337 Bulletin 5 57 



FOSSIL SHELLS. 151 



Length four-fifths, breadth rather less than nine- 

 tenths of an inch. 



Var. a. Length one inch and a tenth, breadth 

 one inch. 



A very distinct species, unlike any other yet 

 described. It varies in proportional length, some 

 being longer than broad, and others broader than 

 long. I adopt Sowerby's generic name, in pre- 

 ference to that of Crassina of Lamarck, for the all- 

 sufficient reason, that it has the priority. The 

 recent shell which I described under the name of 

 Venus castanea, Journ. Acad. Nat. Sc. vol. 2. p. 273, 

 belongs in strictness to this genus. 



2. A. vicina. PI. ix. fig. 6. Trigonal with a 

 distant, somewhat regular, impressed line : lunule 

 much excavated ; apices acute. 



Description. Apices prominent : lunule dilated, 

 deeply excavated, subcordate, separated from the 

 disk, particularly near the beaks, by a subacute 

 angle : beaks prominent, approximate, acute, curv- 

 ed backwards : ligament 7nargin concave : umbones 

 convex. 



Length nine-tenths of an inch, breadth one inch. 



Closely allied to the preceding, but numerous 

 specimens correspond in the much more pro- 

 foundly concave lunule, and in the convexity of 

 the umbones. 



[J. A. N. S., Phila., ist Ser., Vol. IV, 1824.] 



