72 
PALEONTOLOGY OF NEW YORK. 
This species resembles Pterinea reprobus, from the shales of the Chemung 
group at Ithaca, which has straighter and more rounded radii in the left valve, 
with sharper intermediate radii and more elevated, undulating, concentric strise. 
The right valve of that species is more convex with stronger and often dupli¬ 
cating radii; while the byssal sinus is less distinctly' marked. The present 
species differs from Pterinopeden dispandus in its greater length, proportionally 
longer hinge-line, less strongly marked right valve, and want of duplication 
in the radii. 
Formation and locality. This species is common in the shales of the Hamilton 
group at Bear gulf, Schoharie county, N. Y. 
Pterinopecten undosus. 
PLATE II, FIGS. 10-19; and PLATE LXXXII, FIG. 7. 
Aviculopecten undulatus, Hall. MS. 1877. 
“ “ “ Cat. Amer. Palaeozoic Fossils. S. A. Millkr. 1877. 
Not Pecten undulatus, McCoy. Carb. Foss, of Ireland, p. 101, pi. xvii, fig. 12. 1844. 
Not Aviculopecten undulatus, McCoy. 1853. 
Pteiinopeeten undosus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 2, figs. 10-19. 
Jan., 1883. 
Shell larger than medium, rhomboidal or sub-orbicular; left valve moderately 
oblique; right valve scarcely oblique; length one-third greater than the 
height, proportionally shorter in young shells and in the right valve; ante¬ 
rior and basal margins regularly rounded, somewhat produced behind, and 
receding rapidly toward the hinge-line. 
Valves nearly equally convex; the right valve a little less convex, more 
orbicular in outline, with deep byssal notch, and different surface markings. 
When occurring in the softer shales, the valves are often depressed-convex. 
Hinge-line straight, length a little less than the height of the shell, nearly 
central. 
Beak obtuse, rounded, scarcely rising above the hinge-line, anterior to the 
middle of the shell, inclined forward. Umbonal region ample, well-defined, 
subtending a very obtuse angle. 
