TO GEOLOGY. 49 
fect. It has a strong resemblance to Petricola pholadiformis 
(Lamarck) (P. fornicata, Say), but is to appearance entirely 
without teeth, having several irregular folds about the beak. 
It differs also in the exterior folds or ribs, which are, in the 
pholadiformis, marked and situated on the anterior part. 
FAMILY NYMPHACEA. 
GENUS EGERIA. (WNobis.) 
Description. Shell subrotund or subtriangular ; margin 
sometimes crenulate ; hinge variable, sometimes with two 
lateral teeth ; cardinal teeth two in each valve, diverging, 
one bifid ; ligament external. 
Observations. Unable to place a considerable group of 
shells, bearing in the cardinal teeth the same character, 
in any genus with which I am acquainted, I propose to 
found a new genus for them, the proper place of which 
would be between Sanguinolaria and Psammobia. The 
cardinal teeth have some resemblance to the Lutricola of 
Blainville, but that able naturalist does not mention a bifid 
tooth, nor is it so figured in his work. Although there is 
considerable dissimilarity in the form of the several species 
of this genus, some being more round and others triangu- 
lar, the two diverging cardinal teeth, one being bifid, will 
always be found characteristic of the genus. 
G 
