GEN ERA OF SHELLS. 25 



fontinalis, australis, sulcata, striatina, Saratogea, rhom- 

 boidea, partumeia. 



CYRENA. Shell rounded, triangular, turgid 

 or ventricose, solid, inequilateral, covered by 

 an epidermis, beaks decorticated. Hinge with 

 three teeth on each valve ; lateral teeth gener- 

 ally two, one of which is near the cardinal ; 

 ligament exterior, on the widest side. 



The cyrense are generally thick and rather large, 

 shell always covered with a greenish or brown epi- 

 dermis. They are distinguished from cyclas by hav- 

 ing three cardinal teeth in each valve. They have also 

 lateral teeth, one of which is often placed below the 

 corselet. 



(a.) Lateral teeth serrate or dentate. 



C. trigonella, orientalis, cor, fuscata, fluminea, truncata, 

 violacea. 



(b.) Lateral teeth entire. 



C. depressa, Caroliniensis, Bengalensis, Zeylanica. 



GALATHEA. Shell equivalve, subtriangular, 

 covered by a greenish epidermis ; cardinal teeth 

 sulcated, two on the right valve, connivant at 

 base; three on the other, the intermediate 

 one advanced, distinct ; lateral teeth remote ; 

 ligament exterior, short, prominent, turgid ; 

 nymphae prominent. 



The galathea is distinguished from cyrena by the 

 peculiar form of its cardinal teeth. The muscular 

 impressions are lateral, and appear double on each 

 side. 



G. radiata. 



