Lamellibranchiata. 251 



LYRIOPECTEtf CYMBALOK. 



Lyriopecten cymbalon, Hall. Pal. N. Y., vol. v, pt. 5. Plates and Explana- 

 tions: PI. 24, fig. 8. Jan., 1883. 



Shell large, sub-orbicular, not oblique ; height a little less than the 

 longitudinal diameter ; pallial margin regularly rounded, upper 

 anterior margin more inflated than the posterior, which is trun- 

 cated toward the beak. Left valve regularly convex; the basal 

 margin slightly inflected, giving the valve an undue convexity. 

 Eight 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 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 by 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, con- 

 centric 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 striae and 

 undulations of growth. The direction and character of the rays 

 in the specimens described have been affected, apparently, by some 

 injury received during the life of the animal, which has also pro- 

 duced a strong varix of growth. Ligamental area 1.5 mm. 

 wide at the beak, whence it diminishes toward the extremities of 

 the hinge-line, marked by several flat, longitudinal striae. Cartilage- 

 pit broad-triangular, situated under the apex. 



The specimen described is 75 mm. in height, 88 mm. in longitu- 

 dinal diameter, and hinge-line 46 mm. The distance between the 

 centers of two of the large rays at the basal margin is about 17 

 mm. 



The surface markings resemble L. tricostatus, but they are stronger, 

 more rugose, and the rays less numerous ; also the shell is not oblique, 

 and the beak is more obtuse and rounded. It differs from L. macro- 

 dontus and L, magnificus in its greater convexity, deeper byssal sinus, 

 larger posterior ear, and by its surface characters. This remarkably 

 large and fine species shows conspicuously the diminished anterior 



