"CERATOPS beds" OF WYOMING AND MONTANA 285 



but did they do so ? The fact that ammonites, Inocerami, rudistae, 

 and many other groups of invertebrates have no descendants in the 

 Tertiary does not disqualify them as characteristic fossils of the Meso- 

 zoic, neither does the fact that the Cretaceous dinosaurs left no Tertiary 

 descendants disqualify them as characteristic Mesozoic fossils. 



Evidence from the invertebrate fauna. — The two elements of the inver- 

 tebrate fauna, one inhabiting the brackish waters and the other the 

 fresh waters, have already been discussed and the fact has been empha- 

 sized that their local habitats were really distinct, though they might be 

 near each other and might alternately occupy the same territory 

 many times. It is obvious then that an attempt to correlate one 

 locality where there were only fresh-water deposits with another 

 locality where the beds were chiefly of brackish-water origin by count- 

 ing identical species and figuringpercentages would lead to erroneous 

 results. A comparison of the Laramie invertebrates of the Denver 

 Basin with the invertebrates of the " Ceratops beds" of Hell Creek and 

 Converse County is a case of this kind and is of no value whatever. A 

 similar result would be obtained by comparing the fauna of the Poto- 

 mac at Washington with that at the mouth of the same stream. 

 At Washington, although within tidal waters, there is a varied fresh- 

 water fauna, which includes many of the genera that lived in Creta- 

 ceous time, while in the lower course of the river and Chesapeake 

 Bay there are beds of oysters and the few other brackish-water forms 

 usually associated with them. 



The invertebrates of the "Ceratops beds" and indeed practically 

 all of the non-marine forms of the Cretaceous like the plants of the 

 Upper Cretaceous belong to living genera and, taking the world over, 

 it is probable that almost every specific type may be found represented 

 by similar living species. It must be admitted that in themselves, 

 without any reference to stratigraphic occurrence or local geologic 

 history, these fossils could not be depended upon for the discrimina- 

 tion of horizons within the Cretaceous nor for distinguishing between 

 Cretaceous and Tertiary. But when the investigation is confined to 

 a single region and when the geographic and stratigraphic range of 

 non-marine species has been determined their evidence is useful and 

 important. When the brackish-water fauna of a locality like Point of 

 Rocks, Wyoming, is studied and is proved to be of Cretaceous age by 

 means of overlying marine faunas, and when a very similar assemblage 



