86 MOLLUSCA. 
The genus Carpira, of Bruguiére, represented by the 
C.variegata, (Lister, tab. 344, fig. 84.,) consists of equivalve 
free shells, with the hinge furnished with two unequal teeth, 
the one situated under the beak, the other lateral, under 
the anterior margin. 
The chama cor of Linneus appeared to Lamark possess- 
ed of sufficient characters to constitute a distinct genus, 
which he has named Isocarpra. It is an equivalve, free, 
regular, heart-shaped shell, with two cardinal teeth, and a 
separate lateral one, with separate, diverging, involuted 
beaks. It is an inhabitant of the British Seas. 
To Bruguiére we owe the institution of the genus Tri- 
DACNA, which is represented by the chama gigas of Lin- 
neeus, the largest shell in nature. The shell is equivalve 
and free. The hinge consists of two compressed teeth, and 
there is a gape at the lunule. 
From the preceding genus of Bruguiére, Lamark has se- 
parated the chama hippopus of Linnzeus, and formed from 
it a new genus, which he calls A¢ppopus. In its hinge it 
resembles the tridacna, but differs in the structure of the 
lunule, which in this is closed. 
The genus Diceras of Lamark, which he formed from 
the chama bicornis of Bruguiére, approaches the isocardia 
in appearance, but the following character which he assigns, 
is fully sufficient for their discrimination: “ Testa bivalvis 
inzequivalvis, adherens: natibus conicis, maximis, divergen- 
tibus, in spiram irregularem contortis. Dens cardinalis 
maximus, crassus, concavus, auricularis in valvula majore. 
Impressiones duo musculares.” It occurs only in a fossil 
state. 
Before dismissing this Linnzan division of shells, we must 
notice another genus which has beenadded to it by Lamark, 
