LAMELLIBRANCHIATA. 65 



The largest .specimen lias a length of 19 mm., height 29 nun., hinge-line 

 29 mm. 



This species is very similar in form to P. Icetus ; but the hinge-line is pro- 

 portionally longer and the radii stronger and more distinctly alternating. It 

 differs from P. dignatus in its more oblique form, proportionally greater diameter 

 and sharper radii. From P. conspectus it differs in being much more oblique in 

 outline and comparatively narrower. 



Formation and locality. In shales of the Hamilton group, Ontario county, N. Y. 



Pterinopecten spondylus. 



ri.ATE XVII, FIG. 16. 



Pterinopecten spondylus. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 17, fig-. 16. 



Jan., 1883. 



Shell small, sub-rhomhoidal, scarcely oblique ; body of the shell ovate ; length 

 somewhat greater than the height ; pallia! margins regularly rounded, be- 

 coming straight on the posterior slope, which is continuous with the margin 

 of the ear. 



Left valve convex. Right valve not known. 



Hinge-line straight, length greater than the longitudinal diameter of the 

 shell, extended anteriorly. 



Beak obtuse, rounded, directed forward, situated anterior to the middle 

 of the hinge-line. Umbonal region prominent, but not strongly defined on 

 the posterior slope ; subtending an acute angle. 



Ears broad, triangular, flat. Posterior ear much the larger, scarcely 

 defined ; margin slightly concave ; extremity rectangular. Anterior ear 

 defined by a distinct sulcus ; margin convex ; extremity rounded. Byssal 

 sinus shallow. 



Test marked by regular, sharp elevated rays, with broader interspaces 

 which are marked by -finer rays, and concentric foliate lamellae of growth, 

 which are bent backward in crossing the rays, forming short semi-tubular 

 9 



