LAMELLIBRANCHIATA. 17 



.Compared with /.. magmjicus, this species has greater convexity, less elevated 

 radii, and, in general, a comparatively greater longitudinal diameter. It differs 

 from L. cymbalon and L. tricostatus in Burface ornamentation. 



This species, described from only the left valve, is nevertheless so distinctly 

 unlike any others here described that it is readily recognized. 



Formation and localities. In the coarse beds of the Hamilton group, at Ham- 

 ilton, Madison county; and Worcester, Otsego county, N. Y. 



Ltbiopecten cymb llon. 



PLATE \\l\ . lit . 8. 

 hyriupeeten tymbalon, Hall. Pal. N. v.. vol. v, pt. ">. Plates and Explanations: PI. 34, fig. 8. Jan., 1888. 



Shell large, sub-orbicular, nol oblique; height a little less than the Longitudinal 



diameter; pallial margin regularly rounded, upper anterior margin more 

 inflated than the posterior, which is truncated toward the beak. 



Left valve regularly convex; the basal margin slightly inflected, giving 

 the valve an undue convexity. Right valve not known. 



Hinge-line straight, length a little greater than one-half the Longitudinal 

 diameter of the shell, and extending about one-third of its length farther to 

 the posterior than to the anterior margin. 



Beak obtuse, rounded, straight, central : umhonal region defined by its 

 abrupt antero- and post-cardinal margins, and by its convexity. 



Anterior ear very small, narrow-triangular; margin concave; extremity 

 obtuse-angular; byssal notch broad and shallow, not conspicuous. Posterior 

 ear large, triangular, marked bj a deep sinus; margin convex: extremity 

 angular; length twice as great as the anterior ear. 



Test marked by about twelve very strong, continuous, broad, rounded 

 rays, with from one to six smaller ones of variable strength in each of the 

 flat interspaces; crossed by line, sharp, regular, concentric striae n\' growth,. 

 which are elevated into lamellar ridges at irregular intervals. The ears and 

 a broad adjacent space on the body of the valve are destitute a\' rays, but 



show strong Striae and undulations of growth. The direction and character 



