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. 



ISOCARDIA,* Lamarck. 1799. 



BUCARDIA. Lister, 1687. 



CHAMA (spec.) Linn. 



GLOSSUS and GLOSSODEKMA. Poli, 1795. 



TRAPEZIUM: (sp.) Humphr., 1797. 



BUCARDIUM. Megerle, 1811. 



DICEEATIA? Oken, 1815. 



ISOCAKDIUM. 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 Cyprinida 

 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. ISOCARDIA COR, Linnaus. Tab. XV, fig. 9 a b. 



BUCABDIA. Scilla. De Corp. Mar. Lapid., t. xvi, fig. AA, 1670. 

 Lister. Hist. Conch., lib. iii, part ii, fig. Ill, 1687. 

 COMMUNIS. Schum. Essai d'un Nouv. Syst. des Vers. Test., p. 143, pi. xiii, 



fig. 2 a, b, 1817. 



CAKDIUM HUMANUM. Linn. Syst. Nat., ed. 10, No. 82, p. 682, 1766. 

 CHAMA con. Id. ed. 12, p. 1137, No. 154, 1767. 



* Etym. IPOS, like or equal, and cap5ta. 



