LAMELLIBRANCHIATA. 81 



The comparison with P. crenicostatus is given under the description of that 

 species. It resembles the young of Lyriopeden 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, Pennsylvania. 



CRENIPECTEN* Hall. 



Crenipecten amplus. 



PLATE IX, FIGS. 9, 13, 18. 



Crenipecten amplus, Hall. Pal. N. V.. vol. v. pt. 1. Plates and Explanations : PI. 9, figs. 9, 13. Jan., 1883. 

 Crenipecten erenidatus? Hall. Pal. N. Y., vol. v, pt. 1. Plates ami Explanations : 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. Right 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, subtending 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, 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 forms here described under this generic term have in part been referred to Pernopkcten by Pro- 

 fessor 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 discus- 

 sion of the relations of Pernopecten, Entolicm and Crenipecten, in the introduction to this volume. 



11 



