and it is doubtful whether they are really Molluscan. Pelseneer holds that no true fossil Pteropoda occur until the lower Tertiaries.
SECTION 2. GYMNOSOMATA.--Mantle and shell absent in the adult, fins not connected by a lobe, no branchial chamber, head well developed, with two pairs of tentacles, eyes on the posterior pair; cerebral ganglia above the oesophagus; buccal cavity provided with a pair of protrusible “hook-sacs,” radula generally with 4 to 12 hooked laterals, central tooth triangular, jaw in one piece, composed of horny plates, no horny plates in stomach, anus on the right side.
The Gymnosomata are carnivorous, feeding on Thecosomata and other pelagic animals, being provided for this purpose with a formidable buccal armature of hook-sacs and suckers. The intestine, as usual in carnivorous groups, passes straight from the stomach to the anus; the fins are not attached to the head, but to the anterior part of the body. The larva has a straight shell, which disappears in the adult. About 21 species are known, belonging to 7 genera.
FAM. 1. Pneumodermatidae.--Animal fusiform, fins rather small, head prominent, anterior part of buccal cavity protrusible, with suckers on the ventral side, hook-sacs well marked; branchia on right side, skin soft, pigmented. Genera: Dexiobranchaea, no posterior gill, hook-sacs short; Spongiobranchaea, posterior gill circular; Pneumoderma, gill tetraradiate, hook-sacs long.
FAM. 2. Clionopsidae.--Body barrel-shaped, proboscis three times the length of the body, no buccal appendages; hook-sacs short, no lateral gill, posterior gill tetraradiate, skin not pigmented. Clionopsis is the single genus.
FAM. 3. Notobranchaeidae.--Body ovate, buccal appendages conical, no lateral gill, posterior gill with three radiating crests, skin pigmented. Notobranchaea is the single genus.
FAM. 4. Clionidae.--Body long, angulated behind, proboscis short, mouth with two or three pairs of appendages, no jaw, no gills.
Clione limacina is so abundant in Arctic seas as at times to colour the surface for miles. Each of the cephalic appendages has about 60,000 minute pedicellated suckers.
FAM. 5. Halopsychidae.--Body ovate, thick, rounded behind, no gill or proboscis, fins long, narrow, broadened at the ends, epidermis sub-cartilaginoid.
Halopsyche (= Eurybia) has the power of withdrawing its head completely into a sort of pocket, which is closed by an anterior fold of the mantle. There are two long non-retractile buccal appendages.
=Order IV. Pulmonata=
Gasteropoda with two pairs of tentacles, visceral loop euthyneurous, ganglia concentrated round the oesophagus; breathing air by a pallial cavity formed by the union of the front edge of the mantle with the cervical region, sexes united, shell present or absent, no operculum[407] (except in Amphibola).
=Sub-order I.= =Basommatophora.=--Eyes generally at the base of the tentacles, which are not retractile, male and female genital orifices separate, radula (p. 235) multiseriate, shell always present, external. Fresh water or quasi-marine.
FAM. 1. Auriculidae.--Breathing organ a pulmonary sac or true lung; shell spiral, conoidal, internal partitions usually absorbed, aperture more or less strongly toothed. Jurassic----. Genera: Auricula, Carychium, Scarabus, Alexia, Tralia, Plecotrema, Cassidula, Melampus, Leuconia, Pedipes (Fig. 292).
FAM. 2. Otinidae.--Shell auriform, spire very short. Genera: Otina, Camptonyx.--Recent only.
FAM. 3. Amphibolidae.--A pulmonary sac on right side of neck, eyes almost pedunculate, shell turbinate, rudely sculptured, operculate.--Recent. Genus, Amphibola (Fig. 293); subg. Ampullarina.
FAM. 4. Limnaeidae.--Pulmonary sac protected by an external lobe; shell variable, fragile. Jurassic----. (i.) Ancylinae, shell more or less limpet-shaped. Genera: Ancylus, Gundlachia, Latia. (ii.) Limnaeinae, shell spiral. Genera: Limnaea, Amphipeplea, Erinna, Lantzia, Pompholyx, Choanomphalus (with Carinifex). (iii.) Planorbinae, shell sinistral, spire flattened or elevated. Genera: Planorbis, Isidora (= Bulinus).
FAM. 5. Physidae.--Mantle more or less reflected over the shell (radula, Fig. 141C, p. 235); shell sinistral, lustrous. Jurassic----. Genera: Physa, Aplecta.
FAM. 6. Chilinidae.--Lobe of pulmonary sac large, tentacles broad; shell ventricose, rather solid; columella plicate. Miocene----. Single genus, Chilina.
=Sub-order II.= =Stylommatophora.=--Two pairs of retractile tentacles (except in Janella), eyes at the tip of the upper pair, male and female orifices united (except in Vaginulidae and Onchidiidae), no distinct osphradium.
FAM. 1. Testacellidae.--Animal carnivorous, slug-like or spirally coiled, no jaw (whence the name Agnatha, often given to this group), radula with usually few, large, sickle-shaped teeth (p. 232), shell variable, rarely absent, usually external. Cretaceous----. Principal genera: Chlamydephorus (shell a simple plate, internal), Apera, Testacella (slug-like, shell terminal), Strebelia, Streptostyla, Glandina, Salasiella, Petenia, Pseudosubulina, Streptostele, Tomostele, Streptaxis (Fig. 203), Gibbus, Ennea, Daudebardia (Fig. 193), Schizoglossa, Guesteria, Aerope, Paryphanta, Rhytida (subg. Diplomphalus, Elaea and Rhenea).
