MESOZOIO FOSSILS. 607 



Colorado and Montana formations contain several heavy beds of sandstone 

 with closely related littoral faunas. 1 It is evident that the seashore remained 

 in that region throughout nearly all of Upper Cretaceous time, giving the 

 shallow waters and sandy bottom favorable to the continuance of the littoral 

 fauna that was early established there. The Colorado formation is easily 

 recognized in these sections by the occurrence of a number of widely dis- 

 tributed characteristic species, but for some unexplained reason very few 

 of the species that characterize the Montana formation farther east and north 

 occur there. 



This phase of the Cretaceous is well developed on Hams Fork, in 

 western Wyoming, and it extends northward from there nearly to the 

 southern boundary of the Park, for it is well represented in the collection 

 from a sandstone on Glade Creek and at other localities near Snake River 

 in the same region. Fossils are abundant, but only about 20 species 

 were obtained. Judging from the fauna, the horizon is not very far from 

 that of the Colorado shales near Sickle Creek — probably a little above 

 them — and it is provisionally referred to the lower part of the Montana 

 formation. Several of the species occur at Coalville, Utah, and in south- 

 western "Wyoming, and some of them there range down into the Colorado 

 formation. 



More thorough collecting from all the Cretaceous beds exposed in the 

 Yellowstone National Park, and a little farther north and east, will probably 

 give both phases of the Upper Cretaceous faunas in one section and enable 

 us to assign these sandstones to a more definite place in the standard Upper 

 Cretaceous section. 



In the following list of species references are usually given only to 

 the first description and to publications in which the species is figured. 

 For fuller references consult Boyle's Catalogue of American Mesozoic 

 Invertebrates: Bull. U. S. Geol. Surv. No. 102. 



■See, for a fuller discussion of this subject, The Colorado formation and its invertebrate fauna: 

 Bull. U. S. Geol. Surv. No. 106, pp. 37-46. 



