10 PALEONTOLOGY OF NEW YORK. 



Hinge-line straight, extending as far as the anterior margin; length two- 

 thirds the height of the shell; ligamental area narrow. 



Beaks well-defined, not rising above the hinge-line, situated anterior to 

 the middle. 



Posterior ear large, triangular, margin concave ; the ornamentation differ- 

 ing from that on the umbo; defined hy a slight depression and a change of 

 convexity. Anterior ear small, defined by a well-marked sulcus; byssal 

 sinus deep; margin convex near the extremity of the hinge-line. The area 

 of the posterior ear is more than twice that of the anterior. 



Test ornamented by about 35 rounded, radiating eost;e, with intermediate 

 smaller ones, crossed and crenulated by regular stria' of growth, of which 

 there are about 10 in the space of 5 mm. On the ears the radiating lines 

 are present, esjiecially on the anterior ear, and the concentric striae are more 

 crowded than on the body of the shell. 



The characters of the interior are not preserved. 



The largest specimen has a length of 32 mm., and the hinge-line measures 

 23 mm. A smaller specimen, preserving both valves, has a length of 29 

 mm., and height about equal. 



This species is distinguished from A. exacutus by the greater angle subtended 

 by the sides of the umbo, by its greater obliquity, and the less sharp radiating 

 costa' of the surface. In A. Phorcus the length is comparatively greater and 

 hinge-line shorter, the ears smaller, and the strong crenulating stria 1 of 

 growth do not occur. 



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

 Schoharie county ; and Hamilton, Madison county, N. Y. 



AviCULOPECTEN PliORCUS. 

 PLATE V, FIG. 2b 



Ariniltijurtrn Vhorvus, Hall. Pal. N. V., vol. v. pt. 1. Plates and Explanations: PI. 5, fiff. 2'2. Jan., 1S83. 



ShELX of medium size, obliquely ovate; height and length equal; anterior and 

 basal margins regularly rounded, becoming more convex towards the middle 