FAM. 2. Selenitidae.--Shell internal, external, or absent; jaw present, radula Testacellidan, central tooth present. Tertiary----. Genera: Selenites, Plutonia, Trigonochlamys, Pseudomilax (?), Rathouisia (?).
FAM. 3. Limacidae.--Shell present or absent, internal or external, spiral or not, tail often with a mucus pore, jaw (Fig. 107 A, p. 211) with projecting rostrum on cutting edge, radula with central tooth tricuspid, laterals bi- or uni-cuspid, marginals aculeate. Eocene----. Genera: Otoconcha, Urocyclus, Mariaella (subg. Tennentia), Parmarion, Helicarion, Cystopelta, Aspidelus, Estria, Vitrinopsis (subg. Vitrinoidea, Parmella), Damayantia, Nanina (= Ariophanta, including Pachystyla, Rhysota, Hemiplecta, Trochonanina, Euplecta, Orpiella, Xesta, Macrochlamys, Microcystis, Sitala, Kaliella, Durgella, Austenia, Girasia, Parmacochlea, Africarion, Sesara, Macroceras, and others), Vitriniconus, Parmacella, Limax (subg. Amalia and many sections), Vitrina (subg. Vitrinozonites, Velifera), Zonites (subg. Stenopus, Moreletia, Mesomphix, Hyalinia, Gastrodonta, Pristiloma, Poecilozonites, Thyrophorella).
FAM. 4. Philomycidae.--Shell absent, jaw limacidan, radula helicidan, shield covering all the body. Single genus, Philomycus (= Tebennophorus), with subg. Pallifera.
FAM. 5. Helicidae.--Shell present or absent, internal or external; jaw of various types, radula with central tooth tricuspid, equal in size to the first laterals, laterals bi- or tricuspid, marginals smaller, cusped. Eocene----. Principal genera: Oopelta (no shell), Arion (shell absent or formed of calcareous granules), Ariolimax, Geomalacus, Anadenus (subg. Prophysaon), Hemphillia, Cryptostracon, Binneya, Helix (see below), Cochlostyla, Bulimus (subg. Borus, Orphnus, Dryptus, Strophochilus, Pachyotus, and possibly Caryodes, Leucotaenia, Liparus, Livinhacea, Pachnodus, Rachis, Atopocochlis, Cerastus, Clavator belong here, or with Buliminus), Berendtia, Rhodea. Pilsbry proposes[408] to group Helix as follows:
A. Eggs or young very large at birth:
(1) Macroön, incl. Acavus, Pyrochilus (= Phania), Stylodonta, Helicophanta.
B. Eggs or young smaller or minute at birth:
(2) Belogona.--Female genital system with dart sac and mucus gland. Helix [restricted] (with sections Arionta, Campylaea, Chilotrema, Pomatia, Macularia, Tachea, Iberus, Leptaxis, Eulota, Fruticicola, Xerophila; Dorcasia, Acusta, Plectotropis, Aegista, Cathaica, Satsuma, Euhadra; Lysinoe), Gonostoma, Leucochroa, Allognathus, Cochlostyla, Polymita, Hemitrochus (with sections Plagioptycha, Dialeuca, Coryda, Jeanerettia), Glyptostoma, Acanthinula, Vallonia.
(3) Teleophalla.--Female system without accessories, male with flagellum and appendix on penis; no epiphallus. Sagda, Cysticopsis.
(4) Epiphallophora.--Female system without accessories, male with epiphallus on penis; no appendix. Caracolus (with sections Lucerna, Dentellaria, Isomeria, Labyrinthus, Eurycratera, Parthena, Polydontes, Theliodomus, Cepolis), Camaena (incl. Phoenicobius), Obba, Chloritis (incl. Hadra), Papuina, Planispira (subg. Cristigibba).
(5) Haplogona.--All accessory organs absent, jaw soldered into one piece. Polygyra (incl. Daedalochila, Triodopsis, Mesodon, Stenotrema), Endodonta (incl. Libera, Charopa, Gerontia, Therasia, and others), Patula, Trochomorpha, Anoglypta.
(6) Polyplacognatha.--All accessory organs absent, jaw composed of 16–24 separate plates. Punctum, Laoma.
Genera of doubtful position: Strobilops, Ampelita, Pedinogyra, Polygyratia, Macrocyclis, Solaropsis.
FAM. 6. Orthalicidae.--Radula, p. 233. Shell external, large, bulimoid. Single genus, Orthalicus; subg. Liguus, Porphyrobaphe, Corona.
FAM. 7. Bulimulidae.--Radula, p. 233; jaw, p. 211; shell usually external. Genera: Bulimulus (incl. Plecochilus, Goniostomus, Drymaeus, Liostracus, Otostomus, Navicula, Scutalus, Peronaeus, Eurytus, Eudioptus, Plectostylus, Mesembrinus, Mormus, etc.; Thaumastus, Nesiotis), Placostylus (incl. Charis), Amphidromus, Partula, Calycia (?), Peltella (animal limaciform, shell internal), Pellicula, Amphibulimus (incl. Simpulopsis).
FAM. 8. Cylindrellidae.--Radula, p. 233; shell many whorled, long turriculate, last whorl often detached, apex often truncated (Fig. 169, p. 260). Eocene----. Genera: Cylindrella (with sections Callonia, Thaumasia), Leia, Macroceramus, Pineria.
