ART. XXIV -PALEONTOLOGICAL PAPERS NO. 5.-REMARKS ON 

 THE PALEONTOLOGICAL CHARACTERISTICS OF THE CENO- 

 ZOIC AND MESOZOIC GROUPS AS DEVELOPED IN THE GREEN 

 RIVER REGION. 



By C. a. White, M. D. 



1^0 strata of Carbouiferous age in Western North America have yet 

 been found to contain any fossils that are referable to a fresh-water 

 habitat. In the Jurassic strata of different portions of that region, 

 however, some fresh-water Mollusks have been discovered, and a still 

 greater number have been obtained from the strata of different Creta- 

 ceous, Post-Cretaceous, and Tertiary groups, becoming more and more 

 abundant in the order of time. There is sufiQcient evidence to show 

 that over what is now the western part of the continent, the marine 

 condition which prevailed through Palaeozoic time began in Mesozoic 

 time gradually to give place to partially land-locked seas, and finally to 

 great lakes of fresh waters, which changes were caused by the gradual 

 elevation of the continent. In these waters, both marine and fresh, the 

 deposits went on continuously, notwithstanding the important physical 

 changes that were simultaneously in progress, so that we now find an 

 essentially unbroken series of strata ranging from Paleozoic time far 

 into the Tertiary period. Although there existed during the last-named 

 period such immense fresh-water lakes upon that portion of the conti- 

 nent, the areas occupied by fresh water were probably not very exten- 

 sive until after the epoch of the Wahsatch group had well progressed. 

 This inference is drawn from the fact that all the fresh- water deposits 

 that have been discovered among the strata of earlier groups are very 

 limited, and the brackish condition of the waters is known to have pre- 

 vailed for a considerable time after the beginning of the Wahsatch epoch. 



As a partial but not essential modification of the statements just made, 

 it should be mentioned that the character of the invertebrate fossils 

 hitherto found in the Laramie group indicate that few, if any, of its 

 strata were deposited in open sea waters, but rather in wholly or par- 

 tially land-locked brackish waters, with here and there localities where 

 they were nearly or quite fresh. It is a continuation of these conditions 

 that have just been referred to as prevailing in the early part of the 

 Wahsatch epoch, when they finally gave place to a wholly fresh-water 

 condition that continued through all the succeeding epochs. 



During the time of the Laramie epoch and the early part of the 

 Wahsatch, there were, in the vicinity of the Uinta Mountains, the uplift 



