Lamellibkanchiata. 265 



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

 line is proportionally longer and the radii stronger and more dis- 

 tinctly 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 com- 

 paratively narrower. 



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

 county, N. Y. 



Pterinopecten" spondylus. 



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

 tions ; PI. 17, fig. 16. Jan., 1883. 



Shell small, sub-rhomboidal. scarcely oblique ; body of the shell 

 ovate ; length somewhat greater than the height ; pallial margins 

 regularly rounded, becoming straight on the posterior slope, which 

 is continuous with the margin of the ear. Left valve convex. Eight 

 valve not known. Hinge-line straight, extended anteriorly; length 

 greater than the longitudinal diameter of the shell. 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 inter- 

 spaces which are marked by finer rays, and concentric foliate lamellae 

 of growth, which are bent backward in crossing the rays, forming 

 short semi-tubular spines. The radii are less conspicuous on the 

 ears, which also show a few concentric undulations. The ligamental 

 area appears as a simple fold in the hinge. 



The specimen has a length of 15 mm., height 13.5 mm., hinge- 

 line 18 mm. 



This species has the general form and aspect of P. Hermes and 

 P. Icetus, but is proportionally wider, with different and characteristic 

 surface markings. 



Formation and locality. In soft shales of the upper part of the 

 Hamilton group, shore of Canandaigua lake, Ontario county, "N. Y. 

 [Sen. Doc. No. 38.] 34 



