88 University Geological Survey of Kansas. 



amples longer than wide, with all intermediate forms ; hinge 

 always shorter than the greatest breadth of the valves ; anterior 

 outline regularly rounded, or faintly sinuous in the middle. 

 Ventral valve more or less gibbous, with a moderately distinct 

 mesial sinus extending from near the beak to the front ; beak 

 incurved a little beyond the cardinal margin [considerably so] ; 

 ears rather compressed, but not distinctly defined from the swell 

 of the umbo. Dorsal valve moderately concave with a small 

 mesial elevation. Surface of both valves ornamented with 

 numerous rather regular concentric ridges, increasing in size 

 from the beaks toward the front, but becoming again smaller 

 and more crowded in adult shells near the margin ; in the 

 ventral valve these ridges are a little prominent at the margin, 

 separated from each other by smoother spaces, and support 

 numerous small appressed spines, those of the upper row of 

 which are larger and less crowded than the others ; on internal 

 casts, or partly exfoliated specimens, the spines are represented 

 by small tubercles ; surface of the dorsal valve as in the other, 

 excepting that the ridges are represented by little furrows." 

 Interior of the pedicle value with a very small, thin, nearly 

 obsolete mesial septum ; adductor scars long, narrow, larger at 

 posterior extremity, situated well to the back part of the shell. 

 Diductors occupying a large space outside of the adductors, in- 

 dicated by longitudinal folding ridges or parallel lines. In 

 the brachial valve, the cardinal process is long, arched, bifid, 

 slightly enlarged at the extremity; mesial ridge prominent, 

 extending three-fifths the distance to the front margin, and is 

 slightly thickened at the end. Lateral ridges strong, situated 

 just in front of the hinge and parallel to it, but disappearing 

 before the edge of the shell is reached. Adductor scars promi- 

 nent, situated well to the front, varying in outline from nar- 

 rowly elliptical to nearly ovate, with heavy, narrow lobe at the 

 anterior end. Traces of brachial markings very hard to obtain 

 and nearly obsolete. They extend outward from the ante- 

 rior end of the adductor scars and make a broad, subelliptical 

 loop forward to a point which is back of the end of the mesial 

 ridge. Measurements : Moderately large, perfect specimen : 



