Lamellibkanchiata. 249 



shell. Beak of left valve obtuse, not prominent, directed forward, 

 situated about one-third the length of the hinge-line from the ex- 

 tremity of the anterior ear. In the right valve the umbo is often 

 flat, and the angle subtended by its sides is acute, caused by the 

 deep byssal sinus. Posterior ears narrow-triangular, distinguished 

 from the gradually sloping sides of the umbo by their flatness and 

 finer markings ; margins concave ; extremities mucronate. Ante- 

 rior ear of the left valve smaller, triangular ; margins concave ; 

 extremity rectangular. Anterior ear of the right valve convex, 

 triangular, truncated at the extremity ; separated from the umbo by 

 a deep angular sulcus, and a broad longitudinal semi-elliptical byssal 

 sinus, which forms an indentation in the lateral outline of the body 

 of the shell. 



Test of the left valve marked by about 40 strong rounded rays 

 with smaller intermediate implanted rays, crossed by fine, regular, 

 elevated sharp lines of growth, which at intervals are raised into 

 lamellae, and appear in the cast as strong transverse ridges crossing 

 the radii. The furrows between the radii are narrower than the 

 rays. The regular alternation of the radiating lines where seen is 

 a very conspicuous feature. The right valve is marked by more 

 numerous and subdued rays, and their alternation is somewhat 

 obscure. The ears show fine radiating lines and stronger and more 

 crowded concentric striae. 



A medium-sized individual has a height of 47 mm., longitudinal 

 diameter 54 mm., hinge-line 32 mm. A smaller specimen measures 

 in height 35 mm., length 38 mm., hinge-line 24 mm. The largest 

 specimen (imperfect) shows a height of 74 mm., and a length of 

 88 mm. 



In outline and general aspect this species closely resembles L. 

 macrodontus, but differs in the obliquity of the beak, wider anterior 

 ears, and more elevated and numerous rays. It differs from L. mag- 

 niftcus in its outline, larger anterior ears, longer hinge-line, and finer 

 radii of the ears. 



The specimens mostly preserve both valves in connection. A large 

 specimen occurring in the harder argillaceous sandstone attains nearly 

 the dimensions of L. cymhalon but differs conspicuously in the more 

 numerous, strong radii, their gentle curvature toward the anterior 

 side, and the smaller area of surface upon the anterior side of the 

 vertical axis of the valve. 



Formation and locality. In the shales of the Hamilton group, 

 Fultonham, Schoharie county, N. Y. 



[Sen. Doc. No. 38.] 32 



