LAMELLIBRANCHIATA. 59 



degree upon the posterior ear, and somewhat more strongly on the anterior 

 slope and anterior ear. 



Internal characters unknown. 



The specimen figured has a length, across the middle, of 18 mm., height 

 15 mm., hinge-line 22.5 mm. 



In form, this species resembles P. Hermes, but is distinct in its smaller and 

 less defined anterior ear, less strong radii, and more depressed beak. It is 

 much flatter and of different proportions from P. exfoliatus. 



Formation and locality. In the Corniferous limestone at the Falls of the Ohio, 

 near Louisville, Ky. 



Pterinopecten insons. 



PLATE I, FIG. 4. 

 Pterinopecten insons, Hall. Pal. N. V.. vol. v. pt. 1. Plates and Explanations: PI. 1, tig. 4. Jan., 1883. 



Shell small, rhomboidal, slightly oblique ; length somewhat greater than the 

 height; basal margin regularly rounded; the anterior and posterior sides 

 become nearly vertical as they approach the ears. 



Left valve convex, the greatest convexity near the beak. Right valve 

 unknown. 



Hinge-line straight, length one-fourth greater than the length of the shell, 

 extended posteriorly. 



Beak acute, prominent, anterior to the middle of the hinge, directed for- 

 ward. Umbonal region prominent, subtending a right angle. 



Ears triangular ; margins straight ; extremities acute. Posterior ear large, 

 undefined. Anterior ear small, convex, with an undefined fold below the 

 cardinal margin, limited by a distinct rounded sulcus. Byssal sinus slightly 

 marked. 



Test (in a partially exfoliated specimen) marked by regular, sharp, not 

 numerous rays, with broad, flat interspaces, which, in a perfect condition, 

 have probably been marked by fine striae. The rays are obsolete over the 

 posterior slope of the valve and the umbo, and also not preserved on either 

 of the ears. 



