Lamellibbanchiata. 269- 



of the valve; margin convex, in regular continuation with the pallial 

 margin; extremity obtuse-angular. Byssal sinus broad. 



Test thin, marked with numerous fine, alternating, filiform, 

 elevated striae which are crossed by irregular lines of growth. The 

 ornamentation of the ears is somewhat more subdued than on the 

 body of the valve. Muscular impression small, ovate, situated 

 below the middle of the post-cardinal slope. 



A specimen of the left valve has a length of 24 mm., height 19 

 mm., hinge-line 25 mm. A larger example measures 30 mm. in 

 length. 



This species is distinguished for the regular, continuous convex 

 curvature of the outline from the byssal sinus to the posterior ex- 

 tremity of the hinge. In this respect it differs from any other form 

 of this genus here described. The right valve of P. filitextus has a 

 similar outline, but the left valve has been shown to have the margin 

 of the posterior ear concave and the extremity acute-angular. 



Formation and localities. In the shales of the Hamilton group, 

 between Geneseo and Avon, and at Delphi, N. Y. 



Ptebikopectek Vebtumnus. 



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

 tions; PI. 5, figs. 1-8. Jan., 1883. 



Shell large, rhomboidal, oblique; greatest length below the middle 

 of the valve ; length about one-fifth greater than the height ; mar- 

 gins regularly rounded, becoming full behind and sloping rapidly to 

 the hinge-line. Left valve depressed-convex. Eight valve nearly 

 flat. Hinge-line straight, somewhat shorter than the length of the 

 shell. Beaks obtuse, low, anterior to the middle of the hinge-line, 

 directed forward. Umbonal region moderately convex in the left 

 valve, depressed in the right valve, limits defined anteriorly, but not 

 posteriorly. Ears triangular, flat. Posterior ear much the larger, 

 not defined from the body of the valve ; margin gently concave ; 

 extremity acute. Anterior ear small, separated from the umbo by 

 an undefined sulcus, and in the right valve by the marked, angular 

 byssal sinus ; margin concave ; extremity acute, sometimes rounded. 

 Surface of left valve marked by irregularly alternating, strong 

 flexuous, flattened radii; interspaces flat ; crossed by sharp, elevated, 

 concentric striae which are more closely arranged toward the mar- 

 gin of the shell. The same character of surface continues over the 

 ears, and on the posterior ear the radii are finer, nearly equal in 



