Maryland Geological Suuvey 305 



species. The New Scotland variety, however, does not attain the size of 

 the Becraft species, nor has it the obesity, the great shallow ventral sinus, 

 ajid other characters marking R. assimilis. 



Occurrence. — Helderberg Formation, Becraft Member. Cherry 

 Eun, abundant in the Baltimore and Ohio Eailroad cut through North 

 Mountain near its eastern end, West Virginia. 



Collections. — Maryland Geological Survey, U. S. National Museum. 



Ehipidomella musculosa (Hall) 

 Plate LV, Fig. 20; Plate LVI, Figs. 1-4 



Orthis musculosa Hall, 1857, Tenth Ann. Rept. N. Y. State Cab. Nat. Hist., 



p. 46. 

 OrtMs musculosa Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 409, pi. xci, figs. 



1-3; pi. xcv, figs. 1-7,1861. 

 RMpidomella musculosa Hall and Clarke, 1892, ibidem, vol. viii, pt. i, pp. 



190, 210, 225, pi. via, fig. 5. 



Description. — " Shell suborbicular, the length about nine-tenths as 

 great as the width : ventral valve depressed convex, sometimes slightly 

 concave near the front ; beak prominent, equalling or extending a little 

 beyond that of the opposite valve, pointed and slightly incurved: dorsal 

 valve regularly and distinctly convex, most elevated in the central region, 

 sometimes a little depressed towards the front; beak prominent, tri- 

 angular, pointed and incurved; cardinal teeth and process strong; hinge 

 extremely short ; area triangular, scarcely extending beyond the foramen ; 

 foramen large, partly occupied by the prominent cardinal process of the 

 other valve, visceral impression large, fan-shaped, and strong. Surface 

 marked by fine, distinct, radiating strife, those nearest the cardinal margin 

 being curved outwards from the beak; concentrically marked by obscure 

 lines of growth." Hall, 1857. 



Length 3 cm. ; width 3.6 cm. 



Fine examples of this species show them to be large and massive R. 

 assimilis, with the muscular scars relatively larger. Hall compared this 

 form with R. ohlata, and under R. assi7uilis the writer stated that the latter 

 probably developed from the more elongate variety of R. ohlata. This 

 seems to be the line of development, beginning in small specimens and 

 20 



