llKl.DKKHKKd K( »!! M Al' ION IN MaKVLAM) 5 



elevated above the hinge-line, and neatly incurved over the area. Area 

 comparatively large, the length greater than half the width of the shell. 

 Surface very finely and evenly striated. This species bears some resem- 

 blance to 0. [DalmaneUa] elegantula of the Niagara group; but it is more 

 finely and beautifully striated, the dorsal valve is more gibbous, .... 

 and the beak less arched." Hall, 1859. 



Certain individuals referred to this species are much more circular in 

 outline than the forms described by Hall, the cardinal angles being less 

 extended and the valves less elongate. 



Concerning this species Schuchert remarks that it " is easily confounded 

 with DalmaneUa perelegans, but it is always smaller, more obese, and 

 with finer strise. Hall states that these specimens are from the " shaly 

 limestone of Cumberland, but is in error, since all those now found are 

 from the Keyser member, and none have been seen in the iSTew Scotland 

 member." 



Occurrence. — HELDERBEna Formatiox, Keyser Member. Keyser, 

 West Virginia; Cash Valley, Devil's Backbone, Market Street Bridge, 

 Cumberland, section southwest of Eawlings, Pinto, Viaduct Cumberland, 

 Tonoloway, ]\[aryland ; Pleasant Valley, Bedford County, and Hyndman, 

 Pennsylvania. 



Collection. — Maryland Geological Survey. 



Genus RHIPIDOMELLA CEhlert' 

 Enil'IDOMELLA E3IARGINATA (Hall) 



Plate LV, Figs. 1-8 



Orthis oblata var. cviarginata Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, p. 

 164, pi. xa, figs. 4-6, 1861. 



Vescripiion. — " Among the collections from the Lower Helderberg 

 rocks of Cumberland, Maryland, there are numerous specimens of an 

 Orthis, having the same general characters as 0. ohlata, but proportionally 

 narrower and the beak more extended. The dorsal valve is very convex, and 

 the ventral valve is marked by an undefined depression, which, beginning 



' See Hall and Clarke, 1892, Nat. Hist. N. Y., Pal., vol. viii, pt. i, p. 209. 



