678 GEOLOGY OF OHIO. , 



pecially in the posterior half, not so convex, and has'less anterior beaks 

 and 'different surface markings. 



Formation and Locality, Middle beds of the Cincinnati group, at 

 Cincinnati, Ohio, and Covington, Kentucky, where it occurs at an alti- 

 tude of about 350. feet above the bed of the Ohio River. 



Genus WHITELL-A, Ulrich. 



Whitella, Ulrich, 1890, Amer. Geol., vol. vi,p. 176 ; Geol. and Nat. Hist. Surv. Minn., 

 Final Rept, vol. iii, p. 564 (in press). 



The reader may obtain a good' idea of this well marked Lower Silu- 

 rian genus from the Minnesota work above cited. Descriptions of twelve 

 species, nine of them Trenton, the rest from the Hudson River group, 

 are contained therein, 



Wliitella O/iioensis, n. sp: or var. 

 Compare Whitella compressa Ulrich, 1890, Amer. Geol., vol. vi, p. 180. 



Fig. 2. Right valve of Whitella Ohioensis ; Cincinnati group, Waynesville, Ohio. 



Shell large, compressed convex, subrhomboidal in outline, very lit- 

 tle the widest posteriorly. Anterior margin very gently rounded and 

 nearly vertical in the upper half, sharply rounded (almost angular) at the 

 extremity of the hinge, sloping backward below ; the outline from the 

 prominent and strongly rounded post-basal angle to the antero-cardinal 

 angle forms nearly a semielliptical curve; posterior margin slightly 

 oblique, nearly straight in the middle, above curving forward and' rather- 

 gradually merging into the dorsal line. Beaks very prominent but small 

 and not strongly incurved, situated about one-fifth of the length of the 

 shell behind the anterior extremity ; umbonal ridge very little developed, 

 especially as compared with the majority of the species of the genus. 

 Surface with distant subimbricating marks of growth and finer conceu- 



