BIVALVIA. 193 
impression of the posterior adductor, and the mantle mark is quite entire. In all my 
specimens, many of which are presumed to be full grown individuals, the margin is 
quite smooth, and many specimens are marked with coloured bands. There is no 
distinct lunule, but an elongated space on the anterior side that is quite smooth, while 
on the posterior side the lines run up to the margin. It is thin in the young state 
when the ridges are visible on the inside. 
It is very abundant at the above locality, and the valves are often found united. 
TIsocarpiaA,* Lamarck. 1799. 
Bucarpia. Lister, 1687. 
Cuama (spec.) Linn. 
Guossus and GLossopERMA. Poli, 1795. 
TRapEziuM (sp.) Humphr., 1797. 
Bucarpium. Megerle, 1811. 
Diceratia? Oken, 1815. 
Isocarpium. Blainv., 1824. 
Generic Character. Shell equivalved, inequilateral, heartshaped, tumid, generally 
thick and strong, sometimes smooth, often furrowed; umbones prominent and in- 
volute. Hinge with two cardinal and one lateral tooth in each valve. Ligament 
external. Impression of the mantle entire, or without a sinus. 
The animal of this genus is described by Malacologists as closely resembling that 
of Astarte, with a broad, short, triangularly formed foot, and the siphons not extending 
beyond the margins of the shell ; their orifices are fringed, and the edges of the mantle 
double. 
This genus is said to constitute a connecting link between the families of Cyprinide 
and the Cockles, and is also closely allied to Chama and Diceras, particularly in the shell, 
by the peculiar involution of the umbones and the recession of the ligament, by 
which, in its increase, the anterior portion is deserted, and its extension formed on the 
posterior side. 
But few species are known in the living state, and at present not a large number 
of fossils. : 
1. IsocarpIA cor, Linneus. Tab. XV, fig. 9 a—dé. 
Bucarpia. Scilla. De Corp. Mar. Lapid., t. xvi, fig. aa, 1670. 
— Lister. Hist. Conch., hb. ii, part ui, fig. 111, 1687. 
— communis. Schum. Essai d’un Nouy. Syst. des Vers. Test., p. 143, pl. xui, 
fig. 2a, b, 1817. 
CarpDIuM HUMANUM. Linn. Syst. Nat., ed. 10, No. 82, p. 682, 1766. 
CHAMA COR. Id. 33 ed. 12, p. 1137, No. 154, 1767. 
* Etym. isos, like or equal, and capédia. 
