72 PALEONTOLOGY OF SLW YORK. 



This species resembles Pterinea reprobvs, i'roni 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, eoncentric striae. 

 The right valve <>f 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 Pterinopecten 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; ami PLATE LXXXII, JIG. 7 



Ar'n ill" in i tin ii ad iilut us. Hall. MS. 1877. 



" •• Cat. AmiT. Palaeozoic Fossils, s A. Miller. 1877. 



No! Pecten undvlabus, McCoy. Carb. Foss. of Ireland, p. 101, pi. xvii, fig. 12. 1S44. 

 Not AmcvZopeeten undtUatus, McCoy. 1853. 



Pterinopecten nndosus, HALL. Pal. N. V., vol. v. pt. 1. Plates and Explanations: PI. 2, tip*. 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 it very obtuse angle. 



