Maryland Geological Survey 303 



" Tlie vascular area of the ventral valve occupies a comparatively 

 greater space than in shells of 0. ohlala from the Helderberg, and it is 

 more elongated or triangular in form. In the dorsal valve, the double 

 imprints of the adductor muscles are well preserved." Hall, 1859. 



The forms collected in Maryland by the writer differ from those figured 

 by Hall in having the lateral margins well rounded ; giving to the shell a 

 much more circular appearance than the somewhat subtriangular outline 

 of those figured by Hall. 



Rliipidomella emarginata resembles somewhat Duhnanella quadrans and 

 Dalmanella concinna but seldom assiimes the subquadrate shape which 

 characterizes both these species. 



Length of average specimen about 1.5 cm.; width about 1.7 cm. 



Occurrence. — Helderbehg Formation, Keyser Member. Keyser, 

 near Bloomfield, West Virginia; Cash Valley, Cumberland, Maryland. 



Collections. — Maryland Geological Survej^, U. S. National Museum. 



[Maynard.] 



ElIIPIDOMELLA OBLATA (Hall) 



Plate LV, Figs. 9-lG 



Ortnis oblata Hall, 1857, Tenth Ann. Kept. N. Y. State Cab. Nat. Hist., p. 41, 



figs. 1-5. 

 OrtJiis ohlata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 162, pi. x, flgs. 1-22, 



1861. 

 Rhipidomella oblata Hall and Clarke, 1892, ibidem, vol. viii, pt. i, pp. 210, 



225, pi. via, figs. 3, 4. 



Description. — " Shell [in the young state] longitudinally subovate, 

 and varying from circular to transversely oval in its stages of growth, 

 resupinate : ventral valve convex at the beak, flattened in the middle, and 

 concave towards the front : dorsal valve very convex in the middle and 

 towards the beak ; beaks of the two valves nearly equally elevated, that of 

 the ventral valve pointed ; area very small ; foramen large. Surface finely 

 striated; stri^ frequently bifurcating and curving towards the lateral 

 and cardinal margins, concentrically marked by finer striae and stronger 

 lines of growth, which are numerous in the older shell." 



