THE COLORADO FORMATION 87 



characteristic fossil of the Benton limestone in the eastern Colo- 

 rado area, and also speaks of its abundance in certain layers. 

 Inoceramus labiatus has been confused in certain areas with /. 

 deformis and other species of the same genus. This confusion 

 has arisen on account of its reported association with Ostrea 

 congesta. Ostrea congcsta occurs in the Benton, where it is found 

 adhering to a large nearly flat Inoceramus, but is never found 

 adhering to the much smaller species, Inoceramus labiatus. In 

 fact, it is rarely found associated with that species. Ostrea con- 

 gesta occurs also in the Niobrara. It is here found attached to 

 Inoceramus pemiatus, I. concentricus, I. platinus, Radiolites maximus 

 and other large shells. Inoceramus sp., to which the ostreae of 

 the Benton are attached, resemble Inoceramus platinus of the 

 Niobrara, but on account of the extreme brittleness of the Ben- 

 ton shell, due to its transversely fibrous structure, whole speci- 

 mens cannot be obtained for examination. 



But there is a distinction between the forms of the adhering 

 ostreae. Ostrea congesta, var. Bentonensis is a small thin subtri- 

 angular shell which, if permitted to grow uninterrupted, is almost 

 flat. The upper valves are so thin that they are rarely preserved. 

 Ostrea congesta, var. Niobraraensis is a larger, thicker shell, with 

 the lower valve more capacious, and possessing near the hinge 

 area well-marked vertical lines of muscular attachment. The 

 upper valve is also thick, and in many specimens possesses 

 adhering forms of the same species. The differences may not 

 be marked enough to be considered specific differences, but they 

 are sufficiently well developed to distinguish the two forms. It 

 is well to bear in mind then that there is a species of Inoceramus 

 in the Benton which possesses adhering forms of ostreae, and 

 that there is a similar species in the Niobrara possessing them, 

 and that therefore Ostrea congesta adhering to Inoceramus cannot 

 be taken as a criterion for either group unless the ostreae are 

 properly differentiated. 



The shale group. — Resting upon the limestone group is a bed 

 of shales called the Ostrea shales on account of the abundance 

 of that fossil in them. These shales are argillaceous, so much 



