47 
POTAMIDES. B, ogniart . 
Gen. Char. A turreted, 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 genus so nearly allied to Cerithium that it is very 
difficult to distinguish it, nevertheless it is desirable so 
to do, since the shells composing it are inhabitants of 
fresh water, and have probably a structure suitable to 
the necessary difference 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 
Brogniart so long ago as in 1810 , we were unwilling 
to adopt it for fossil species, until we came to examine 
Ruccinum rigidum of Grander, 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 associatipn, we 
