GENERAL OBSERVATIONS. 85 



No true or exclusively marine species have been discovered in these 

 brackish-water Tertiary strata, and it is probable that the waters in which 

 they were deposited were previously cut off from the open sea, but yet 

 retaining to a great degree their former saltness. The genera thus far dis- 

 covered are Ostrea, Anomia, Corbula, Corbicula (Leptesthes), Cyrena (Velori- 

 tma), and Neritina, besides the more exclusively fresh-water genera Unio, 

 Goniolasis, Viviparus, Tulotoma, and Leioplax? 



The final cliange to a wholly fresh, from the brackish water condition, 

 which was never to be resumed, was so gradual that no physical difference 

 appears in the strata accumulated under both conditions ; but from and 

 after that change a uniformity of molluscan type prevailed through all the 

 subsequent epochs of the Tertiary period, as represented in the Plateau 

 Province, that is really remarkable. It is especially so if, as Professor 

 Powell has suggested from stratigraphical considerations, these Cenozoic 

 groups represent the whole of what is generally known as the Tertiary 

 period. Here and there, at different places in each of the Tertiary groups, 

 except the Brown's Park group, which, because of its barrenness of fossils, 

 is not included in this discussion, a few locally restricted species have been 

 found, amounting to a considerable number in the aggregate. But prevail- 

 ing at numerous horizons through all these groups after the brackish -water 

 condition had ceased, and often in. great profusion, are the three molluscan 

 genera UniOj Viviparus, and Goniobasis, which are almost invariably imme- 

 diately associated and almost as invariably without other faunal associates 

 except occasionally a large discoid Planorlis. 



The Unios are of several species, which are denned by characters simi- 

 lar to those upon which accepted recent species of the genus are established; 

 but I have been unable to discriminate with entire satisfaction more than 

 one species each of Goniobasis and Viviparus among these prevailing forms 

 in the northern part of the Plateau Province, from the upper part of the 

 Bitter Creek Group to the top of the Bridger Group, inclusive, with the pos- 

 sible exception of Viviparus Wyominfjensis Meek, from the Bridger Group. 

 The Planorbis just mentioned I have usually referred to P. spectabUis Meek, 

 but in the Bitter Creek Group, especially south of the Uinta Mountains, a 

 variety occurs with fewer and broader volutions, in which respect it cor- 



