41 



subtruncate posteriorly ; beaks nearly central, small, in- 

 curved and not much elevated. Surface marked with very 

 fine, regular, closely arranged, concentric striae, which on 

 the posterior umbonial slopes are crossed by a few simple, 

 stronger radiating lines or costee, passing from the beaks ob- 

 liquely to the lower posterior margin ; behind these radia- 

 ting lines there is a kind of corselet or area, apparently only 

 marked with the same fine concentric striae seen on other 

 parts of the shell. 



Length and height each '15 inch ; breadth '12 inch. 



The small size and nearly circular form of this shell, to- 

 gether with its smooth area, behind the radiating lines or 

 costae, will serve to distinguish it from any species known 

 to me at this time. 



Ph^ladomya (Goniomya) borealis. — Shell ovate, rather 

 compressed, anterior end rounded ; posterior extremity 

 rounded or subtruncate ; cardinal border sloping a little 

 from the beaks ; base forming a broad gentle curve ; beaks 

 nearest the anterior end, small, not much elevated. Surface 

 ornamented by about twenty-two costae, which pass down 

 the sides of the shell from the dorsal region, in front and 

 behind the beaks, and meet at rather acute angles along the 

 umbonial slopes, and down the flanks ; though some of those 

 nearest the extremities reach the base before uniting to form 

 an angle. 



Length about 1*70 inches ; height 1*17 inches ; breadth 

 *65 inch. 



This shell very closely resembles a species described by 

 Dr. Hayden and myself from the upper Cretaceous rocks of 

 Nebraska, under the name of Goniomya Americana (See 

 Proceed. Acad. Nat. Sci., Phil'a, vol. 8, p. 81), but is pro- 

 portionably deeper from the beaks to the base than that 

 shell, and the costae on the posterior side differ in curving 

 forward, instead of being directed obliquely backwards as 

 in G. Americana. Other differences would probably be seen 

 on comparing better specimens. 



