47 

 POTAMIDES. Brogniart. 



Gen, Char. A tiirretcd, univalved shell ; aper- 

 ture nearly semicircular, without a canal in 

 the upper angle, contracted at the base into 

 a short slightly truncated beak; outer lip 

 dilated ; operculum corneous. 



A oENus SO nearly allied to Cerithiuni that it is very 

 difficult to distinguish it, nevertheless it is desirable so 

 to do, since the shells composing it are inhabitants of 

 freshwater, and have probably a structure suitable to 

 the necessary diiference in their economy. The principal 

 differences hitherto noticed between the Genera are the 

 following : Potamides has a corneous epidermis, and is 

 frequently decollated or eroded .; Cerithium has an ex- 

 tremely thin epidermis, if any, and is generally perfect ; 

 Potamides has a very short, not recurved beak, and has 

 no well defined, reflected canal at the upper angle of the 

 mouth, but sometimes a groove in the lip in place of it ; 

 Cerithium has a recurved beak, and often a distinct canal 

 at the upper angle of the mouth, and generally the form 

 of the aperture in Potamides is proportionally shorter 

 than in Cerithium. 



Although this genus was separated from Cerithium by 

 Broguiart so long ago as in 1810, we were unwilling 

 to adopt it for fossil species, until we came to examine 

 Buccinum rigidum of Brander, when we were so much 

 struck by its similarity to Potamides ater of Brogniart, 

 that we were constrained to admit his genus. Could 

 we with certainty determine fresh water or marsh shells 

 among fossils, by inference from their association, we 



