142 Systematic Paleontology — Middle Devonian 



Leptaena depressa Hall, 1852, Pal. N. Y., vol. ii, p. 62, pi. xxi, fig. 8; p. 257, 



pi. liii, fig. 6. 

 Leptaena depressa Rogprs, 185S, Geol. Penna., vol. ii, pt. ii, p. 823, fig. 630. 

 Strophomena rugosa Hall, 1859, Pal. N. Y., p. 195, pi. xix, fig. 1. 

 Strophomena rhomboidalis Billings, 1861, Canadian Jour., vol. vi, p. 336, 



flgs. Ill, 112. 

 Strophomena rhomboidalis Hall. 1867, Pal. N. Y., vol. iv, p. 76, pi. xii, figs. 



16-18; p. 414, pi. XV, figs. 15, 16. 

 Leptaena rhomhoidalis Hall and Clarke, 1892, Pal. N. Y., vol. viii, pt. i, p. 



279, pi. viii, figs. 17-31; pi. xv A, flgs. 40-42; pi. xx, figs. 21-24. 

 Leptaena rhomhoidalis Schuchert, 1897, Bull. U. S. Geol. Surv., No. 87, p. 240. 

 Leptaena rhomhoidalis Clarke, 1903, N. Y. State Mus., Bull. 65, p. 239. 

 Leptaena rhomhoidalis Grabau and Shimer, 1907, N. Am. Index Fossils, vol. 



ii, p. 226, figs. 273a, b. 



Description. — Shell of more than medium size, semielliptical or sub- 

 quadrate varyiBg in its proportions of length and breadth; hinge-line 

 straight and equal to greatest width of shell ; cardinal extremities mostly 

 rectangular, sometimes rounded and sometimes salient; the valves are 

 geniculated and the proportions of the flattened part or disc and the 

 recurved portion are very variable. The surface of the flattened part is 

 marked by strong concentric, undulating elevations, which are parallel 

 with the curve of geniculation and are bent outwards and often become 

 obsolete on the cardinal angles; these elevations are most prominent on 

 the part parallel to the front maj'gin ; they are very variable in number, 

 ranging from six to sixteen. The entire surface is covered by radiating, 

 fairly coarse, uniform striae. The ventral valve is slightly convex near 

 the umbo, but flat or even somewhat depressed between the umbo and 

 geniculation; dorsal valve generally corresponding in its concavity with 

 the convexity of the ventral, but differing in its depth. 



The Maryland specimen consists of a single fragment of a ventral 

 valve; but the strong concentric wrinkles and even striae leave no doubt 

 as to the correctness of this identification. The species may be readily 

 determined by its shape, the deep concentric wrinkling of the disc, the 

 abrupt geniculation of both valves toward the dorsal side, and the uni- 

 form, sharply marked striae. 



Length of average specimen from the Columbus limestone at Colum- 

 bus, Ohio, 22 mm.; width, 32 mm. 



