82 University Geological Survey of Kansas. 



lar curve, but the middle of the front is generally rather dis- 

 tinctly sinuous. Ventral valve gibbous, the greatest convexity 

 being usually behind the middle, and the curve to the beak 

 more rapid than to the front, provided with rather deep mesial 

 sinus ; posterior lateral slopes descending nearly vertically to 

 the ears ; umbonal region moderately prominent, and usually 

 projecting rather distinctly beyond the hinge, as seen in look- 

 ing down upon the shell when lying with the dorsal valve be- 

 neath ; beak small, strongly curved, but scarcely passing beyond 

 the cardinal margin ; surface ornamented with generally rather 

 obscure, somewhat variable radiating costse, which are often 

 obsolete in the umbonal region, or, in some examples, over much 

 of the valve farther forward, in other specimens quite distinct 

 to the beak, sometimes bifurcating, and in other instances 

 coalescing to form larger, faintly denned ribs in front ; fine, in- 

 distinct marks of growth are also sometimes seen, and occa- 

 sionally very obscure traces of small, concentric wrinkles may 

 be observed near the beak ; spines stout, erect, long, scattering, 

 and arranged in quincunx. Ventral valve distinctly concave, 

 or following nearly the curve of the other, and provided with a 

 small mesial ridge corresponding with the sinus of the latter ; 

 surface marked as in the other valve, but apparently always 

 without spines." Proboscis short, trilobate ; two ridges extend 

 laterally from its base nearly parallel to the hinge line, until 

 the border of the visceral region is reached, where they bend 

 abruptly forward, and in very old individuals form a ridge 

 entirely around the visceral and brachial cavity, but in many 

 adult specimens it fades out before it reaches the front side of 

 this cavity ; the lateral borders of the shell are often striated 

 from this ridge to the margin ; mesial ridge faint, extending 

 nearly to the front of the viscero-brachial area. Adductor scars 

 small, not prominent, ovate, placed close beside the mesial 

 ridge, well to the back of the shell, directed obliquely for- 

 ward ; directly in front of these the brachial markings extend 

 toward the antero-lateral margin of the brachial area, near 

 which they enlarge ; in front of these the surface of the valve 

 is marked by many or few (according to the age of the shell) 