FAM. 9. Pupidae.--Radula, p. 233; shell external, spire usually long, aperture often narrowed, more or less toothed, often with internal lamellae. Carboniferous----. Genera: Anostoma (Fig. 154, p. 248), Hypselostoma (Fig. 202A, p. 302); Anastomopsis (Cretaceous), Lychnus (Cretaceous), Boysia, Odontostomus (incl. Tomigerus), Buliminus (incl. Petraeus, Napaeus, Zebrina, Mastus, Chondrula, Ena, and perhaps Rachis, Pachnodus, Hapalus, and others), Pupa (incl. Torquilla, Pupilla, Sphyradium, Leucochila, etc.), Zospeum, Vertigo, Megaspira, Strophia, Holospira, Eucalodium (incl. Coelocentrum), Coeliaxis, Perrieria, Balea; Rillya (Eocene), Clausilia (with many sub-genera), Rhodina (?).
FAM. 10. Stenogyridae.--Radula, p. 234; shell long, spiral, shining, more or less translucid, apex blunt, sometimes decollated. Eocene----. Genera: Stenogyra (subg. Rumina, Obeliscus, Opeas, Melaniella, Spiraxis, Leptinaria, Nothus, Subulina, Glessula), Ferussacia (subg. Cionella, Azeca), Caecilianella (subg. Geostilbia). Achatina (shell large, ventricose, columella strongly truncate), with the sub-genera Perideris, Limicolaria, Columna, Pseudachatina, Homorus, probably belongs to a distinct family.
FAM. 11. Achatinellidae.--Radula, p. 234; shell small, bulimoid, indifferently dextral or sinistral. Genera: Achatinella (subg. Auriculella, Amastra, Carelia), Tornatellina.
FAM. 12. Succineidae.--Radula, p. 234; lower pair of tentacles wanting or small; shell internal or external, thin, spiral or not, last whorl large. Eocene----. Genera: Succinea, Homalonyx, Hyalimax, (?) Lithotis, (?) Catinella.
FAM. 13. Janellidae.--Radula, p. 234; animal slug-like, no lower tentacles, shell an internal plate. Single genus, Janella (= Athoracophorus), with subg. Aneitea.
FAM. 14. Vaginulidae.--Radula, p. 234; animal slug-like, covered with a coriaceous mantle, lower tentacles bifid, genital orifices widely separated, male behind the lower right tentacle, female on inferior median part of right side, anus and pulmonary orifice nearly terminal; shell absent. Single genus, Vaginula (= Veronicella).
FAM. 15. Onchidiidae.--Body oval, mantle thick, often warty, sometimes set with “eyes” (p. 187), two tentacles, genital orifices widely separate, anus and pulmonary orifice as in Vaginula; no shell. Genera: Peronia, Onchidium, Onchidiella. The family appears to be an instance of Pulmonata reverting to marine habits of life.
XVI. CLASSES SCAPHOPODA AND PELECYPODA
CLASS SCAPHOPODA
Head rudimentary, mantle edges ventrally concrescent, forming a tube opening before and behind, and covered with a shell of the same shape; sexes separate.
The Scaphopoda form a small but very distinct class, whose organisation is decidedly of a low type. The body is usually slightly curved, the concave side being the dorsal; muscles near the posterior end attach the body to the shell. The foot, which can be protruded from the anterior or wider aperture, is rather long, pointed, and has sometimes two lateral lobes (Dentalium), sometimes a terminal retractile disc (Siphonodentalium), sometimes a retractile disc with a central tentacle (Pulsellum). The cephalic region, as in Pelecypoda, is covered by the mantle. The mouth is situated on a kind of projection of the pharynx; the buccal mass, containing the radula (p. 236), is at the base of the foot, and the intestine branches forward from the front part of the stomach. The liver (Fig. 299) is paired, and consists of a number of symmetrical, radiating coeca. There are no eyes, but on each side of the mouth are small bunches of exsertile filaments (captacula), which appear to act as tactile organs for the seizing of food. There is no special respiring apparatus, heart or arterial system, breathing being conducted by the walls of the mantle. The nervous system has already been described (p. 205).
Two kidneys open on either side of the anus. The genital gland is large, occupying nearly all the posterior part of the body, the sexual products being emitted through the right kidney. The veliger has already been figured (p. 131, Fig. 44). The embryonic shell is formed of two calcareous laminae, which subsequently unite to form the tube.
With regard to their general relationships, the Scaphopoda resemble the Gasteropoda in their univalve shell, and in the possession of a radula; while the pointed foot, the non-lobed velum in the veliger, the generative system, the bilateral symmetry of the organs generally, and the absence of any definite head, eyes, or tentacles, are points which approximate them to the Pelecypoda.
The Scaphopoda are known from Devonian strata to the present time. They are found at a depth of a few fathoms to very deep water. The only three genera are Dentalium, Siphonodentalium (subg. Cadulus), and Pulsellum, which differ in the structure of the foot, as described above.
CLASS PELECYPODA
Cephalic region rudimentary, mantle consisting of two symmetrical right and left lobes, covering the body and secreting a bivalve shell hinged at the dorsal margin; no radula, sexes usually separate. Reference has already been made to the reproductive system (p. 145), breathing organs (p. 164 f.), mantle (p. 172), nervous system (p. 205), digestive system (p. 237 f.), and nomenclature of the various parts of the shell (p. 269 f.).
