ISOCAllDIA. 



Lam. — Hist. Nat. des Anim. sans vert. vi. p. SO. 



« ^ » ■ 



TESTA aequivalvis, cordata^ ventricosa ; umboni- 

 bus distantibus^ divaricatis^ involutis. Denies 

 cardinales duo^ compressi^ intrantes^ uiius sub 

 umbone recurvus. Dens lateralis anticus^ elon- 

 gatus. Ligamentum externum^ divaricatum, seg- 

 mentis sub umbonibus decurrentibus. Impres- 

 siones muscular es duse^ laterales^ distantes. 



Arranged by Linne in his Genus Chama^ and by Bru- 

 guiere among his Carditce, but dissevered from both by 

 Lamarck, and we think with reason, because its involute, 

 divaricate umhones^ and its, consequently, dichotomous liga- 

 ment running in each valve to the point of the umbo, serves 

 to distinguish it from Bruguiere's other Carditce. The same 

 circumstances serve as good discriminating marks between 

 Isocardia and all other bivalves, if we except Bruguiere's 

 Chamay and Lamarck's jD/ccr«5 ; but these are both inequi- 

 valve, and attached by the outside of the shell ; there is 

 therefore no danger of its being confounded with them. 



The two valves of Isocardia are alike : when closed, so 

 that both the umbones are seen in front, they have a cordate 

 general form,- and are each of them very ventricose : their 

 umbones are distant, turned backwards and involute. Car- 

 dinal teeth two in each valve, compressed ; that one nearest 

 the apex turned in under the umbo. There is only one 

 lateral tooth in each valve, which is also compressed and 

 elongated, and is placed just before the ligament, which is 

 external, divided into two segments at its posterior extre- 

 mity, one of which is decurrent to the point of the umbo in 

 each valve. There are also in each valve two lateral, dis- 

 tant, muscular impressions. 



