WHITE ON DISTRIBUTION OF MOLLUSCS. 723 



those that have been frequently designated as the'" Bear Eiver Estuary 

 Beds", and sometimes as the "Sulphur Creek Estuary Beds"; together 

 with the coal-bearing series that is seen to rest upon them in the Valley 

 of Bear Eiver, northward from Evanston, Wyoming. 



It will be seen that Ostrea uiyomingensis is indicated with doubt as 

 occurring in the Judith Eiver Group. This reference is made because 

 of the probable identity of Ostrea glabra Meek & Hayden, with 0. 

 icyomingensis Meek, and the doubt is expressed because the proof ui)on 

 that point is not entirely satisfactory. The former species, as identified 

 in the Lignitic strata east of the Eocky Mountains in Colorado, is there 

 found to be connected by associated intermediate forms with shells that 

 cannot be distinguished from the typical forms of 0. wyomingensis, and 

 therefore no doubt is expressed upon that point as regards that region. 

 This species is not only found in the strata of the other regions indi- 

 cated in the table, but in various localities within the great Green Eiver 

 Basin west of the Eocky Mountains it is found to range through the 

 whole series of Laramie strata, a thickness of not less than 3,500 feet. 

 I am also a little in doubt as to the real identity of Campelonia multi- 

 lineata in the Bitter Creek series ; but all the other species embraced 

 in the table are probably correctly identified. Not only has the Ostrea 

 icyomingensis the great vertical range in the Laramie Group which 

 has just been mentioned, hat Aiiomia micronemay Bracliydontcs regularise 

 Melania wyomingensis^ and probably other species also, have an equally 

 great vertical range; embracing, in fact, the whole thickness of the 

 Laramie strata in the great Green Eiver Basin, which thickness proba- 

 bly reaches a maximum of 4,000 feet. 



It is a well-known fact that the aggregate thickness of the Laramie 

 strata east of the Eocky Mountains in Colorado is much less than it is 

 in either of the other regions here named. But those eastern strata 

 appear to represent the whole Laramie period, because they contain all 

 the species just mentioned that are known to range through the whole 

 series west of the mountains, where it has its maximum thickness, and 

 they also contain certain species associated in the same layers that ap- 

 l)ear to characterize the Fort Union and Judith Eiver beds respectivelj', 

 in the Upper Missouri Eiver region, and not there associated together 

 in either. 



The distribution of species in the Laramie Group, on both sides of the 

 Eocky Mountains, is too conspicuously shown by the table to need com- 

 ment. 



In the foregoing discussion only the species that are common to the 

 strata of two or more of the districts here discussed have been con- 

 sidered. Therefore, only the faunal relationships between these regions, 

 and not their differences, are shown. To show the latter, a considera- 

 tion of all the species yet discovered in the strata of this great group 

 is necessary. The characteristics of all the known species of the dis- 

 tricts named, except a part of those of Bear Eiver Valley, are in har- 