The shape of the shell, in many Pelecypoda, involving as it does the position, size, and number of the adductor muscles, is probably due to mechanical causes, depending on the habits and manner of life of the individual genus. Thus in a typical dimyarian or two-muscled bivalve, e.g. Mya (Fig. 300, A), the adductor muscles lie well towards each end of the long axis of the shell, with the hinge about midway between them. In this position they are best placed for effectually closing the valves, and since they are nearly equidistant from the axis of motion, i.e. from the hinge, they do an equal amount of work, and are about equal in size. But in a form like Modiola, where the growth of the shell is irregular in relation to the hinge-line, the anterior muscle is brought nearer and nearer to the umbones, where its power to do work, and therefore its size, becomes less and less. But the work to be done remains the same, and the posterior muscle has to do it nearly all; hence it moves farther and farther away from the hinge-line, and at the same time gains in size. In shells like Ostrea, Pecten, and Vulsella, the anterior muscle, having drawn into line with the hinge and the posterior muscle, becomes atrophied, while the posterior muscle, having double work to do, has doubled its size.[409]
The development of the foot, again, largely depends upon habits of life. It is well developed in burrowing forms, while in sessile genera (Ostrea, Chama, Spondylus) it becomes unnecessary and aborts. Even in Pecten, which does not become sessile, but has ceased to use the foot as an organ of progression, a similar result follows. Forms which burrow deeply often “gape” widely, sometimes at one end only, sometimes at both. Venus, Donax, Tellina, Mactra, which are shallow burrowers, do not gape; Solen, Lutraria, and to a less degree Mya, burrow deeply and gape widely. In order to burrow deeply the foot must be highly developed, and the larger it becomes, the more will it tend to keep the valves apart at the place where it is habitually protruded. Burrowing species always remain in communication with the surface by means of their siphons, the constant extension of which tends to keep the valves apart at the end opposite to the foot. Burrowing species, again, tend to burrow in such a way as to descend most easily, and not be impeded by their own shells; in other words, they act as a wedge, and descend with their narrowest part foremost. But the burrowing organ, the foot, has to follow suit, and gradually draws round to the narrowest part of the shell, so that the habitual deep burrower, such as Lutraria, lies with its long axis exactly at right angles to the surface, its siphons protruding from, and keeping open, the uppermost or posterior margin of the shell, and the foot producing the same effect upon the lower or anterior margin. The deeper the burrower, the more elongated does the shell become, until, through forms like Pholas and Saxicava, we arrive at Solen, the most highly specialised burrower of all, in which the breadth of the shell is equal throughout, and no obstructive curve exists to impede its rapid ascent or descent.
The Pelecypoda have been classified in various ways; by the completeness or sinuation of the pallial line, depending on the absence or presence of siphons, by the number of adductor muscles, by the character of the hinge-teeth, and by the number of the branchiae. For various reasons, none of these methods have proved entirely satisfactory. That adopted here was suggested by Pelseneer, and depends upon the character of the branchiae themselves, as suggesting successive stages of development (p. 166 f.).
=Order I. Protobranchiata=
Branchial filaments not reflected, the two rows inclined at a right angle (more or less), ventral surface of foot more or less flattened, byssogenous apparatus little developed, a single anterior aorta, kidneys distinct, sexes separate, each genital gland opening into the corresponding kidney.
FAM. 1. Nuculidae.--Labial palps very large, rows of branchial filaments at right angles to one another, mantle edges open, siphons contracted, foot disc-shaped, elongated; shell equivalve, oval, or produced, interior generally nacreous, hinge with numerous saw-like teeth. Silurian ----. Principal genera: Nucula (heart dorsal to the rectum); Palaeoneilo (Devonian), (?) Sarepta, Leda, Yoldia, Malletia; Tyndaria (Upper Tertiary), Lyrodesma (Silurian), Actinodonta (Silurian), Babinka (Silurian).
FAM. 2. Solenomyidae.--Labial palps united, one row of branchial filaments pointing dorsally, the other ventrally; mantle edges in great part united postero-ventrally, a single siphonal orifice with two very long tentacles, foot proboscidiform, with a round denticulate disc at the end; shell equivalve, resembling a Solen, with a strong corneous periostracum; no hinge-teeth, ligament internal. Single genus, Solenomya. (?) Cretaceous ----].
=Order II. Filibranchiata=
Rows of branchial filaments parallel, pointing ventrally, reflected, and provided with interfilamentary ciliated junctions, foot usually with a well-developed byssogenous apparatus.
=Sub-order I.= =Anomiacea.=--Heart dorsal to the rectum, a single aorta, foot small, anterior adductor very small; shell ostreiform, no hinge-teeth, fixed by a calcified byssus traversing the right valve (Fig. 173, p. 262).
FAM. Anomiidae.--Jurassic ----. Genera: Anomia, Placunanomia; Carolia (Eocene), Placuna; Hypotrema (Jurassic), Placunopsis (Oolite).
=Sub-order II.= =Arcacea.=--Mantle edge open, both adductors well developed, heart with two aortae, branchiae free, without interlamellar junctions, no siphons; renal and generative apertures distinct.
FAM. 1. Arcadae.--Mantle edge with composite eyes; shell round or trapezoidal, solid, often with stout bushy periostracum; ligament often external, on a special area; hinge with numerous lamelliform teeth. Ordovician ----. Principal genera: Arca (incl. Barbatia, Scaphula, and Cucullaea), heart dorsal to rectum; Pectunculus, Glomus, Limopsis; Trinacria and Nuculina (Tertiary).
FAM. 2. Trigoniidae.--Foot large, hatchet-shaped, with ventral disc; no byssus, mantle edge with ocelli; shell sub-trigonal, hinge-teeth few, strong; interior violet-nacreous. Devonian ----. Genera: Trigonia; Myophoria and Schizodus (Trias), Cyrtonotus (Devonian).
=Sub-order III.= =Mytilacea.=--Mantle edges fused at one point, anal orifice distinct, anterior terminal adductor small, one aorta, branchiae with interfoliary junctions, genital glands penetrating the side of the mantle and opening by the side of the kidneys.
