LAMELLIBRANCHIATA. 21 



d'Orbigny), the valve i.s more convex, beak more obtuse, and the concentric 

 marking forms a prominent feature. 



Formation and locality. In the Chemung group at Conewango, Cattaraugus 

 county, N. Y. 



AviCULOPECTEN PLENUS. 



PLATE XXIV, FIG. 3. 



Avk-uhqiecleu pJenua. Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explanations: PI. 24, tig-. 3. Jan., 1883. 



Shell sub-orbicular, sub-equilateral ; transverse axis nearly vertical to the 

 hinge-line ; height one-sixth less than the length, giving a marked fullness 

 to the outline of the sides. 



Left valve convex, the convexity equal to one-sixth the length of the 

 shell. The point of greatest convexity distant one-third the length from 

 the beak. Right valve not <)l)served. 



Hinge-line straight, nearly central, the length more than two-thirds the 

 length of the shell. 



Beak obtuse, situated centrally with regard to the hinge-line, and also to 

 the body of the shell ; umbo rounded. 



Ears sub-equal, triangular. Posterior ear mucronate at the extremity ; 

 margin deeply concave. Anterior ear acute at the extremity, and the margins 

 convex; separated from the uml)o by an abrui^tly sloping sulcus extending 

 to the beak. 



Surface ornamented with about 130 regular, rounded, close, filiform radii, 

 crossed by very fine, sharp, concentric striae. The surface markings are 

 jiresent on the ears, the radii distinct and stronger than on the adjacent 

 portions of the shell. The rays become more frequent and finer from the 

 basal margin in both directions towards the ears. 



Interior not preserved in the specimen described. 



The height of the specimen is 25 mm., length 30 nun., hinge-line 21 mm. 

 On the lower margin there are 9 rays in the space of 5 mm. 



