272 Thirty-fifth Report oisr the State Museum. 



upper part, subtending an acute angle. Ears triangular. Posterior 

 ear much the larger, flat ; margin concave ; extremity slightly acute, 

 obscurely defined. Anterior ear small, convex, defined by a well- 

 marked groove or sulcus ; margin rounded ; extremity acute. Byssal 

 sinus distinct in the left valve, and a deep rounded notch in the 

 right valve. 



Test of left valve marked by alternating stronger and finer rounded 

 radii, crossed by sharp, elevated, concentric lines of growth and 

 more distant undulations, with imbricating lamellose lines. These 

 markings are continued over the ears, much subdued on the right 

 valve, and obsolete upon the umbo. Ligamental area marked by 

 two or three furrows, and a central cartilage-pit. 



The specimen described has a length of 25 mm., height 30 mm., 

 hinge-line 25.5 mm. 



This species is doubtfully arranged with the present group of forms. 

 In some aspects it resembles P. conspectus, but is specifically very 

 distinct. It might be compared with the young of some large forms- 

 of Lyriopecteis" or with Aviculopecte;^, but the extent of the hinge- 

 line, and the undefined limitation of the posterior ears, do not assimi- 

 late it with the ordinary characters ascribed to those genera. 



Formation and locality. In the Chemung group near Elmira, N. Y. 



Pterikopecten" imbecilis. 



Pterinopecten imbecilis, Hall. Pal. N. Y., vol. y, pt. 1. Plates and Explana- 

 tions ; PL 24, fig. 1. Jan. 



Shell small, rhomboidal, oblique, much longer than high ; anterior 

 and basal margins rounded; posterior margin produced. Left valve 

 moderately convex. Right valve not known. Hinge-line equal to, 

 or greater than, the length of the body of the shell. Beak obtuse, 

 directed forward, rather prominent. Umbonal region ample, de- 

 fined anteriorly ; subtending an angle of about 90°. Ears very un- 

 equal, triangular. Anterior ear small, defined by a distinct sulcus. 

 Posterior ear large, undefined ; margin concave ; extremity angu- 

 lar. 



Surface marked by numerous strong, simple radii, with wider 

 interspaces which have usually finer intermediate radii ; crossed by 

 fine, equal concentric lines of growth, and stronger, more distant 

 lamellae, which are elevated and arched backwards over the larger 

 rays, producing nodes. 



The specimen has a length of 15 mm., height 11 mm., hinge-line 

 about 15 mm. 