FAM. Mytilidae.--Byssus well developed, shell more or less equivalve, oval, broad; hinge-teeth evanescent. Devonian----. Principal genera: Mytilus?, Myalina, Septifer, Modiola, Lithodomus, Crenella, Dacrydium, Myrina, Idas, Modiolaria, Modiolarca.
=Order III. Pseudolamellibranchiata=
Mantle edges entirely open, foot little developed, anterior adductor usually aborted, branchial filaments reflected, with interlamellar junctions, which are sometimes vascular; genital glands opening into the kidneys or close to the apertures of the kidneys.
FAM. 1. Aviculidae.--Foot long, tongue-shaped, byssogenous apparatus well developed, branchiae concrescent with the mantle, adductor muscle sub-central, at times a small anterior adductor, siphons absent; shell usually inequivalve, dorsal margin straight, often very long, winged, lateral teeth much prolonged; structure of shell cellular, inside prismatic, outside nacreous. Palaeozoic ----. Principal genera: Avicula, including Meleagrina, Malleus; Vulsella (no wings or hinge-teeth); Perna, including Crenatula, Inoceramus (ligaments in a number of fossettes); Aucella and Monotis (Palaeozoic and Secondary); Pterinaea and Ambonychia (Palaeozoic); Pinna; Aviculopinna (Carboniferous).
FAM. 2. Prasinidae.--Shell very small, umbones anterior, incurved, anterior side depressed, hinge-teeth replaced by dentiform projections of the lunule fitting into corresponding grooves. Recent. Single genus, Prasina.
FAM. 3. Ostreidae.--Heart generally ventral to the rectum, branchiae concrescent with the mantle, no byssus; shell inequivalve, fixed by the left valve, form irregular. Jurassic ----. Genera: Ostrea; Heligmus (Oolite), Naiadina (Cretaceous), Pernostrea (Jurassic).
FAM. 4. Pectinidae.--Byssus usually absent, mantle edge open, duplicated, folded back, with pallial ocelli; branchiae not concrescent with the mantle; shell with unequal “ears” at the umbo, hinge-teeth lamelliform, often obscure. Silurian----. Principal genera: Pedum, Chlamys, Hinnites, Hemipecten, Amussium, Pecten; Aviculopecten (Palaeozoic), Crenipecten.
FAM. 5. Limidae.--Mantle edge as in Pecten, tentaculate; shell sub-equivalve, eared, fixed by a byssus or free. Carboniferous----. Genera: Lima (Fig. 85, p. 179), Limea.
FAM. 6. Spondylidae.--Foot with a peduncular appendage, no byssus, numerous pallial ocelli; shell fixed by right valve, surface often very spinose, two cardinal teeth in each valve. Jurassic----. Genera: Plicatula, Spondylus; Terquemia (Lias).
FAM. 7. Dimyidae.--Shell ostreiform, fixed, hinge with or without symmetrical teeth, two muscular impressions. Single genus, Dimya (Tertiary).
=Order IV. Eulamellibranchiata=
Mantle edges united at one or more points, branchiae with interfilamentary junctions which are always vascular, genital glands not opening into the kidneys, usually two adductor muscles.
=Sub-order I. Submytilacea.=--Mantle edges more or less open, anal orifice distinct, usually no siphons, pallial line usually simple, cardinal and lateral teeth well marked.
FAM. 1. Carditidae.--Foot with a byssus or groove, branchiae large, unequal; shell equivalve, solid, radiately grooved, one or two oblique cardinal teeth, one or two laterals. Silurian ----. Principal genera: Venericardia, Cardita, Carditella, Carditopsis, Milneria; Pleurophorus (Palaeozoic), Anodontopsis (Silurian).
FAM. 2. Astartidae.--A short anal siphon, labial palps large; shell triangular, thick, ligament external, hinge with two or three cardinals in each valve, laterals obscure.? Devonian ----. Principal genera: Astarte; Pachytypus (Jurassic), Plesiastarte (Eocene), Parastarte, Woodia, Opis (Secondary strata), Prosocoelus (Devonian).
FAM. 3. Crassatellidae.--Mantle with anal orifice or open; shell equivalve, thick, subtriangular, ligament in an internal fossette, hinge with two cardinals, laterals produced. Cretaceous----. Principal genus, Crassatella.
FAM. 4. Cardiniidae.--Shell equivalve, oval or triangular, ligament external, cardinal teeth small, laterals fairly strong. Devonian----Oolite. Principal genera: Cardinia, Anthracosia, Carbonicola, Anoplophora.
FAM. 5. Cyprinidae.--Anal and branchial orifices complete, papillose, foot thick; shell variable, equivalve, thick, umbones often spiral, hinge-teeth very variable, ligament external. Jurassic----. Principal genera: Cyprina; Pygocardia (Crag), Veniella (Cretaceous), Venilicardia (Secondary strata), Anisocardia (Jurassic), Isocardia, Libitina, Coralliophaga; Basterotia (Eocene). The families Pachydomidae (Palaeozoic) and Megalodontidae (Palaeozoic--Secondary) are probably related to the Cyprinidae.
FAM. 6. Aetheriidae.--Anal orifice complete, foot absent, labial palps large; shell irregular, free or fixed, no hinge-teeth. Fluviatile, recent only. Genera: Aetheria, Mülleria, Bartlettia.
