IIi:i.Di;iMJEl!G FORIMATION IN ]\[aRYLAND 7 



the extremities of the hinge-line; the foramen is broad and spreading, but 

 filled by a callosity of the ventral valve, which has a narrow groove at its 

 summit for the protrusion of the pedicle ; the apex of the dorsal valve is 

 often, and perhaps always, perforated. The flatter portions of both valves 

 are strongly marked by concentric undulations, which are crossed by fine 

 strige. On the deflected portions there are no undulations, the striee alone 

 marking the surface. Sometimes the shell is nearly flat, the deflected 

 portion being either very narrow, or not at all conspicuous. The un- 

 dulations are variable in number, even in shells of the same size, and are 

 not to be relied upon as characteristic ; and in very old shells they are not 

 so strong as in younger ones, or those of medium size. The striae crossing 

 the undulations are likewise variously prominent in difi^erent individuals, 

 frequently bifurcating, and in well-preserved surfaces are crossed by fine 

 concentric striae. The interior structure is always peculiar and suSiciently 

 characteristic, though the exterior characters are very closely simulated by 

 a different shell in the shaly limestone of the Helderberg. 



" This species has a wide range, occurring in the Clinton group and 

 ranging to the Upper Helderberg limestones." Hall, 1852. 



This species is found also in the Clinton and Oriskany of Maryland. 



Occurrence. — Helderbeeg Formatiox, Keyser IMe^mber. Devil's 

 Backbone, Corriganville, Cash Valley, Pinto, Tonoloway, Maryland; 

 Hyndman, Pennsylvania. Coeymans Member. Dawson, Corriganville, 

 Devil's Backbone. New Scotland Member. 21st Bridge, Dawson, 

 Corriganville, Miller's Spring, Ernstville, Maryland; Cherry Eun, West 

 Virginia. Becraft Member. AVarren Point, Pennsylvania. Oriskany 

 Formation, Eidgely Member. Miller's Spring, near Cumberland. 



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



Genus STROPHEODONTA Hall 



Stropheodonta varistriata (Conrad) 

 Plate LIX, Figs. 1, 2 



Strophoviena varistriata Conrad, 1842, Jour. Acad. Nat. Sci. Phila., vol. viii, 



p. 255, pi. xiv, fig. G. 

 Strophodonta varistriata Hall, 1859, Pal. N. Y., vol. iii, p. 180, pi. viii, figs. 



1-16; pi. xvi, figs. 1-8, 1861. 

 StropJieodonta varistriata Schuchert, 1897, Bull. U. S. Geol. Survey, No. 87, 



p. 427. 



