Maryland Geological Survey 167 



valve gibbous towards the umbo, depressed convex in the center and 

 flattened towards the front; tlie front margin straight, or without sinus; 

 delthyrium very broad, nearly twice as wide as high. Dorsal valve the 

 more gibbous, the greatest convexity above the middle, longitudinally 

 marked by a median depression which is sometimes obsolete; the interior 

 shows a prominent cardinal process, which is continued in a strong median 

 ridge for about one-half the length of the valve. Surface marked by fine, 

 radiating, bifurcating striae, which are crossed by finer concentric ones, 

 and by more distant subimbricating lamellose lines of growth; minute 

 tubular openings occur on the surface of the striae; the external striae 

 usually mark the inner margins of the valves. 



Hall noted the close agreement of this species with R. vanuxemi, but 

 stated that it differed in being more ovate in shape, the cardinal extrem- 

 ities are less rounded, the sides slope almost directly nearly to the middle 

 of the shell and the dorsal valve is more gibbous. He referred speci- 

 mens from Cumberland, Md., to this species and figured an internal 

 impression of a ventral valve which was stated to be " probably of this 

 species." ' Comparatively few specimens of this species were found in 

 Maryland; but there are a few impressions which show at least a part of 

 its distinctive characters and apparently ought to be referred to this 

 species. As already stated the species closely resembles R. vanuxemi but 

 these impressions differ from that species in their more ovate outline, 

 greater convexity of valves, and more pointed posterior end. 



Length, 32 mm. ; greatest width, 32 mm. ; width on hinge-line, 9 mm. 



Occurrence. — Eomney Formation, Hamilton Member. Williams 

 Road, f mile east of Queen City Hotel, Cumberland. 



Collections. — Maryland Geological Survey; New York State Museum; 

 American Museum of Natural History. 



EhIPIDOMELLA PENELOPE Hall 



Plate XIV, Figs. 6-9 



Orthis penelope Hall, 18G0, Thirteenth Rep. N. Y. State Cab. Nat. Hist., p. 79, 



figs. 1, 2. 

 Orthis penelope Hall, 1867, Pal. N. Y., vol. iv, p. 50, pi. vl, figs. 2a-2m. 



' Pal. N. Y., vol. iv, pi. vii, fig. 4i. 