FAM. 7. Unionidae.--Foot large and thick, no byssus, anal siphon short, branchial orifice complete or not, siphon present or absent, embryo of certain groups passing through a glochidium stage (p. 146); shell equivalve, sometimes very thick, nacreous within, hinge variable. Fluviatile. Jurassic----. Principal genera: Unio (subg. Arconaia), Monocondylaea, Pseudodon, Anodonta, Solenaia, Mycetopus, Mutela, Spatha, Hyria, Castalia, Leila.
FAM. 8. Dreissensiidae.--Both siphons prominent, foot tongue-shaped, byssiferous; shell mytiliform, with small internal septum. Genera: Dreissensia; Dreissensiomya (Tertiary). The common Dreissensia polymorpha Pall. was distributed over large parts of Europe in later Tertiary times. From unknown causes it died out, and has during the past two hundred years been regaining its position, migrating N. and W. from its original habitat, the Caspian, by the Volga and its Oka confluent.
FAM. 9. Modiolopsidae.--Shell mytiliform, ligament exterior, hinge-teeth small, rather numerous. Palaeozoic----. Principal genera: Modiolopsis, Cyrtodonta, Mytilops, Ptychodesma.
FAM. 10. Lucinidae.--Anal orifice sometimes with a siphon, branchial orifice complete or not, sometimes a single branchia; foot very long, vermiform, no byssus, anterior adductor long; shell rounded, equivalve, blanched, hinge with two cardinals and two laterals in each valve, sometimes toothless, ligament more or less internal. Silurian----. Principal genera: Lucina, Corbis, Axinus, Diplodonta, Montacuta.
FAM. 11. Ungulinidae.--Anal orifice complete, foot vermiform, no byssus, two branchiae; shell equivalve, subcircular, hinge-teeth variable, no laterals, adductor impressions long, continuing the pallial line. Tertiary----. Single genus, Ungulina.
FAM. 12. Unicardiidae.--Shell equivalve, round or oval, cardinal shelf large, a single cardinal in each valve, ligament external. Carboniferous--Cretaceous. Genera: Unicardium, Scaldia, Pseudedmondia.
FAM. 13. Kellyellidae.--Anal siphon prolonged, no marked branchial orifice; shell very small, oval or round, anterior lateral very strong, under the cardinal. Eocene----. Genera: Kellyella; Allopagus and Lutetia (Tertiary), Turtonia.
FAM. 14. Erycinidae.--Mantle edges with three apertures, branchial orifice on the buccal margin, foot long, broadened, with a byssus, animal usually viviparous. Tertiary----. Genera: Erycina, Kellia, Pythina, Lasaea, Lepton.
FAM. 15. Galeommidae.--Mantle edges more or less reflected over the shell, apertures and foot as in Erycinidae; shell thin, equilateral, hinge with few teeth or none. Tertiary----. Genera: Galeomma, Scintilla, Sportella, Chlamydoconcha, Hindsiella, Ephippodonta (Fig. 32, p. 81).
FAM. 16. Cyrenidae.--Siphons short, foot large, no byssus; shell equivalve, subtriangular, with periostracum, hinge with two or three cardinals, laterals present; animal hermaphrodite, viviparous. Fresh or brackish water. Jurassic----. Genera: Cyrena, Corbicula (subg. Batissa, Velorita), Sphaerium (= Cyclas), Pisidium, Galatea, Fischeria.
The families Cyrenellidae (single genus, Cyrenella) and Rangiidae (single genus, Rangia) are probably to be placed here.
=Sub-order II. Tellinacea.=--Siphons long, separate, foot and labial palps very large, pallial sinus deep, two adductor muscles.
FAM. 1. Tellinidae.--External branchial fold directed dorsally, foot with byssogenous slit, but no byssus, branchiae small; shell compressed, equivalve, ligament external, at least two cardinals in each valve, laterals variable. Cretaceous----. Principal genera: Tellina (with many sections), Gastrana.
FAM. 2. Scrobiculariidae.--Animal as in Tellina; shell orbiculate or long oval, equivalve, hinge-teeth weak, ligament in an internal cavity. Tertiary----. Principal genera: Scrobicularia, Syndosmya, Theora, Cumingia, Semele.
FAM. 3. Donacidae.--External branchial fold directed ventrally; shell equivalve, subtriangular, solid, smooth, two or three cardinals in each valve, laterals variable, ligament external. Jurassic----. Genera: Donax, Iphigenia, Isodonta.
FAM. 4. Tancrediidae.--Shell donaciform, ligament external, cardinals usually two in each valve, posterior laterals strong. Trias----. Genera: Tancredia (Secondary strata), Hemidonax.
FAM. 5. Cardiliidae.--Shell heart-shaped, hinge as in Mactridae, posterior adductor resting on a myophore or shelf. Single genus, Cardilia. Tertiary----.
FAM. 6. Mesodesmatidae.--Mantle edges largely united, with three orifices, foot byssiferous or not; shell regular or irregular, usually one cardinal and strong lateral teeth. Tertiary----. Genera: Mesodesma, Ervilia.
FAM. 7. Mactridae.--External branchial fold directed ventrally, siphons fused, foot tongue-shaped; shell equivalve, triangular-oval, hinge with ligament in an internal fossette, another portion external, a bifurcated cardinal tooth in the left valve, fitting into a branching tooth in the right valve, laterals present. Jurassic----. Genera: Nactra, Harvella, Raëta, Eastonia, Heterocardia, Vanganella.
=Sub-order III.= =Veneracea.=--Branchiae slightly folded, foot compressed, siphons generally short, pallial line variable, two adductor muscles.
