LAMELLIBRANCHIATA. 47 



Compared with L. magnificus, 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 surface 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. 



Lyriopecten cymbalon. 



PLATE 'SXIV, FIG. S. 

 Lyi-itypecten i-ymbaloit. Hall. Pal. N. Y., vol. v, pt. .5. Plutes an<l Explanations: PI. 24, tig-. 8. Jan., 1883. 



Shell large, sub-orbicular, not oljlique ; 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 ; umbonal region defined by its 

 al^rupt antero- and post-cardinal margins, and hy its convexity. 



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

 obtuse-angular ; l>yssal notch broad and shallow, not conspicuous. Posterior 

 ear large, triangular, marked ))y 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 fine, sharp, regular, concentric striae of 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 of rays, but 

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



