LAMELLIBRANCHIATA. 71 



fig. 22 of pi. 17, is imperfect at the posterior extremity of the hinge, and 

 thus resembles the present form more than it would if properly restored. 



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

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



Pterinopecten Vertumnus. 



PLATE V, FIGS. 1-8 ; and PLATE LXXXIII, FIGS. •_>, 3. 



Pterinopecten Vertumnus, Hall. Pal. N. Y., vol. v. pt. 1. Plates and Explanations: PI. 5, figs. 1-S. 



Jan.. 1883. 



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

 length about one-fifth greater than the height ; margins regularly rounded, 

 becoming full behind and sloping rapidly to the hinge-line. 



Left valve def>ressed-convex. Right 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 stria? 

 which are more closely arranged towards the margin of the shell. The 

 same character of surface continues over the ears, and on the posterior ear 

 the radii are finer, nearly equal in strength, and crenulated by the concen- 

 tric striae. On the right valve the rays are more equal and finer than on the 

 opposite valve ; conspicuous on the posterior ears and subdued on the 

 anterior. 



Three specimens have respectively the following dimensions : length 

 38, 40, 41 mm., height 30, 33, 29 mm., hinge-line 35, 35, 32 mm. 



