86 MOLLUSCA. 



The genus CARDITA, of Bruguiere, 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 Linnaeus appeared to Lamark possess- 

 ed of sufficient characters to constitute a distinct genus, 

 which he has named ISOCARDIA. 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 Bruguiere we owe the institution of the genus TRI- 

 DACNA, which is represented by the chama gigas of Lin- 

 naeus, 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 Bruguiere, Lamark has se- 

 parated the chama hippopus of Linnaeus, and formed from 

 it a new genus, which he calls hijqpopus. 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 Bruguiere, approaches the isocardia 

 in appearance, but the following character which he assigns, 

 is fully sufficient for their discrimination : " Testa bivalvis 

 inaequivalvis, 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 Linnaean division of shells, we must 

 notice another genus which has been added to it by Lamark, 



