80 University Geological Survey of itansas. 



Meek's description (in part) : " Shell of medium size, wider 

 than long, very convex ; hinge margins about equaling the 

 greatest breadth of the valves. Pedicle valve exceedingly gib- 

 bous, and very strongly incurved, with a deep rounded sinus 

 extending from near the beak to the front, to which it imparts 

 a sinuate outline ; umbo prominent and strongly incurved, so 

 as to pass somewhat within the hinge margin ; ears well de- 

 fined, arched and rather distinct from the abrupt swell of the 

 umbo, from which they are sometimes separated by a small 

 ridge or fold. Brachial valve flattened in the visceral region, 

 and more or less abruptly curved or geniculated toward the 

 front and anterior lateral margins, the former of which usually 

 shows a small mesial ridge. Surface of both valves ornamented 

 with distinct, rather unequal, depressed and rounding radiat- 

 ing costae, which sometimes bifurcate, or, in other instances, 

 two or more of them coalesce in front of the visceral portion, 

 to form a larger one ; crossing all of these, on the visceral re- 

 gion, are numerous, well-defined, concentric wrinkles, produc- 

 ing a distinct reticulated appearance, while the whole surface 

 of the ventral valve is sometimes provided with a few scatter- 

 ing, rather stout, erect spines, somewhat regularly arranged in 

 quincunx. Sometimes nearly all the spines, excepting those on 

 the lateral regions, apparently wanting." The internal mark- 

 ings of the ventral valve consist of long, narrow, arborescent 

 adductor scars mesially situated along the vaulted part of the 

 shell, with, exterior to these, the long linear folds, or line-like 

 diductor scars. The beak, as shown in a cast, is quite pointed, 

 possessing a depression on either side running obliquely back- 

 ward, near the posterior end of which there is an elevation, in 

 front of which the pit is deeper than back of it. Above this de- 

 pression is a rounded ridge on each side of the mesial sinus. 

 The proboscis of the dorsal valve extends beyond the hinge 

 line, and is backed by three radiating ridges, one of which ex- 

 tends directly forward to the front of the flattened area, 

 rounded, not prominent ; one extends antero-laterally on each 

 side of the mesian ridge, running nearly outward until the 

 edge is approached, when it curves forward, joining the border 



