LAMELLIBRANCHIATA. 63 



radiating striae, which increase by interstitial addition, and become obsolete 

 on the npper part of the umba The radii are also seen on the ears. 



Internal characters not observed. 



The largest specimen has a length of 19 mm., height 15 nun., hinge-line 

 19 mm. Another specimen measures 17 mm. in length, 14 mm. in height, 

 with the hinge-line 18.5 mm. 



This species closely resembles P. l&tus, but differs in being less oblique, with 

 greater height, more acute beak, fewer and stronger radii, and a characteristic 

 concentric marking. From P. Hermes it difiers in being less oblique, with the 

 posterior ear less extended, and with finer rays upon the ears. From P. con- 

 spectus it difters in its greater length, and distinctly different surface markings 

 of the left valve. 



Formation and locality. In the Marcellus shale at Bloomfield, Ontario 

 county, N. Y. 



PtERINOPECTEN LiETUS, n. sp. 



PLATE I, FIG. l:!. 



Plerinopecten laiitas (in cn-dr), Hali,. P:il. N. Y., vtil. v, )]t. 1. Plates and Exiilanations : PI. I, tiff. 13. 



.Ian., 18S3. 



Shell small, sul^-rhomboidal, regularly rounded ; length one-sixth less than the 

 height. 



Left valve depressed-convex. Right valve unknown. 



Hinge-line straight, longer than the greatest length of the valve, extended 

 anteriorly beyond the margin of the shell. 



Beak obtuse, anterior to the middle, directed forward, not prominent. 

 Umbonal region scarcely defined, subtending about a right angle. 



Ears unequal, triangular. Posterior ear much the larger, undefined ; mar- 

 gin very slightly concave ; extremity rectangular. Anterior ear smaller, 

 not distinctly defined from the 1jody of the shell ; margin above the sinus 

 convex ; extremity acute. Byssal sinus moderate. 



Entire surface marked l)y distinct, close, rounded, duplicating radii, crossed 

 by fine, concentric lines of growth. The surface of the umbonal region is 



