24 GENERA OF SHELLS. 



SECT. III. CONCHIFERA LAMELLIPODA. 

 CONCKLE. 



Three cardinal teeth, at least, in one valve ; as many or fewer 



in the other ', sometimes lateral teeth. 

 The conchaB constitute one of the most beautiful and 

 numerous families of the conchifera. Their shells are 

 equivalve, orbicular or transverse, always regular, free, 

 and in general very close, especially at the sides. They 

 are more or less inequilateral, and seldom have true, 

 radiating ribs. 



CONCHJE FLUVIATILES. 



Like the naiadae, these shells are covered with a kind 

 of greenish epidermis, which turns more or less brown, 

 and is often decorticated at the beaks. They are distin- 

 guished from them by the hinge, which has cardinal 

 teeth, like the venus, near the hinge. 



CYCLAS. Shell ovate-globose, transverse, 

 equivalve ; beaks protuberant, cardinal teeth 

 very small, sometimes nearly obsolete ; some- 

 times two on each valve, one of which is 

 folded, sometimes only one, plaited or lobed on 

 one valve, and two on the other ; lateral teeth 

 transversely elongated, compressed, lamelli- 

 form; ligament exterior. 



The cyclades are small shells, with thin valves, and 

 never have three teeth on either. The beaks are never 

 eroded. They are grayish green or yellowish : some 

 smooth, others transversely striated with lightish col- 

 ored bands. 



C. rivicola, cornea, lacustris, obliqua, calyculata, obtusalis, 



