LATER MESOZOIC INVERTEBRATE FAUNAS 423 
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 molluscan 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. 
