MO'LLUSCA. 547 



basal margin gently convex anteriorly and straight or slightly 

 concave posteriorly ; postero-basal extremity sharply rounded or 

 subangular ; posterior margin slightly convex, obliquely truncate ; 

 postero-cardinal margin long and straight, joining the posterior 

 margin in an obtusely rounded angle. From the beak to the 

 postero-basal angle, a rather broadly rounded umbonal ridge 

 extends in a nearly straight line; the postero-cardinal slope is 

 nearly flat posteriorly, becoming a little convex towards the beak ; 

 the cardinal margin is sharply inflected to form the sides of 

 the narrow but rather deeply excavated escutcheon. In front of 

 the umbonal ridge the shell is flattened or slightly sinuate from 

 "the beak obliquely backward to the basal margin ; in front of the 

 beak the shell is inflected along the cardinal margin to form the 

 rather broad, concave lunule. Surface of the shell marked with 

 moderately fine, but rather strong and regular concentric lines of 

 growth, which become obsolescent back of the umbonal ridge. 



In the internal casts the beaks are more prominent, erect and 

 rather widely separate, the shell is rather more pointed posteriorly 

 and the muscular and pallial impressions are of moderate strength. 



Remarks. — There seems to be some uncertainty as to the origin 

 of the type specimen of this species. In the original description 

 it is said to be from the "Deep Cut, Delaware and Chesapeake 

 Canal," which would make its horizon about that of the Merchant- 

 ville clay-marl. This type specimen is preserved in the collection 

 of the Philadelphia Academy of Science, and is labeled in Gabb's 

 own hand-writing, "Crosswick's N. J.," according to Johnson, 

 which would make its horizon the Woodbury clay. The species 

 is a very characteristic one, and shows so little variation, that it 

 is not possible to mistake it, and in all the collections made since 

 the species was described, it has been found only in the Manasquan 

 marl. It seems probable, therefore, that the type specimen also 

 came from this formation, notwithstanding the fact that the 

 original author of the species recorded it from elsewhere. The 

 two different localities assigned tO' the specimen by Gabb would 

 indicate some uncertainty as to its true origin in that author's 

 own mind. 



The species was placed in the genus Btea by Conrad, and al- 

 though the hinge-structure has not been observed to demonstrate 



