43 



cuneate posteriorly ; buccal end sloping (with a gentle out- 

 ward curve) obliquely backwards from near the beaks al- 

 most to the middle of the prominently rounded base ; anal 

 half narrowing abruptly on the lower side. Corselet broad 

 lanceolate, nearly equaling the greatest length of the shell ; 

 deeply arcuate, and marked longitudinally by two impressed 

 lines on each side, and on the middle elevated part, by dis- 

 tinct transverse costse. Beaks nearly terminal, slender, ele- 

 vated, incurved and approximate. Surface ornamented by 

 about twenty-two strongly elevated, sharp, transverse costse, 

 which on the anterior part of the shell have a few distantly 

 separated nodes. Concentric lines of growth rather distinct ; 

 posterior muscular attachment and pallial line strongly im- 

 pressed. 



Length, 2*30 inches ; height 1*50 inches ; breadth 1'40 

 inches. 



This well marked species will be at once identified by its 

 narrow, elevated, nearly terminal beaks, and the obliquity 

 of its gibbous umbonial slopes. Its sharply elevated costse 

 are also remarkable for having only on the anterior end of 

 the shell a few distantly separated nodes. On the buccal 

 half, the costse are stronger, more distant, and pass obliquely 

 downwards and forwards from the corselet to the front bor- 

 der, while those further back pass obliquely downwards and 

 backwards to the base and posterior basal margins. 



The specific name is given in honor of Dr. John Evans, 

 U. S. Geologist of Oregon. 



Thracia ? occidentals. — Shell elliptical, compressed, 

 thin and fragile ; right valve a little the most convex ; beaks 

 nearly central, small, not much elevated, in close contact ; 

 extremities narrowly and subequally rounded ; base form- 

 ing a regular broad elliptical curve ; cardinal border sloping 

 a little from the beaks. Surface (of worn specimens) re- 

 taining faint lines of growth. 



Length 2*08 inches ; height 1*25 inches ; breadth *52 inch 



