LATER MESOZOIC INVERTEBRATE FAUNAS 195 



marine faunas in the Mesaverde, Eagle, Claggett, and Judith River 

 formations. The last-named formation in its typical area has a 

 considerable fauna with a number of species that are not known in 

 other horizons, associated with others of wider range. 



The Laramie fauna, which is the last of the conformable Upper 

 Cretaceous series, does not differ materially from the non-marine 

 faunas that preceded it except in specific details. The brackish-water 

 and freshwater elements of its faunas are, of course, seldom mingled 

 in the same stratum but alternate with each other. The brackish- 

 water species have survived from earlier formations in the same 

 region by living in the marine waters or advancing with the sea margin 

 when the submergence came. The freshwater types must have been 

 preserved in the streams of the adjacent lands when marine or even 

 brackish waters covered the larger part of their habitat. A consider- 

 able number of freshwater types were thus enabled to survive into 

 the Tertiary and there are some Laramie species that continue without 

 perceptible change in the Fort Union or earliest Eocene. With the 

 brackish-water forms of the Laramie the case is different. They 

 could not exist for any appreciable period much above sea-level and 

 when the final uplift came that drained the interior region and brought 

 the Cretaceous to a close, the last oysters and other brackish-water 

 mollusks of the interior region died. Hence in areas of non-marine 

 deposition where the line between Cretaceous and Eocene has not 

 been sharply drawn, because the erosion plane that is supposed to 

 separate them has not yet been located, the occurrence of an oyster- 

 bed, or a stratum full of Corbula, is sufficient evidence that the rocks 

 are still Cretaceous and below the major unconformity that separates 

 Cretaceous from Tertiary. 



The very few freshwater shells that are known from the Denver 

 and Livingston formations in their type areas are not distinctive, but 

 the beds which bear the Triceratops vertebrate fauna in Converse 

 County, Wyoming, and the strata with the same vertebrates in eastern 

 Montana, locally known as the "Hell Creek Beds," have a highly 

 differentiated moUuscan fauna of Unios, and other freshwater shells 

 which is much more closely related to the preceding Cretaceous faunas 

 than to that of the typical Fort Union which follows. The evidence 

 of the invertebrates as well as of the vertebrates is strongly in favor 

 of assigning these so-called ''post-Laramie beds" to the Cretaceous. 



