319 Bulletin 5 49 



FOSSIL SHELLS. 1 43 



rather coarsely wrinkled, and on the. umbo with 

 small, regular undulations ; umbo flattened : apex 

 subacute, not prominent, placed a little behind the 

 middle of the shell : hinge teeth distinct, promi- 

 nent : fosset much dilated ; a profound groove on 

 the anterior inner margin : edge not crenated ; on 

 the exterior anterior margin and submargin are 

 two undulations, of which the former is less dis- 

 tinct and more acute. 



Width rather less than one inch and nine-tenths 

 to four inches and three-fifths, length less than one 

 inch and three-tenths to about three inches. 



The species to which this is most closely allied, 

 is the compressa, Lam. Its compressed form, and 

 the somewhat elevated lines on the anterior part 

 of the shell, seem to be very similar ; but that 

 species is proportionally much broader behind, the 

 beaks are much nearer the posterior extremity, 

 and the inner edge is crenated. A perfect valve 

 was purchased at the sale of the late Professor 

 Barton's collection by Mr. Watson, and by him pre- 

 sented to the Academy. Its locality was not known, 

 but a fragment which I dissected out of a mass con- 

 taining fragments of Perna, belonging to the col- 

 lection of Mr. Finch, has decided this question. 



ISOCARDIA. 



1. fraterna. PI. xi. fig. i. a. Hinge 2. b. Back 

 view. Cordate-globose, slightly oblique, with ra- 

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



