194 T. W. STANTON 



corals, Rudistae, Actaeonella, etc., which suggest the Cretaceous 

 of Jamaica. It may be considered a reef facies of the Ripley fauna. 



All the late Cretaceous marine faunas that have been briefly men- 

 tioned are still typically Mesozoic, although it is true that they con- 

 tain many generic types that continue on through the Tertiary. The 

 succeeding Tertiary faunas, whether on the Pacific coast, the Gulf 

 border, or the Atlantic coastal plain, show a very striking change 

 from the Cretaceous faunas that immediately precede them. The 

 specific types are practically all different. 



N on-marine later Cretaceous faunas. — In the Rocky Mountain 

 region throughout later Cretaceous time there was a great develop- 

 ment of freshwater and brackish-water deposits alternating with 

 marine formations. They are usually coal-bearing, and yield inver- 

 tebrate faunas frequently associated with land vertebrates and plants. 



The invertebrate fauna of the Dakota sandstone is too meager 

 to be of much value. It consists of a few brackish-water species 

 with Unio and a few other freshwater shells in other strata and at 

 the top some marine species that probably really belong with the 

 succeeding Colorado fauna. The freshwater species show relation- 

 ship through the genus Pyrgulifera with the fauna of the Bear River 

 formation which is apparently about on the horizon of, or a little 

 later than, the Dakota. The Bear River fauna is distributed over a 

 considerable area in southwestern Wyoming, and is unique among 

 western non-marine faunas in that it contains a number of types 

 that have left no descendants in later formations of the region. The 

 most distinctive forms are freshwater species, but the fauna also con- 

 tains brackish-water elements. The submergence beneath the Colo- 

 rado sea which immediately followed the deposition of the Bear River 

 formation seems to have been so complete in this region that the fresh- 

 water forms were not able to survive. But in the Colorado group itself 

 along the western margin of the sea, especially in Utah and western 

 Wyoming, there are intercalations of coal-bearing beds which contain 

 a few Unios and other freshwater shells and brackish-water types 

 like Ostrea, Anomia, Corbula, and Modiola, some of which recur 

 in identical or closely similar forms at several horizons to the top of 

 the Cretaceous. 



In the Montana group there are local more or less distinctive non- 



