Logan. J The Invertebrates, Benton Group. 453 



the greater convexity of the valves, and other evidences of 

 mature growth." 



This fossil has been found in the Septaria horizon only, and 

 there only in a fragmentary condition. Complete specimens 

 are difficult to obtain, on account of the extreme friable condi- 

 tion of the Septaria nodules in which they occur. The fossil is 

 found associated with I. tenuirostratus and I. fragilis, in the 

 septaria on the Saline south of Plainville. Fragments have 

 also been found in the septaria of Oak creek, in Smith county. 



iHoceranitis gilbert ii White. Plate xcn, figs. 1, 3. 



Innceramus qilbertii White, 1876, Geol. Uinta Mts., p. 113; 1879, 11th Ann. 

 Rep. U. S. "Geol. Surv. Terr., p. 285, pi. in, ff. 3 and 4; Stanton, 1892, Bull. 

 U. S. Geol. Surv. No. 106, p. 79, pi. xiv, ff. 3 and 4. 



Description: "Shell irregularly subovate in marginal out- 

 line, the transverse diameter being greater than the vertical; 

 front more or less flattened ; valves nearly or quite equal, the 

 left one, if either, the larger, both of these gibbous, and some- 

 time quite ventricose ; umbones broad and elevated ; beaks 

 very near the front, incurved, but not projecting beyond the 

 front margin ; front nearly straight vertically, or sometimes 

 more rounded, in the former case forming a nearly right angle 

 with the hinge ; front margin rounded below to the basal mar- 

 gin, which is broadly convex for more than half the length of 

 the shell ; postero-basal margin extended obliquely upward, 

 with a slight emargination, to the posterior extremity, which 

 is abruptly rounded to meet the downward-sloping postero-dor- 

 sal margin ; dorsal margin straight, its length equaling more 

 than half the long diameter of the shell. Upon each valve 

 there is an obscure radiating depression or ill-defined furrow 

 extending from the umbonal region to the postero-basal border, 

 and ending there at the emargination before mentioned. Sur- 

 face marked by the usual lines of growth, and also by numerous, 

 extravagant, irregular folds or wrinkles. Length, seventy-five 

 mm. ; height, fifty mm." 



Inoceramus (jilbertii occurs not only in the Septaria horizon 

 but also in the Fort Benton limestone group. 



