I 



432 NON-MARINE FOSSIL MOLLUSCA. 



These two species are represented on Plates 17 and 14 respectively. 

 Some imperfect specimens of a form that seems to be specifically identical 

 with U. cryptorhynchus have also been discovered in the Laramie strata 

 at Black Buttes station. The correctness of this identification is ren- 

 dered the more probable, because of similar identification of other mol- 

 luscan species in both regions, one of which has already been mentioned. 



Of the Uniones that are yet known from the Judith Kiver beds, two 

 species remain to be noticed namely, U. primcevus and U. senectus 

 White,* which are represented on piates 14 and 19 respectively. 



These two species differ much in external form, but both are marked 

 by small rugose plications upon the postero-dorsal portion of the sur- 

 face, a feature which is not unusual among living North American 

 species of Unio, but which is possessed by few of the known fossil 

 species. 



The aspect of these shells is so suggestive of certain features pre- 

 sented by some of the living forms of Unio just referred to, that it is 

 only their fossilized condition which at first view conveys to the mind 

 an impression of their great antiquity. 



One other species, Unio prisons Meek & Hayden,t has also been ob- 

 tained fro m the Laramie strata of the Upper Missouri Eiver region ; 

 but this one is from the Fort Union beds which occur in a part of that 

 great region which lies far to the eastward of that in which the Judith 

 Eiver beds are found. This species is of an 'ordinary oval outline, with 

 a moderately short front, but without any noteworthy peculiarities. It 

 is represented on Plate 14 by a copy of Mr. Meek's original figure. 



In a single stratum of the Laramie Group at Black Buttes station, in 

 Southern Wyoming, nearly a dozen species of Unio have been found, 

 among which are some of the most interesting fossil forms of that 

 genus that have ever been discovered: Some of these species have the 

 peculiarly shortened front, together with a transversely oval form, of 

 which mention has already been made; but in their general character- 

 istics others of them are peculiarly like certain forms that are now liv- 

 ing in the waters of the Mississippi drainage system. 



Among these species from the Black Buttes locality is Unio couesii, 

 White,! the largest species of that genus which has ever been found in 

 North American strata. It is represented on Plate 16. Its large size, 

 massive test, and general aspect strongly recall certain of the large 

 Uniones that are found living in the waters of the Wabash and other 

 rivers of the Mississippi drainage system. 



Associated with U. couesii is another species, which is nearly as large, 

 but of different form, namely, U. endlichi White, which is represented 

 on Plate 15. 



* An. Rep. Sur. Terr, for 1878, Part I, pp. 69, 70, pi. 29, fig. 3, and pi. 28, fig. 1. 



tU. S. Geol. Sur. Terr., vol. ix, p. 516, pi. 43, fig. 8. 



{An. Rep. U. S. Geol. Sur. Terr., for 1878, Part I, p. 64, pi. 27, fig. 1. 



$ An. Rep. U. S. Geol. Sur. Terr., for 1878, Part I, p. 66, pi. 26, fig. 1. 



