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. 



C R E N I P E C T E N ,* Hall. 

 Crenipecten amplus. 



PLATE IX, FIGS. 9, 13, 18. 



Orenlpecte7i amplux. Hall. P;il. N. V., vcl. v. jit. 1. PLites and Kxiilanations : P!. 9, figs. 9, 13. Jan., 1883. 

 Crenipecten crenidatuJi ? UALh. Pal. N. Y., vol. v, pt. 1. Plate.s and Explanaticm.s : PI. 9, fig. 18. Jan., 1883. 



Shell larger than medium, broadly and o])liquely ovate ; height about one- 

 sixth greater than the length ; margins regularly rounded, more extended 

 posteriorly. 



Left valve ver^^ 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. Umlional 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 jiart been leferred to Pebnopecten by Pro- 

 fessoi- Winchell ; but a critical study of the type species of that geiuis i-hows that it possesses a large central 

 cartilagp-pit with a crenulated hinge-)ilate on each side below the hinge-margin, which characters alone ai-e 

 sufticient to distinguish the genus and exclude the foi-ms here described uadei' Crenipecten. See discus- 

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



11 



