83 



Locality. — On the Missouri river, nearly opposite the 

 northern boundary of Missouri. 



Paxopaea Cooperi. 



Shell when young rather oblong, and having its valves 

 but moderately convex ; becoming more gibbous, or almost 

 subcylindrical with age ; anterior end narrowly rounded 

 and more compressed than the other ; posterior end very 

 widely gaping, obliquely and distinctly truncate ; basal 

 margin almost straight or forming an exceedingly gentle 

 curve, rounding up gradually in front, and meeting the 

 truncated posterior edge behind (in old shells), so as to 

 form a distinct angle. Dorsum slightly concave and not 

 declining behind the beaks, sloping abruptly in front ; 

 beaks depressed, subangular behind, closely incurved and 

 not oblique, located nearly half way between the middle 

 and the front. Hinge straight, or slightly concave behind 

 the beaks, provided (in the cast) with a narrow, deep, dis- 

 tinctly circumscribed cardinal area. Surface of cast orna- 

 mented by concentric marks of growth which form on the 

 umbones, small somewhat regular undulations. Muscular 

 and pallial impressions unknown. Length 2*57 inches ; 

 height 1*37 inches ; breadth 1*12 inches ; breadth of pos- 

 terior hiatus 1 inch. 



We name this species in honor of Dr. J. G. Cooper, Sur- 

 geon and Naturalist of the North Pacific "Wagon Road 

 Survey, to whom we are indebted for the specimen here 

 described, as well as for several of the same species found 

 by Major Hawn, near the Smoky Hill fork of Kansas river. 



Locality — Near Helena in North-Eastern Kansas. 



Nautilus eccentricus. 



Shell rather small, subdiscoidal ; volutions rounded and 

 not embracing ; umbilicus very large and shallow, showing 

 all the inner whorls ; transverse section of the septate por- 



