CONCHACEA. 335 



The river Conchacea are not very numerous in species, 

 neither are such as inhabit lakes, or fens and marshes. They 

 may be recognized by having their shells covered with a thick 

 hard epidermis, generally much eroded towards the umboes. 

 The hinge has two or more distinct lateral teeth, whicli 

 peculiarity is not discoverable in the marine. 



Three genera pertain to the Tluviatile Conchacea, namely, 

 Cyclas, Cyrena, and Galathcea ; but no particulars of inte- 

 rest are associated with them. The last genus was instituted 

 by Bruguiere, for the reception of a singular mollusk inhabit- 

 ing the great rivers of Africa. 



Shells of the Marine Conchacea differ from those of the 

 Fluviatile, in being mostly destitute of epidermis, and in 

 having but one lateral tooth, often none ; but the cardinal 

 teeth are similar. An immense series of marine mol- 

 lusks pertain to this division : the animal inhabitants 

 are nearly similar, and scarcely any decided change of 

 character is perceptible in their testaceous coverings. The 

 five genera are, Cyprina, Astarte, Vemis, Cytherea, and 

 Pullastra. 



. Cyprina may be said to partake of the characters of 

 Cyrena and Venus, thus serving to establish a close affinity 

 between the fluviatile and marine sections of the family. 



