LX INTRODUCTORY REMARKS. 



the fresh-water forms, and at other places a species of Hydrobia and one of 

 a similar new genus that may have lived in brackish water. 



As already intimated, the shells of this group are generally found in a 

 remarkably fine state of preservation, looking, as they do, nearly as fresh as 

 dead examples of existing species with the epidermis removed. Although in 

 most cases they are nearly allied to our living North American forms, in a 

 few instances they much more nearly resemble Chinese and Indian species, 

 our Viviparus trochiformis being almost exactly like certain angular varieties 

 of V. Bengatensis from China, with which they have been compared. 



Seeing how very conflicting is the evidence in regard to the ages of this 

 and the Judith River groups, and some similar deposits of the far-west, as 

 derived from the reptilian, molluscan, and vegetable remains, the question 

 naturally suggests itself whether fossils of these kinds, from such brackish- and 

 fresh- water deposits, can be fully relied upon in deciding in regard to their 

 parallelism with particular formations elsewhere. When reptilian remains, 

 pronounced by Professor Cope decidedly Cretaceous types, are found directly 

 associated with numerous remains of plants as confidently considered Tertiary 

 types by Professor Lesquercux, and fresh- and brackish-water shells also occur 

 in the same association, which, from their modern affinities, would be by any 

 palaeontologist referred to the Tertiary, it is quite evident that the testimony 

 of all cannot be right. Yet it does not prove that either or any of the author- 

 ities are necessarily in error in their verdicts in regard to the affinities of the 

 fossils as interpreted in the light of all past experience. It shows, however, 

 that something has yet to be learned in regard to the geological range of cer- 

 tain types of the Reptilia and fossil plants, and that the affinities of new 

 species of fresh- and brackish-water shells to recent forms, especially when 

 the fossil species are yet unknown in any well-established horizon, are not 

 always safe guides. It was too recently that vegetable palaeontologists found 

 it necessary to admit the existence of a decided Tertiary flora (according to 

 all previous experience) in Kansas and Nebraska, at a horizon of one thou- 

 sand to fifteen hundred feet below the top of well-marked Cretaceous strata, 

 for us to feel quite sure that something farther in this department may not 

 remain to be learned, and when known may not require other modifications of 

 views. On tin' oilier hand, the occurrence of Eocene types of vertebrates, 

 along with Dinosaurian remains, on the branches of Milk River, already men- 

 tioned, shows that we cannot be quite sure that a longer lease of life may not 



