222 Thirty-fifth Report ox the State Museum. 



an obscure carination, and an abrupt change in the surface charac- 

 ters to subdued striae. The anterior ear is well-defined by the sulcus 

 extending from the angular byssal sinus. 



Test ornamented by about forty strong, sharp, continuous rays, 

 alternating in size with broader and concave interspaces, crossed by 

 fine, sharp, crenulating concentric striae. The ears show finer con- 

 centric striae and a few rays. Pallial line impressed, continuous, ex- 

 tending parallel to the margin of the shell about half way from the 

 beak, terminating near the center of the posterior side, in a sub- 

 circular, muscular impression, marked with regular concentric striae. 



The largest specimen has a height of 35 mm. A medium-sized 

 specimen has a height of 26 mm.; hinge-line 24 mm. 



This species differs from the young of A. princeps, in its more acute 

 beak, smaller anterior ears, and in having rays on both ears ; the rays on 

 the body of the shell are also stronger. It resembles A. formio in 

 outline, but has more sharply elevated rays, and less strong concentric 

 striae. A. pecteniformis has obtuse rounded beaks, and ears not marked 

 by rays, which are distinguishing features of that species. 



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

 Monteith's Point, Ontario county, and Hamburg, Erie county, N. Y. 



AviCULOPECTEST FORMIO. 



A'oiculopecten formio, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: 

 PI. 5, figs. 20, 21. Jan., 1883. 



Shell of medium size, obliquely ovate, extended behind ; length 

 equal to or somewhat less than the height. Valves depressed, 

 slightly convex. The right and left valves are very similar; the 

 right being somewhat less convex, and having finer surface markings. 

 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 ora- 

 mentation differing from that on the umbo ; defined by 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 costae, with inter- 

 mediate smaller ones, crossed and crenulated by regular striae of 

 growth, of which there are about 10 in the space of 5 mm. On the 

 ears the radiating lines are present, especially on the anterior ear, 



