358 GEOLOGY OF THE BLACK HILLS. 



at the extremity and ventricose on the surface; beak moderately large, 

 pointed, incurved, and extending' above the cardinal line; body of the shell 

 quite prominently convex, especially on the umbo; anterior margin of the 

 shell slightly projecting in the upper part so as to leave a faint sinus below 

 the anterior wing, and regularly rounding obliquely backwards and down- 

 ward to the basal border below, with which it forms an almost regular curve 

 to near the posterior basal extremity, which is sharply rounded. Right 

 valve smaller than the left, very depressed-convex, becoming distinctly 

 flattened toward the posterior margin and wing ; beak small, pointed, but 

 not projecting beyond the hinge line; posterior wing large; anterior wing 

 small, and separated from the body of the shell by a depressed groove. 



Surface of the right valve faintly marked by a few distinct radii. 

 That of the left valve marked by a varying number of distant, elevated, 

 raised radii, flattened on their surfaces and separated by interspaces of 

 twice or thrice their width, bearing finer lines. 



Messrs. Meek and Hay den refer this species with doubt to Avicula mun- 

 steri Bronn, and at the same time suggest the name A. (0.) mucronata, which 

 we have adopted on the evidence of their suggestion, not having been able 

 to compare them with European specimens. 



Formation and locality. — In shaly limestones of Jurassic age, on the 

 tops of the highest hills, two miles south of the Belle Fourche River, near 

 Bear Lodge Butte, Black Hills. 



Genus GERVILLIA De France. 

 GEBVILLIA BECTA. 



Plate 4, fig, 3. 



Gervillia recta Meek, U. S. Geol. Surv. Terr., vol. ix, p. 60. 



A single small right valve of a species of Gervillia occurs in the 

 collection, which, so far as we can judge, appears to belong to the above- 

 named species. The valve is very triangular in form, oblique and depressed 

 convex, the body of the shell being scarcely distinct from the rather large 

 posterior wing, and entirely destitute of any surface markings other than 

 the ordinary lamellose concentric lines common to all the species of the 

 genus ; hinge line straight and about two-thirds as long as the extreme 



