LAMELLIBRANCHIA TA. 
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 figs. 6, 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 LXXXIII, FIG. 10. 
Crenipecten impolitus , Hall. Pal. N. Y., vol. y, pt. 1. Plates and Explanations: PI. 9, tig - . 14. Jan., 18S3. 
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 the 
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 16 mm., width 26, and 19 mm., hinge-line 9 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. 
