LAMELLIBRANCHIATA. 83 



All the specimens are in the condition of casts or much exfoliated, and the 

 surface markings are therefore obscure. 



The species was originally founded upon specimens of the left valve; the 

 right valves shown in tigs. (i. 7 and 8, pi. 9, occur in the same association, 

 and have been identified as of the same species. 



This species differs from C. obsoletus in its proportionally greater convexity 

 and length, and the obliquity of the transverse axis. It is also longer than 

 C. amplus. 



Formation and locality. In shales of the Chemung group at Rockville, Alle- 

 ghany county, N. Y. 



Crenipecten impolitus. 



PLATE IX. FIG 14; AND PLATE I.AXXIII, FIU. 10. 

 Crtiiipecttii impolitic*, Hall. Pal. N. Y.. vol. v. ]it. 1. Plates and Explanations: PI. 9, tig. 14. .Ian.. 1883. 



Shell of medium size, obliquely sub-ovate ; length somewhat less than the 

 height; anterior and basal margins regularly rounded, produced behind and 

 extending thence in nearly a direct line to the beak. 



Left valve depressed-convex. Right valve not known. 



Hinge-line straight, anterior, less than half the length of the shell. 



Beak obtuse, rounded, low, inclined a little forward. Umbonal region 

 scarcely defined, subtending about a right angle. 



Ears nearly equal ; margins concave. The posterior ear extends along the 

 posterior margin as a narrow expansion. In well-preserved specimens tin- 

 ears are defined by sulci. Byssal sinus inconspicuous. 



Obscure evidences of concentric striae appear in the casts. 



Internal characters unknown. 



Two specimens have respectively the following dimensions : length 24 

 and 10 mm., width I'll, and 19 mm., hinge-line and 7 mm. 



In general form and convexity this species resembles C. crenulatus, but it is 

 narrower above, with the hinge-line comparatively much shorter. 



