Lamellibranchiata. 231 



convex, beak more obtuse, and the concentric marking forms a 

 prominent feature. 



Formation and locality. In the Chemung group at Oonewango, 

 Cattaraugus county, N. Y. 



AviCULOPECTEH PLEXUS. 



Aviculopecten plenns, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions: PI. 24, fig. 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. 

 Eight valve not observed. 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 umbo by an abruptly 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 mark- 

 ings are present 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 toward the ears. 



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

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



In form, this species bears considerable resemblance to A. duplicatus; 

 but the surface markings are quite different, and the extremities of 

 the hinge-line are more acute in this species. In A. scdbridus the 

 beak is not so well-defined, the ears usually more extended, and the 

 coarser rugose radii o.f that species are characteristic. This form 

 resembles some varieties of A. fasciculatus in the surface characters, 

 but differs in the obliquity of the shell and form of the ears. It 

 differs from A. Idas in the more rotund form, more numerous and 

 elevated rays, and larger ears. The single valve described appears to 

 be well distinguished from all the other species, and possesses charac- 

 ters too remote to be united with any of those described from right 

 valves alone. 



Formation and locality. In arenaceous shales of the middle Che- ' 

 mung group, near Elmira, N. Y. 



