240 Thirty-fifth Eeport on the State Museum. 



Test thin, marked by fine sharp radii, with wider interspaces, 

 which show one, two or three finer rays, crossed by fine crenulating 

 lines of growth. The same characters of marking extend over the 

 ears. 



The specimen is 14 mm. in length, 12 mm. in height, with hinge- 

 line 15 mm. 



This species, in surface markings, bears a slight resemblance to a 

 young individual of Aviculopecten princeps ; but the undefined ears 

 and proportionally longer hinge-line distinguish it. 



Formation and locality. In the upper layers of the Corniferous 

 limestone, Young's farm, Williamsville, Erie county, N. Y. 



Section I. 

 Aviculopecten ignotus. 



Aviculopecten ignotus, Hall. Pal. N. T., vol. v, pt. 1. Plates and Explana- 

 tions: PI. 1, fig. 2. Jan., 1883. 



Shell of medium size, transversely and obliquely sub-ovate; height 

 about five-sixths of the length; margins regularly rounded, full on 

 the anterior and posterior sides. Left valve slightly convex, nearly 

 flat, Right valve unknown. Hinge-line straight, central, length 

 nearly equal to the height of the valve. Beak obtuse, rounded, 

 low, directed toward the anterior. Posterior ear large, broad-trian- 

 gular, separated from the side of the umbo by a wide shallow sulcus ; 

 margin slightly concave; extremity obtuse-angled. Anterior ear 

 less than one-half the width of the posterior, narrow -triangular, 

 separated from the umbo by a deep byssal sinus and a distinct 

 sulcus, extending to the apex of the beak; margin convex; ex- 

 tremity rounded. 



Test thin, about .5 m. in thickness; marked with about twenty- 

 five irregular, rounded radii, which are crossed by very fine con- 

 centric striae, and occasionally interrupted by varices of growth. 

 The rays are conspicuous over the center of the valve, becoming 

 obscure toward the margins. The ears show the concentric strise 

 only. 



A specimen of the left valve measures 25 mm. in height, 29 mm. 

 in length, and the hinge-line 21 mm. 



This species in general expression resembles A. tenuis, but is dis- 

 tinct in its smaller anterior ear, more obtuse beak, orbicular form, 

 and more irregular and stronger rays. Their geological positions are 

 widely separated. 



