78 INVERTEBRATE PALEONTOLOGY. [WHITE. 



ively are not in all cases clearly defined by faunal characteristics, even 

 although they may be separated by sufficiently distinct physical characters 

 by the field geologist 



The periods are, as a rule, very clearly separated from each other by 

 faunal characteristics, there being a partial exception in the case of those 

 of the upper group of the Cretaceous period and the lower group of the 

 Tertiary. But these facts will be noticed under appropriate heads on fol- 

 lowing pages. 



Our investigations further show that certain faunal characteristics which 

 have hitherto been relied upon to fix the geological age of strata of marine, 

 brackish and fresh water origin, respectively, are not parallel. In other 

 words, our collections contain types or forms of fresh and brackish water 

 origin that have been regarded' as peculiar to the Tertiary period, which 

 were obtained from strata that underlie those containing such marine types 

 as are universally regarded as peculiar to Mesozoic age; showing conclu- 

 sively, that so-called Tertiary fresh and brackish water types and Cretaceous 

 salt water types co-existed. The land shells also that have been obtained 

 from strata herein classified as Cretaceous, are of Tertiary or even of still 

 more recent type. These facts have made it especially difficult to fix the 

 period of our terrestrial, and fresh and brackish water fossils with satisfactory 

 precision, if they were- new species and obtained from unique and isolated 

 localities. 



A striking peculiarity of the strata of the Plateau Province, is the large 

 proportion among them of fresh and brackish water deposits. So far as at 

 present known, all the strata of Carboniferous age are of marine origin, the 

 first unmistakably fresh water accumulations yet discovered in the Plateau 

 Province being of Jurassic age. The only species obtained from these fresh 

 water strata is a f/m'o, which is one of ordinary recent type; as is also 

 another species of Unio and some Viviparine shells, described by Meek and 

 Hayden, from the valley of the Upper Missouri River. These facts again 

 illustrate the comparatively small value that can be placed upon fresh water 

 invertebrate forms as indices of the passage of geological time. 



Most of the Mesozoic strata were evidently deposited in water that was 

 salt by virtue of having been a part of, or in communication with, the open 



