100 



GENUS. ISOCARDIA. 



Equivalve, heart-shaped, ventricose ; beaks very dis- 

 tant, divergent and spirally turned to one side ; 

 hinge with two primary flattened teeth situated 

 under the beak, and with an elevated lateral one 

 under the external ligament. 



I. COR. Lam. l.Turt. B. p. 193. t. 14. F p. 

 419. CHAMA C.Lin. 1137. D.p. 212. Ch.f. 483. 

 E. t. 232. /. l.Blain. t. 69. /. 2. Don. 4. t. 

 134. Pen. 4. p. 214. Mont. p. 134. and Sup. p. 

 50 Lin. T. S.p. 9l.Turt.D.p.32.f. \l.Wern. 

 Soc. 1. p. 385. t. 48. /. 483. Heart-shaped, globose, 

 fulvous under a reddish epidermis, nearly smooth; 

 the beaks paler, twisted into a single flat volu- 

 tion, both fronting the hinder side. 4...3i. Dublin 

 Bay. 



TRIBE. ARCACEA. 



Teeth small, numerous and disposed in each valve in a 

 line, which is either straight or interrupted ; the 

 teeth mutually inserted. 



GENUS. ARC A. 



Transverse, subeguivalve, inequilateral; beaks remote, 

 separated by the area of the ligament ; hinge linear 



