82 INVERTEBRATE PALEONTOLOGY. [WHITE. 



the other periods represented in the collections. The entire absence of all 

 articulate and molluscoidean species from all the Cretaceous collections is 

 quite remarkable. All four of the groups of strata are represented in the 

 collections, those from the Henry's Fork Group containing the smallest num- 

 ber of species. 



The groups as a rule appear to be separated by good paleontological 

 as well as physical characteristics. Those of the Henry's Fork and Sulphur 

 Creek Groups, however, are similar, and the dividing line between the Salt 

 Wells and Point of Rocks Groups appears to be indistinct as regards generic 

 and family types; but, with very few exceptions, the species have not been 

 found to pass from one group to another. The exceptions thus far noticed 

 are those of species that have* a very wide geographical as well as an un- 

 usually great vertical range. 



The Henry's Fork and Sulphur Creek Groups, so far as they have been 

 examined, appear to have been wholly open-sea deposits, no genera of 

 brackish water habitat, except Ostrea and Anomia, which are also open-sea 

 forms, having been discovered in any of the strata. The Sulphur Creek 

 Group is also remarkable for containing nearly all the Cretaceous Cephalo- 

 poda of the collections; the only exceptions being a specimen of Baculites 

 ovatus Say, from the' Salt Wells Group in the valley of Red Creek, Utah, 

 and species of Scaphites from the same group two miles northwestward from 

 Salt Wells Station, Wyoming. 



The Salt Wells Group is riot only remarkable for its paucity of Ceph- 

 alopods, but also in consequence of the fact that from among its strata we 

 obtain the first Cretaceous fresh- water forms. Even among the fossils of 

 some of its more distinctively marine strata, we obtain such forms as are 

 usually found in the brackish and fresh water Cretaceous and Tertiary de- 

 posits of that region. The earliest of the fresh or brackish water Cretaceous 

 deposits that have been discovered in the Plateau Province occurs among 

 strata of this group near Coalville, Utah, a fortunate exposure of which 

 afforded Mr. Meek, a few years ago, species of the genera J7nio, Cyrena, 

 Phym, Nerit'ma, &c., all having a remarkably modern aspect; but the strata 

 which contained them are immediately overlaid by those which contain 

 Inoccramus, Grypliea, Anchura, and other distinctively Mesozoic forms. 



