GENERA OF SHELLS. 31 



(b.) Shell more longitudinal than transverse. 

 C. phrenilica, crassicosta, rufescens, calyculata, subaspera, 

 nodulosa, sinuata, concamerata, aviculina, citrina, sublaevi- 

 gata, corbularis, lithophagella, crassa. 



Cypricardia. Shell free, equivalve, inequi- 

 lateral, obliquely or transversely elongated. 

 Hinge with three teeth beneath the beaks, 

 and a lateral tooth prolonged under the corse- 

 let. 



The cypricardise differ from the carditœ, which they 

 resemble in general form, by having, like the venus, 

 three teeth, instead of one only, under the beaks. 



C. Guiniaca, angulata, rostrata, corralliophaga. 



HiATELLA. Shell equivalve, very inequila- 

 teral, transverse ; gaping at the superior mar- 

 gin. Hinge with a small tooth on the right 

 valve, and two oblique teeth, somewhat longer, 

 on the left ; ligament exterior. 



H. arctica. 



IsocARDiA. Shell equivalve, heart shaped, 

 cordate, ventricose ; beaks distant, diverging, 

 rolled spirally to one side ; cardinal teeth two, 

 compressed, entering, one of which is curved 

 and sunk under the beak ; lateral tooth elon- 

 gate, under the corselet ; ligament exterior, 

 forked on one side. 



I. cor, arietina, Moltkiana, semisulcata. 



