The pearl-oysters (Aviculidce) are also very abundant fossils : but owing to the frequent repetition of similar forms, it is difficult to determine the genera with any degree of cer tainty by the aid of external characters alone. The Silurian species mostly belong to the genus Pterinca (Goldfuss), and are broadly winged, and have the hinge-area striated length wise, and a few diverging hinge-teeth. Amboncychia (Hall) resembles Inoceramus, and ranges from the Silurian to the carboniferous strata (fig. 13, 3). The Silurian genus Cardiola is ridged like a cockle ; and Posidonomya, which is found in all the pakeozoic rocks, is very thin and concentrically fur rowed (fig.13, 2). Many other genera have been proposed whose characters are even more imperfectly understood. Monotis (Salinarius), one of the common shells of the trias, has no anterior ear. Pteroperna (Lycett), an oolitic form, has a winged shell, with numerous small anterior teeth and long posterior laminfe. The genus Gervillia (fig. 14, 4), ranging from the carboniferous limestone to the chalk, consists of elon gated shells, with several cartilage-pits in the ligamental area. Bakewellia, found in the Permian, has an anterior muscular impression like Arca. The recent genus Perna commenced in the Has or preceding formation, and exhibits great variety of shape. Pulvinites Adansonii (fig. 14, 3) appears to have been a Perna with a byssal foramen like Anomia ; and Inoce ramus (fig. 14, s), characteristic of the cretaceous strata and oolites, differs from Perna chiefly in form, the larger valve being sometimes completely involute, and resembling a Nautilus. The genus Pinna, which appears to belong to this family, although provided with two adductor muscles, occurs fossil in the Devonian and all subsequent strata. Some of the oolitic species, distinguished by the name Trichiles, are inequi valve and irregular, and attain a thickness of more than an inch, resembling mineral masses of fibrous carbonate of lime.
Amongst the Mytaidce are many Silurian species distin guished by their large, round, anterior muscular scar (Modio &psis, Hall), and others which have a straight hinge-line and plaited valves (Orthonotus, Conrad). Myalina has the cartilage-groove repeated (fig. 13, 4), and is found in the upper palTozoic rocks. Sometimes the anterior adductor is supported on a shelf, as in the recent Septifera and Dreissenia. True Mytili and Modioks abound in the oolitic strata. Dreissenia, now confined to the rivers of the Aralo-Caspian region, or only naturalized in Western Europe, was represented by many species, and some of large size, in the eocene of Hampshire and miocene of Vienna.
Fossil Arcades are far more numerous than the recent shells, and mostly belong to the division Cuculima, of which a single species survives in the Coral Sea. The palTozoic Arks have anterior teeth like Arca, and posterior teeth like Cuculloca, and differ from both in the reduction of the hinge area to a narrow tract corresponding with the posterior half only in the recent shells. The casts of Ark-like shells in the Silurian rocks are further distinguished by a deep furrow behind the front muscular impression. These constitute the genus Ctenodanta (Salter), which has hinge-teeth like Nucula, and a prominent external ligament (fig. 13, s). Some of the oolitic Arks, with a byssal sinus, and the posterior teeth very long and parallel, form a sub-genus called Macrodon (fig. 14, 6).
Others, with prominent umbones, teeth like Nucula, and a striated ligamental area, form the genus Isoarca of Munster (fig. 14, 7). Above 200 species of Nucula and Leda are known only as fossils, and range through all the rock systems. The palatozoic species are anomalous in form, and when better understood, will certainly be considered distinct as genera.
Yoldia is a newer tertiary form characteristic of high northern latitudes ; and Solenella occurs fossil in Patagonia and New Zealand. The problematic genus Solentya is supposed to have existed in the carboniferous period. Pectuneuli appear first in the cretaceous strata, being less ancient than Limopsis, which occurs in the Bath oolite. A member of the latter genus found in the Belgian eocene has the ligamental area entirely behind the cartilage-pit, and is called Nucunella by d'Orbigny. The " Stalagmium” of Conrad Myopara, Lea) is identical with Crenella (T. Br.), a sub-genus of Modiola, found in the cretaceous and tertiary strata.
The Trigoniadce are represented in the lower Silurian strata by Lyrodesma (fig. 13, 6), a shell with several radiating hinge-teeth, striated transversely ; and in the upper palozoics by Axinus (fig. 13, 7) and several other imperfectly-known genera. Sehizodus occurs in the permian at Garford (with Turbo and Rissoa), near Manchester. The trial contains true Trigcntice associated with the genus Myophoria (fig. 14, s), which has the umbones turned forwards, and a posterior hinge-tooth. The only member of this family which has yet been found in tertiary strata is the little genus Verticordia (Wood) of the crag. No Trigonice have been met with, although 100 species are known in the secondary rocks, and two are still living on the coasts of South Australia.
Fresh-water mussels (Unionicice), of large size and various form, occur in the Wealden formation, and are not generically distinguishable from recent shells ; but those of the coal measures and older rocks are extremely problematic, and may even belong to marine genera.
Of the genus Chama there is one species in the upper greensand and chalk of England, and another in the London clay. Elsewhere they are more abundant, amounting to thirty species. Closely allied to Chama is the Diceras (Lam.), of which the remarkable casts attracted attention at an early period (fig. 14, I). They are found in the coral rag of France and Germany, and resemble the horns of some animal. The shell is attached by the umbo of either valve, indifferently, like some of the recent Chamas. The posterior adductor muscle is supported on a prominent ridge (as in Paehydesmo, Mega lodon, and the recent Cardillo), which causes a spiral furrow in each horn of the cast. The shells which succeed Diceras, in the lower cretaceous strata, have the right valve usually much smaller than the left, and in one instance (fig. 14, z) it is like the operculum of a spiral univalve. The only British species of this group is Requienia Lonsclalii, found in the ironsand of Bowood. In France, and also in Texas, another form occurs, with the attached valve simple and conical, like a Hippurite. The ligamental groove is straight, and the umbo of the free valve marginal.