MARYLAND GEOLOGICAL SURVEY 237 



Occurrence. MARTINSBURG SHALE (Eden division). Molds and casts 

 showing the specific characters quite plainly are found at Jordans Knob 

 one and one-half miles northeast of Fort London, Cowans Gap, five miles 

 northeast McConnellsburg, and Tuscarora Mountain two and one-half 

 miles southeast McConnellsburg, Pennsylvania. In Maryland the same 

 horizon on Eickard Mountain, Washington County, has afforded 

 specimens. Not uncommon in the Eden and Maysville strata of the 

 Ohio Valley. 



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



Order PROTREMATA 

 Superfamily ORTHACEA 

 Family BILLINGSELLIDAE 



Genus EOORTHIS Walcott 



EOORTHIS DESMOPLEURA (Meek) 



Plate XXVII, Figs. 1-5 



Orthis desmopleura Meek, 1872, Hayden's U. S. Geol. Surv. Wyoming, p. 295. 

 Orthis (Plectorthis) desmopleura Walcott, 1905, Proc. U. S. Nat. Mus., 



vol. xxviii, p. 261. 

 Eoorthis desmopleura Walcott, 1912, Mon. U. S. Geol. Surv., vol. li, p. 777, 



pi. xcvi, figs. 1, la-r. 



Description. " This shell has the general form and external characters 

 of E. wichitaensis (Walcott). It differs in being less convex and in the 

 details of the radiating ribs. Eoorthis desmopleura differs from E. rem- 

 nicha (N. H. Winchell) in its uniformly smaller size, less convexity, and 

 in the details of the radiating ribs. The ribs have a wide range of varia- 

 tion, but when from the same character of matrix they are all of the same 

 type and the shells grade from one to the other. The ventral valves of 

 young shells 2 to 3 mm. long are highly convex and usually appear to be 

 a little longer than wide ; if in such shells the surface striae are in sharply 

 elevated fasiculae, the result is to all appearances a rhynchonelloid 

 shell. 



