314 Systematic Paleontology 



Description. — " Length and width nearly equal ; inferior valve ventri- 

 cose; superior valve deeply concave; radii sharp, prominent, subtuber- 

 culated, much more prominent on the upper than on the lower half of the 

 valves, where they greatly bifurcate and become fine and very numerous ; 

 umbo convex, the summit slightly elevated ; hinge angles slightly salient." 

 Conrad, 1842. 



Length 3.6 cm. ; width 3.3 cm. 



This well-known species has heretofore been reported from the Scho- 

 harie grit, Onondaga, Hamilton, and Chemung formations, and now its 

 range is extended downward into the Upper Oriskany. The four ventral 

 interiors cannot be distinguished from Hamilton specimens, but as yet 

 no dorsal valves have been seen in the Oriskany. The species nearest 

 related to S. demissa in the Lower Oriskany is S. schuchertanum, but its 

 striae are finer and somewhat bundled and the valves are decidedly more 

 arched. 



Occurrence. — Orlskany Formation, Ridgely Member. North fork 

 of south branch of the Potomac River, Pendleton County, West Virginia. 



Collection. — U. S. National Museum. 



Stropiieodonta (Leptostrophia) begkii (Hall) 

 Plate LVII, Figs. 12, 13 



Strophodonta b ckii Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 191, pi. xxii, 



figs, lo-l/, 1861. 

 Stropheodonta (Leptostrophia) iecki Hall and Clarke, 1892, ibidem, vol. vili, 



pt. 1, p. 288, pL xiii, figs. 23, 24. 



Description. — " Shell semielliptical or subquadrate : length sometimes 

 equal to the width (though usually from two-thirds to three-fourths as 

 great). Ventral valve very depressed convex: beak very small. Dorsal 

 valve flat or a little concave near the hinge, slightly convex near the front. 

 Hinge-line crenulated, generally equal to the greatest width of the shell, 

 but sometimes less. Area linear, confined to the ventral valve. Foramen 

 small, linear, usually closed. 



" Surface marked with strong, regular, closely arranged, bifurcating, 

 radiating strife, crossed by fine obscure concentric lines, and more or less 



