Maryland Geological Survey 309 



striae. On the deflected portions there are no undulations, the strise 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 =;hells 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 strige crossing 

 the undulations are likewise variously prominent in different individuals, 

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

 concentric striae. The interior structure is always peculiar and sufficiently 

 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. — Helderberg Formation. Keyser Mejiber. Devil's 

 Backbone, Corriganville, Cash Yalley, 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 Kun, West 

 Virginia. Becraft Member. Warren Point, Pennsylvania. Oriskany 

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



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



[Maynard.] 



Lert.ena rhomboidalis var. ventricosa (Hall) 

 Plate LVI, Fig. 18; Plate LVII, Fig. 1 



Strophomena depressa var. ventricosa Hall, 1857, Tenth Ann. Rept. N. Y. 



State Cab. Nat. Hist., p. 55. 

 Strophomena rugosa var. ventricosa Hall, 1859, Nat. Hist. N. Y., Pal., vol. iii, 



p. 417, pi. xciv, figs. 2e, 2/, 3. 

 LepUrna rhomboidalis var. ventricosa Hall and Clarke, 1892, ibidem, vol. viii, 



pt. i, pi. xva, fig. 43. 



Description. — " Shell transversely oblong, subsemicircular; length and 

 breadth sometimes nearly equal ; front often straight in the middle, and 

 parallel to the hinge-line; ventral valve extremely ventricose, scarcely 



