250 Thirty-fifth Report on the State Museum. 



LiYRIOPEOTEtf MACRODONTUS. 



Lyriopecten macrodontus, Hall, S. A. Miller. Cat. Pal. Foss. Cincinnati. 

 1877. 



Lyriopecten macrodontus, Hall. Pal. N. Y., vol. v, pt. 1. Plates and Explana- 

 tions : PI. 4, fig. 9; pi. 8, figs. 9, 10. Jan., 1883. 



Shell large, sub-circular ; in some large specimens becoming trans- 

 versely broad-ovate, from the posterior extension of the valve; 

 height from one-sixth to one-eighth less than the length; margins 

 regularly rounded, more convex at the posterior lateral margin, and 

 extending in nearly a straight line to the beak. Left valve de- 

 pressed, moderately convex. Right valve unknown. Hinge-line 

 straight, nearly central, varying in length with the age of the shell. 

 Beak obtuse, rounded, directed slightly toward the anterior, not 

 rising above the hinge. Anterior ear small, about one-third the 

 length of the posterior, triangular, not well-defined; margin 

 straight or slightly concave; extremity obtuse. Posterior ear large, 

 triangular, defined by the absence of the strong radii, and by a shal- 

 low, undefined depression reaching to the beak; margin deeply 

 sinuate ; extremity acuminate. 



Surface marked by from 30 to 40 broad, rounded radii, with 

 smaller intermediate rays, crossed by fine striae of growth. Some 

 fragments of shell, adhering to the internal mould, have a thick- 

 ness of one millimeter. Muscular impression large, sub-circular, 

 concentrically striated, situated posterior to, and below theumbonal 

 region. Cartilage-pot very small, triangular beneath the apex of 

 the beak, not extending to the hinge-margin. Ligamental area 

 usually marked by five strong, elevated ridges, which curve outward 

 at the beak diverging from, and partially inclosing the cartilage - 

 pit, and continuing toward the extremities of the ears; the inner- 

 most ridges disappear before reaching the extremity of the area. 



The largest specimen is 85 mm. high, 98 mm. long, and the 

 hinge-line 50 mm. Another specimen, somewhat different in pro- 

 portions, is 68 mm. in height, 75 mm. long, with hinge-line of 

 40 mm. 



Compared with L. magnificus, this species has greater convexity, 

 less elevated radii, and, in general, a comparatively greater longitu- 

 dinal diameter. It differs from L. eymbalon 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 Hamilton, Madison county, and Worcester, Otsego county, N. Y. 



