OEETAOEOUS FOSSILS. 140 



while the anterior ventral region is abruptly cuneate; anterior margin 

 rounded in outline; base with a shallow semiovate outline, being usually 

 slightly more prominent in advance of the middle; posterior margin long, 

 and very obliquely truncated, with a slightly convex outline, from the hinge 

 to the posterior basal extremity, which is prominent and subangular; hinge- 

 margin very short, or little more than equaling one-third the length of the 

 valves; posterior umbonal slopes prominent, and more or less angular from 

 the beaks to the posterior basal angle; cardinal area short and rather nar- 

 row; beaks moderately prominent, incurved, and placed one-fourth to one- 

 third the length of the valves from the anterior margin. Surface ornamented 

 by moderately distinct lines of growth, crossed by obscure radiating costse 

 that are wider than the mere linear furrows between. 



Length, 1.50 inches; height, 1 inch; convexity, about 0.70 inch. 



I only know this shell from casts that do not show the hinge. They 

 bear impressions, however, in some instances, of a ridge or projection along 

 the margin of the posterior muscular scar, as in CuculMa. From the gen- 

 eral form of the shell, and its very short hinge-margin, I can scarcely doubt 

 that it belongs to Mr. Conrad's group Trigonarca ; but, until the hinge can 

 be seen, its relations to that genus cannot be satisfactorily determined. I 

 know of no very nearly allied species. 



Locality and position. — East Canon Creek, Wasatch Range, Utah ; in a 

 whitish sandstone of Cretaceous age. 



Genus AXIN^A, Poll. 



AxiN^A Wyoming-ensis, Meek 



Shell attaining a medium size, subcircular or very slightly longer than 

 high, gibbous, thick, and strong; beaks rather prominent; basal margin 

 semicircular, and rounding regularly upward in front into the anterior border, 

 which rounds to the cardinal edge, so as scarcely to produce more than a 

 very slight angularity at the connection of the two above; posterior margin 

 making a slightly broader and similar curve, excepting that it is faintly 

 sinuous above the middle; hinge -plate thick, moderately arched, with denti- 

 cles rather strong, nearly straight, or a little curved, numbering about eight 

 or ten well-developed ones in front, and seven or eight behind, with per- 



