Lamellibranchiata. 277 



This species is distinguished by its sub-orbicular form, slight ob- 

 liquity, nearly equal length and height, and comparatively short 

 hinge-line. The comparison with P. crenicostatus is given under the 

 description of that species. It resembles the young of Lyriojpecten 

 orbiculatus, but the details of the characters are quite different. 



Formation and localities. In the Chemung group at Hobbieville 

 and Elm Valley, Alleghany county ; .New Albion, Cattaraugus county, 

 N, Y., and Mansfield, Tioga county, Pa. 



CKENIPECTEN,* Hall. 

 Crenipecten amplus. 



Crenipecten amplus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations : 

 PI. 9, figs. 9, 13. Jan., 1883. 



Crenipecten crenulatusf Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 9, fig. 18. Jan., 1883. 



Shell larger than medium, broadly and obliquely ovate ; height about 

 one-sixth greater than the length; margins regularly rounded, 

 more extended posteriorly. Left valve very convex. Eight valve 

 not known. Hinge-line straight, about equal to one-half the length 

 of the shell, situated a little anterior to the middle. Beak obtuse, 

 erect, prominent. Umbonal region very convex, well-defined, sub- 

 tending a right angle. Ears nearly equal ; margins gently concave. 

 Posterior ear somewhat the larger, defined by a sulcus. Anterior 

 ear defined by a sulcus which is more distinct and abrupt than in 

 the posterior ear. Byssal sinus shallow. 



Surface of the cast marked by fine, irregular, concentric striae, 

 with obsolescent, fine radiating lines. Ligamental area narrow and 

 distinctly crenulate. 



The largest specimen is 33 mm. in length, 38 mm. m height, 

 hinge-line 16 mm. 



This species differs from P. crenulalus in its proportionally greater 

 height, rotundity, and less obliquity. Although known only as a cast, 

 it is distinctly different from other species. 



Formation and locality. In shales of the Chemung group at 

 Kockville, Alleghany county, N. Y. 



*The forms here described under this generic term have in part been referred to Perno- 

 pecten by Professor Winchell, but a critical study of the type species of that genus 

 shows that it possesses a large central cartilage-pit with a crenulated hinge-plate on each 

 side below the hinge-margin, which characters alone are sufficient to distinguish the 

 genus and exclude the forms here described under Crenipecten. See discussions of the 

 relations of Pernopecten, Entolium and Crenipecten, in the introduction to Pal. N. Y., 

 vol. v, pt. 1. 



