LAMELLIBRANCHIATA. 9 



dued stride. The anterior car 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, crenu- 

 lating concentric stria?. The ears show finer concentric stria? and a few rays. 



Pallial line impressed, continuous, extending parallel to the margin of the 

 shell about half way from the beak, terminating near the centre of the 

 posterior side, in a sub-circular, muscular impression, marked with regular 

 concentric stria?. The cast preserves traces of the exterior markings, but 

 presents no definite characters of the hinge. 



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

 has a height of 26 mm. ; length 27 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 stria?. A. pecieniformis 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. 



AviCCLOPECTEN FORMIO. 



PLATE V, FIGS. 'JO, U. 



Aviculopecten formiu, Hall. Pal. N. Y., vol. v. pt. 1. Plates and Explanations : PI. 5, tigs. 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 somew r hat less convex, and having finer surface 

 markings. 



