187 



A general idea of the hinge of this genus may be gained 

 from PL vii. fig. 22. 



10. Crassina. — A suborbicular, transverse, close, equi- 

 valved, subinequilateral bivalve ; the hinge with two strong 

 diverging teeth on the right valve, and two unequal teeth in 

 the other ; the ligament external, and in the longest side. 



This shell is distinguishable from a small crassatella by 

 the situation of the ligament ; and, from venus, by having 

 only two teeth on each valve. 



Lamarck forms the family of coiicIkz for the reception of 

 the following shells : — 



Fresh-water shells — Cyclas, Cyrene, ^iwd Galathea, 

 . Sea shells — Cyprijia, Cytherea^ Venus ^ and Venericardia. 



1. Cyclas. — An oval, rather tumid, transverse, and equt- 

 valved bivalve ; the hinge teeth very small, sometimes hardly 

 any ; sometimes two on each valve, of which one is divided 

 into two ; and sometimes there is only a plicated or lobated 

 one on one valve, and two on the other ; the lateral teeth 

 are compressed, and extended transversely ; the ligament 

 is external. 



The recent shells are very thin and fragile, and some- 

 times transparent. 



2. Cyrena. — A roundish, trigonal, solid, thick, tumid, 

 inequilateral shell ; the hinge with three teeth on each 

 valve, the lateral ones always two, and of which one often 

 approaches the cardinal ones ; the ligament external, at- 

 tached to the largest side. 



These have been confounded with the cyclades. They 

 are not known in Europe in a recent state ; some of the 

 species have their lateral teeth serrulated. — PL viii. fig. 14. 



3. Galathea. — A subtrigonal, equivalved bivalve ; the 

 hinge teeth grooved ; two in the right valve, united at their 

 base ; and three in the other valve, the intermediate one 