FAM. 1. Veneridae.--Siphons free or partly united, foot seldom byssiferous; shell solid, equivalve, hinge usually with three cardinal teeth, laterals variable. Jurassic----. Principal genera: Cytherea, Circe; Grateloupia (Tertiary), Meroe, Dosinia (= Artemis), Cyprimeria, Cyclina, Venus, Clementia, Lucinopsis; Thetis (Cretaceous), Tapes, Venerupis.
FAM. 2. Petricolidae.--Animal perforating rocks; shell oval, slightly gaping behind, two or three cardinals, no laterals, pallial sinus well marked. Recent----. Genera: Petricola, Naranio.
FAM. 3. Glaucomyidae.--Siphons long, united, foot small; shell produced, thin, hinge with three cardinals, no laterals, pallial sinus well marked. Recent----. Genus, Glaucomya (incl. Tanysiphon).
=Sub-order IV.= =Cardiacea.=--Branchiae much folded back, mantle edges with three apertures, foot cylindroidal, more or less produced, siphons present or absent, one or two adductor muscles, pallial line variable.
FAM. 1. Cardiidae.--Siphons rather long, foot long, no byssus; shell equivalve, more or less radiately ribbed, hinge with one or two cardinals in each valve, laterals variable, ligament external, two adductors. Brackish water or marine. Devonian----. Genera: Byssocardium and Lithocardium (Tertiary), Conocardium (Palaeozoic), Cardium (with many sections, including Hemicardium), Limnocardium (subg. Didacna, Monodacna, Adacna).
FAM. 2. Lunulicardiidae.--Shell equivalve, very inequilateral subtriangular, anterior margin short or truncated, with a deep lunule. Single genus, Lunulicardium (Palaeozoic).
FAM. 3. Tridacnidae.--Mantle orifices widely separated, foot short, byssiferous, no anterior adductor; shell equivalve, large, thick, usually gaping in front, one cardinal tooth and one or two posterior laterals in each valve, no pallial sinus. Miocene----. Genera: Tridacna, Hippopus. The muscular power of the great Tridacna is immense. Once caught between their gaping valves, a man’s hand or foot can scarcely be withdrawn. Two valves of T. gigas in the British Museum weigh respectively 154 and 156 lbs.
FAM. 4. Chamidae.--Mantle orifices widely separated, foot short, no byssus, both adductors present, ovary invading the mantle lobes; shell fixed, irregularly inequivalve, umbones spiral, ligament external, cardinal teeth often a mere ridge, anterior lateral strong, nearly central, no pallial sinus. Jurassic----. Genera: Chama; Diceras (Jurassic), attached by one umbo, umbones very prominent, teeth strong; Heterodiceras (Jurassic), Requienia (Cretaceous), left valve widely spiral, attached by the umbo, right valve small, fitting on the other as an operculum, teeth obsolete; Toucasia, Apricardia, Matheronia (all Secondary strata).
The four succeeding families require special study in a work on Palaeontology.
FAM. 5. Monopleuridae.--Shell very inequivalve, left valve operculiform, right conical or spiral, fixed at the apex, ligament prolonged in external grooves. Cretaceous----. Genera: Monopleura, Valletia.
FAM. 6. Caprinidae.--Shell very inequivalve, thick, free or fixed by apex of right valve, which is spiral or conical, left valve spiral or not, often perforated by radial canals from the umbo to the free margin. Neocomian and Cretaceous----. Principal genera: Plagioptychus, Caprina, Ichthyosarcolites, Caprotina, Polyconites.
FAM. 7. Hippuritidae (= Rudistae).--Shell very inequivalve, externally as in Caprinidae, umbo central in left valve, no ligament proper, left valve with strong hinge-teeth and grooves, two adductor impressions on prominent myophores, shell structure of the two valves differing. Cretaceous only. Single genus, Hippurites (Fig. 309, B).
FAM. 8. Radiolitidae.--Shell inversely conical, biconical, or cylindrical, general aspect of Hippurites, umbo of left valve central or lateral, right valve with a thick outer layer, often foliaceous, umbonal cavity partitioned off by laminae. Cretaceous only. Genera: Radiolites, Biradiolites.
=Sub-order V.= =Myacea.=--Branchiae much folded back, mantle edges usually with three openings, foot compressed, siphons large, united or not, two adductor muscles, pallial line variable.
FAM. 1. Psammobiidae.--Siphons long, not united, foot large, not byssiferous; shell equivalve, long, oval, slightly gaping at the ends, ligament external, prominent, two cardinal teeth in each valve, no laterals, a deep pallial sinus. Jurassic----. Genera: Psammobia, Solenotellina, Sanguinolaria, Asaphis, Elizia, Quenstedtia (Jurassic).
FAM. 2. Myidae.--Pedal orifice small, siphons long, united in great part; shell inequivalve, gaping at one or both ends, periostracum more or less extensive, ligament internal, resting on a prominent shelf; hinge-teeth variable. Cretaceous----. Genera: Mya, Tugonia, Sphenia, Corbula, Lutraria (for which latter some propose a separate family).
FAM. 3. Solenidae.--Foot long, powerful, more or less cylindrical, no byssus, siphons usually short, united or not, branchiae narrow; shell equivalve, long and narrow, gaping at both ends, with periostracum, umbones flattened, ligament external, hinge-teeth variable.? Devonian----. Genera: Solecurtus, Pharella, Pharus, Cultellus, Siliqua, Ensis, Solen, Orthonota (?), Palaeosolen (?).
FAM. 4. Glycimeridae.--Pedal orifice very narrow, siphons long, united in great part, often covered with periostracum; shell more or less equivalve, gaping at both ends, hinge toothless or with two weak cardinals, ligament external; animal free or perforating. Cretaceous----. Genera: Glycimeris, Saxicava, Cyrtodaria.
