80 COLORADO FORMATION AND ITS INVERTEBRATE FAUNA, [bull.106. 



nearly or quite equal, the left one, if either, the larger, both of them 

 gibbous, and sometimes 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 nearly a right angle with the hinge; front 

 margin rounded below to the basal margin, which is broadly convex lor 

 more than half the length of the shell; postero-basal margin extending 

 obliquely upward, with a slight emargination, to the posterior extrem- 

 ity, which is abruptly rounded to meet the downward-sloping postero- 

 dorsal margin ; dorsal margin straight, its length equaling more than 

 half the long diameter of the shell. Upon each valve there is an ob- 

 scure radiating depression, or ill-defined furrow extending from the 

 umbonal region to the posterobasal border, and ending there at the 

 emargination before mentioned. 



" Surface marked by the usual lines of growth, and also by numerous 

 extravagant, irregular, concentric folds or wrinkles. 



"This species belongs to a section of the comprehensive genus Inocer- 

 amus that Brongniart designated under the name Catillus. It is a 

 peculiarly well-marked species and readily distinguishable from any 

 other published species from American strata. 



"Transverse length of an average-sized specimen, 7J cm ; height from 

 base to hinge, 5 cm . 



" Position and locality. — Cretaceous strata, probably of the Fox Hills 

 group ; near Last Chance creek, southern Utah, where it was collected 

 by Mr. G. K. Gilbert," 



At the time the above description was written the author of the 

 species referred to the Fox Hills group almost all of the Coalville sec- 

 tion, a large part of which is now known to belong to the Colorado. 

 In many respects this species resembles some examples of Inoceramus 

 flaccidus, which certainly come from the Colorado formation. For 

 these reasons it is now placed with doubt in the Colorado fauna. 



Inoceramus flaccidus White. 

 PI. xiii, Fig. 1. 



Inoceramus flaccidus White, 1876, U. S. Geog. & Geol. Sur. West of tlie 100th Meridian, 

 vol. iv, p. 178, PL 16, Figs, la and 1&. 



Original description : 



" Shell large, iriegularly subovate in marginal outline, exclusive of 

 the ears, valves subequal. not much inflated; wing moderately large, 

 well defined at its inner side by an auricular furrow; hinge line not 

 very long, nearly at right angles with the front of the shell, and only a 

 little oblique with the axis ; a more or less distinct, but somewhat irregu- 

 lar farrow extending the whole length of the shell from the posterior side 

 of the umbo to the posterobasal margin, giving each valve an obscurely 

 bilobed appearance; crenulated face of the hinge narrow, crenulations 



