82 



small, deep, ovate, and placed near the buccal edge, bound- 

 ed posteriorly by a distinct ridge extending down nearly 

 perpendicularly from the beak ; while a second much smaller 

 impression is also usually visible just above the latter. 



Onr specimens being internal casts, of course do not show 

 satisfactorily either the hinge or surface characters, some of 

 them however, appear to retain faint traces of two or three 

 small radiating ridges on the posterio-dorsal regions. The 

 hinge, as near as can be determined, does not appear to be 

 as in Cardinia or Cypricardia, but more like that of Mr. 

 King's genus Plenrophorns, to which we have provisionally 

 referred it. 



This species appears to differ from the last in being 

 broader at the anterior end, while the reverse is the case 

 with the last. 



Locality. — Near mouth Smoky Hill fork of Kansas river — 

 Major Hawn's collections. 



Lyonsia (Penop^a) concava. 



Shell small elongate ; valves moderately convex in the 

 umbonial region ; anterior extremity rounded ; posterior 

 end more narrowly rounded, or faintly subtruncate, com- 

 pressed, and somewhat gaping ; base forming a very gentle 

 curve, rounding up a little more gradually behind than in 

 front ; dorsum concave, and nearly parallel to the base ; 

 hinge line long, nearly straight, or a little concave j beaks 

 rising somewhat above the hinge, but not much elevated, 

 incurved and located about half-way between the middle 

 and the anterior end. Surface of cast, marked near the 

 border by a few small, irregular concentric wrinkles ; pal- 

 lial and muscular impressions unknown. Length 0*66 inch ; 

 height 0*25 inch ; breadth # 32 inch. 



This is a very neat, handsome little shell, which varies 

 somewhat in form at different ages, being a cast, its generic 

 characters are left in doubt. 



