172 REPORT UNITED STATES GEOLOGICAL SURVEY. 



Denver, gave me a fine specimen that lie informed me came from Horse 

 Tail Greek, a tributary of South Platte River, some seventy-five miles 

 eastward from Greeley, Colo. Besides these specimens from strata, east 

 of the mountains, I collected others from the original locality in the Bit- 

 ter Creek series at Clack Buttes Station ; at two or three localities in 

 Yampa Eiver Valley ; and at one locality in White River Valley. A care- 

 fid comparison of all these specimens, collected at the localities just named, 

 on both sides of the Rocky Mountains, shows M. wyomingensis Meek and 

 M. larunda White to be one and the same species. It must therefore 

 take the name originally given it by Meek, which is here adopted. 



At the Crow Creek locality it was found in bed No. 5 associated with 

 purely fresh-water forms, but more plentifully in bed No. 3, where it was 

 found associated with Corbula, Corbicula, &c, but no species were found 

 in that bed that are regarded as purely fresh-water forms. It was found 

 at all the localities named west of the mountains, with similar associates, 

 by which I infer that this species was capable of living in waters that 

 were at least in some degree saline. 



No. 21. Viviparus prudentia White. 



This species has been found only at the Crow Creek locality, and only 

 in bed No. 5 of that section. It is an unusually short species, and in 

 general aspect it recalls the living species, V. intertexia Say. It is de- 

 scribed in the bulletin of the United States Geological and Geographical 

 Survey of the Territories, vol. iv, p. 710. 



No. 22. Tulotoma thompsoni White. 



No other species of this genus has yet been discovered in American 

 strata, and only one living North American species is known. Its claims 

 to be regarded as a distinct genus from Viviparus will be discussed else- 

 where. T. thompsoni was first discovered by me in the upper strata of 

 the Bitter Creek series of the Laramie Group, at Black Buttes Station, 

 Wyoming, and described in Powell's Report on the Geology of the Uinta 

 Mountains, p. 134. It was then found to occupy a thin layer immediately 

 above one that at the same locality is crowded with Corbicula, but the 

 Tulotoma apparently had no other than purely fresh-water associates. 

 At the Crow Creek locality it was found only in No. 5 of the section there, 

 its associates being purely fresh-water forms, except Volsella (Brachy- 

 do'tites) regularis, which occurs in that bed as already mentioned, but 

 perhaps accidentally. 



It has hitherto been discovered at only these two localities, one on 

 each side of the Rocky Mountains, but this fact shows that it was a 

 widely distributed species, and it is found in large numbers at both 

 localities. The identity of the species at these two distant localities is 

 unmistakable, but there appears to be a greater variation among the 

 examples from the Crow Creek locality than among those vobtained at 

 Black Buttes Station, but the latter are mostly very imperfect. One of 

 the variations observable among the examples obtained in the valley of 

 Crow Creek is a tendency of the nodes to become obsolete, even upon 

 the last volution. The aspect of those examples in which the obsoles- 

 cence of the nodes is greatest is closely like that of the typical forms of 

 Viviparus trocMformis Meek & Hay den, thus suggesting the possible 

 passage of a molluscan species from one recognized generic form to 

 another without any clearly definable change of specific characters. * 



* Dr. M. Nemnayr, of Vienna, lias shown a similar gradation to exist bet'syeen cer- 

 tain Miocene Tertiary forms from Sclavonia which American conchologists would not 

 hesitate to refer respectively to Viviparus and Tulotoma, but all of Avhich he refers to 

 the former genus. The paleontologist is of course confined to the study of the shell 



