Lamellibranchiata. 223 



and the concentric striae are more crowded than on the body of the 

 shell. 



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 costae of the surface. In A. Phorcus the length 

 is comparatively greater and hinge-line shorter, the ears smaller, and 

 the strong crenulating striae 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 Phorcus. 



Aviculopecten Phorcus, Hall, Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 

 PI. 5, fig. 22. Jan., 1883. 



Shell of medium size, obliquely ovate ; height and length equal ; 

 anterior and basal margins regularly rounded, becoming more con- 

 vex toward the middle of the posterior side, and thence concave to 

 the beaks, giving the shell an appearance of considerable amplitude 

 behind. Left valve regularly and moderately convex. The right 

 valve is unknown. Hinge-line straight, about one-half the length 

 of the shell. Beak rounded, extending to and situated near the 

 center of the hinge-line ; umbonal angle smaller than usual, being 

 about 90°. Ears comparatively small, triangular, with concave 

 margins ; posterior ear the larger, separated from the body of the 

 shell by a difference in convexity and the absence of radiating lines; 

 anterior ear defined by a sulcus. 



Test marked by about 60 fine, rounded, radiating costae, with a 

 few intermediate smaller ones, and fine striae of growth. The rays 

 are absent" on the ears, but the lines of growth are more distinct 

 than on the body of the shell. 



The specimen described has a height of 37 mm., and a greatest 

 length of 26 mm. 



This species differs from A. formio by its smaller ears, their distinct 

 character, and the general surface markings. It resembles young in- 

 dividuals of A. princeps, but the comparative size and extent of the 

 ears are very different, while the body of the shell is more constricted 

 at the base of the ears. A. fasciculatus is very similar in the general 

 form of the body of the shell ; but the length is greater than the 