FAM. 5. Gastrochaenidae.--Foot small, cylindrical, no byssus, branchiae narrow, siphons long; shell perforating or cemented to a shelly tube, gaping widely on the anterior and ventral sides, no hinge-teeth, a deep pallial sinus. Cretaceous----. Genera: Gastrochaena, Fistulana (tube with a median diaphragm, perforated by the siphons).
=Sub-order VI.= =Pholadacea.=--Mantle edges largely closed, siphons long, united, foot short, truncated, disc-shaped, ligament absent, two adductor muscles; animal perforating.
FAM. 1. Pholadidae.--Organs contained within the valves, ctenidia prolonged into the branchial siphon, shell more or less gaping, thin, dorsal margin in part reflected over the umbones, one or more dorsal accessory pieces, no hinge-teeth, an interior apophysis proceeding from the umbonal cavity. Jurassic----. Genera: Pholas, Talona, Pholadidea (posterior extremity of the valves prolonged by a corneous appendage, a passage to the long tube of Teredo), Jouannetia, Xylophaga, Martesia; Teredina (Eocene).
FAM. 2. Teredinidae.--Animal vermiform, ctenidia mainly within the branchial siphon, siphons very long, with two calcareous appendages (“pallets”) near the anterior end, shell very small, continued into a long calcareous tube, valves deeply notched, internal apophysis as in Pholadidae. Lias----. Single genus, Teredo (Fig. 310).
=Sub-order VII.= =Anatinacea.=--External branchial fold directed dorsally, not reflected, sexes united, ovaries and testes with separate orifices, mantle edges largely united, byssus usually absent, two adductor muscles, pallial line variable, shell usually nacreous within.
FAM. 1. Pandoridae.--Siphons short, largely united, foot tongue-shaped; shell free or fixed, inequivalve, semi-lunar, or subtriangular, ligament often with calcareous ossicle, pallial line complete or with slight sinus. Cretaceous----. Genera: Pandora, Myodora, Myochama.
FAM. 2. Chamostreidae.--Shell fixed, Chama-like, thick, umbones spiral, ligament with ossicle. Single genus, Chamostrea.
FAM. 3. Verticordiidae.--Siphons not prolonged; shell heart-shaped, umbones prominent, spiral, ligament with an ossicle, pallial line complete. Miocene----. Genera: Verticordia, Mytilimeria, Lyonsiella.
FAM. 4. Lyonsiidae.--Foot short, byssiferous, siphons short, separate, shell inequivalve, hinge-teeth usually absent, ligament and ossicle in an internal groove. Eocene----. Single genus, Lyonsia.
FAM. 5. Ceromyidae.--Shell inequivalve, large, heart- or wedge-shaped, hinge toothless, ligament internal in one valve, external in the other. Secondary strata----. Genera: Ceromya, Gresslya.
FAM. 6. Arcomyidae.--Shell equivalve, thin, surface finely granulated, hinge toothless, cardinal edge dentiform, ligament external. Secondary and Tertiary strata----. Genera: Arcomya, Goniomya, Pleuromya, Machomya.
FAM. 7. Anatinidae.--A fourth (?byssal) pallial orifice, siphons long, separate or fused; shell thin, sometimes inequivalve, exterior often granulose, ligament often with ossicle, hinge toothless or with lamellae. Jurassic----. Genera: Anatina, Plectomya (Secondary), Periploma, Cochlodesma, Thracia, Tyleria, Alicia, Asthenothaerus.
FAM. 8. Grammysiidae.--Shell equivalve, oval, ligament external, cardinal margin straight, toothless, pallial line complete. Palaeozoic----. Principal genus, Grammysia, with many other genera of toothless hinge, but whose exact affinities are uncertain.
FAM. 9. Praecardiidae.--Shell thin, equivalve or not, radiately ribbed, margins dentated, sub-umbonal area as in Arca, hinge toothless. Palaeozoic----. Principal genus, Praecardium.
FAM. 10. Pholadomyidae.--A fourth pallial orifice, siphons very long, united, foot small; shell thin, equivalve, with radiating ribs, ligament external, hinge toothless, pallial line sinuate. Jurassic----. Single genus, Pholadomya.
FAM. 11. Clavagellidae.--Foot rudimentary, siphons long, united, contained in a long calcareous tube; shell small, one or both valves soldered in the tube, tube with a centrally fissured disc at the anterior end, more or less frilled at the posterior end. Cretaceous----. Genera: Clavagella, Brechites (= Aspergillum).
=Order V. Septibranchiata=
Mantle edges united at three points, branchiae replaced by a muscular septum extending from the anterior adductor to the point of separation of the siphons, septum with symmetrical orifices.
FAM. 1. Poromyidae.--Branchial septum with groups of lamellae between the orifices, labial palps large, foot long and narrow, siphons short, papillose, separated, animal hermaphrodite; shell small, slightly inequivalve, rounded or oblong, nacreous within. Eocene----. Genera: Poromya, Silenia.
FAM. 2. Cuspidariidae.--Siphons long, united in part, foot short, animal dioecious; shell small, slightly inequivalve, rostrate, not nacreous, each valve with ligamentary cartilage spoon-shaped, with a calcareous ossicle, cardinal and lateral teeth present or absent. Jurassic----. Single genus, Cuspidaria, with many sections.
BRACHIOPODA
I. RECENT BRACHIOPODA
BY
ARTHUR E. SHIPLEY, M.A.
Fellow and Tutor of Christ’s College, Cambridge