LAMELLIBRANCHIATA. (Bivalve Shells.) More than a third part of the known fossil shells are ordinary bivalves (Conchifera, Dh.) They amount to nearly 6000, while the recent species scarcely exceed half that number. Nevertheless, it is a group which attains its maxi mum in the present seas. The genera are seven times more numerous in the newer tertiary than in the oldest geological system ; and the number of species found in the entire Silurian series is less than 100, while the chalk contains 500, and the miocene 800. Out of 150 genera, 35 have become extinct, Paleozoic Bivalves.
s. Posidonomya Becheri ; Carboniferous, Hesse.
besides numerous sub-genera. The families Cyprinidee, tam, and Anatinidtr, have passed their maximum ; the Trigoniadce are nearly extinct ; and the Hippuritidce have no living representatives.
The monomyary bivalves, and others with an open mantle, attain a degree of importance at an early period ; and with them some of the burrowing families (Ifyacida and Ana tinidce) ; while the highest organized siphonated shells (e.g., Veneridce and Tellinidce), unknown in the older rocks, are most abundant now.
The family Ostreidce, distinguished from the Pectens and Anomice by resting on the left valve, contains two fossil forms. Of these, Exogyra resembles an oyster with spiral umbones, directed backward, or to the left hand ; it is an attached shell, characteristic of the cretaceous strata. The genus Gryphoca (fig. 140) abounds in the oolites, and is gregarious, but unattached, the umbo of the larger valve being curved inward like a claw. A single Ostrea occurs in the carboniferous lime stone, after which the species become abundant, and are with difficulty distinguishable from the smooth and plaited, or " cocks-comb," oyster of the present day.
Several curious modifications of Anomia and Placuna have been obtained in a fossil state. Limanomia (Bouchard) has ears like Lima, and is attached to shells and corals of the Devonian age. Placunopsis (M. and L), found in the oolites,
has a transverse ligamental groove, which, like the umbo of the upper valve, is some way within the margin of the shell. And Carolia (Cantr.), a tertiary form of Placuna, has a byssal plug passing through a foramen like that of Anomia when young, but closed in the adult.
Fossil Pectinidce are very numerous. Some of them in the carboniferous limestone (e. g., P. Sowerbyi) cannot be distin guished generically from the living Pectens, and retain diverg ing bands of colour. But the greater part of these old species are somewhat aviculoid in form (fig. 13, z), and their hinge area is grooved with cartilage-furrows, like those of Area.
The most beautiful forms occur in the chalk and greensand, and resemble the recent scallop (Janira, Schum.) y in the Secondary Bivalves.
t. Grypluea arcuata, Lam.; Lists, Charmouth.
s. Pecten (Neithea) quinquecostata, Sby. ; Chalk, Sussex.
5. Inoceramus salcatus, Park.; Gault, Folkestone.
6. Cucullsea (Macrodon) Hirsonensis, D'Arch.; Great °elite, Min chinhampt,on.
7. Isoarca cordiformis, Schloth.; Cora/Ilan, Nattheim.
inequality of their valves, but are further characterized by the possession of articulating hinge-teeth like Spondylus. These constitute the genus Neithea (fig. 14, 2). Plicatulce exist in the trial and oolites, along with shells referred dubiously to Hinnites and Spondylus. True Hinnites (a sub-genus of Pecten) are cha racteristic of the miocene. Spondyli appear in the greensand and chalk. Some of them (like the so-called " Plagiostonta spin me) are unattached ; others resemble the recent deep-water S. Gussonii, and have been called " Dianchorw." The inner layer, including the hinge of these shells, is seldom preserved. Lima proboscidea first appears in the lower oolite, and reappears in the great oolite, and in the Kelloway rock. Lima duplicata, and some other oolitic species, have two ranges of little hinge teeth, but not like those of the recent species of Limcca. The large and smooth or striated Limas of the oolites have been called Plagiostcrma, a name originally given by Llhwyd.