60 NON-MARINE FOSSIL MOLLUSCA. 



Among the species which occur in the coal-bearing beds near Evans- 

 ton, Wyo., several of which species have already been herein noticed, 

 there occurs a slender form which I have described as Eydrobia recta* 

 Jt is represented on Plate 27. 



Another species was obtained by Professor Powell from certain strata 

 in Utah, which he then referred to the Bitter Creek Group, but which 

 may probably prove to be of Laramie age. This form was described 

 by me under the name of Hydrobia utaliensis.] It is represented on 

 Plate 27. 



The only remaining species to be noticed under the Rissoidaj is Bythi- 

 nella gregaria% Meek; which was obtained from the fresh -water Eocene 

 strata at Pacific Springs, Southern Wyoming, by Dr. Hayden. This 

 species is also represented on Plate 27. 



VIVIPARID.*:. 



Among the non-marine gasteropods of North America, no family is 

 more conspicuous than the Viviparidse, and, although this family ranks 

 high in its class, it seems to have been fully established, essentially as 

 it exists to-day, in the Jurassic period, and its origin was doubtless 

 much earlier. It seems to have existed in greatest abundance in the 

 Laramie aud Eocene periods, but that is perhaps largely due to the fact 

 that the conditions of their existence and preservation were greater 

 then. After the Eocene, the preservation of the family doubtless took 

 place in rivers, the great lacustrine waters of the continent having then 

 passed away, or what remained seemed to have been uncongenial to 

 Viviparine life. 



The small collection of fresh-water shells which was obtained by 

 Meek & Hayden from near the Black Hills, and by them referred, with 

 some doubt, to the Jurassic period, has already been mentioned in con- 

 nection with Unio nucalis and Planorbis vetemus. Two other species 

 were included in that collection, namely, Viviparus gillianus and Lio- 

 placodes vetemus § Meek & Hayden, both of which forms are illustrated 

 on Plate 3. 



Besides the original specimens of Lioplacodes no other representatives 

 of the genus, either fossil or recent, have ever been discovered, and it 

 therefore falls into the category of extinct types, a subject which will 

 be briefly discussed on subsequent pages. These two species, being of 

 Jurassic age, are the oldest members of the Viviparidse that are yet 

 known in North American strata. 



~No member of this family of Cretaceous age is yet known; and the 

 next to be mentioned is a very large species from the Bear Elver 



'Powell's Rep. Geology of the Uinta Mountains, p. 132. 

 t Powell's Geology of the Uinta Mountains, p. 132. 

 t An. Rep. IT. S. Geol. Sur. Terr, for 1878, Part I, p. 48, pi. 19, fig. 6. 

 § For diagnosis of Lioplacodes, and description and figures of L. vetemus and Vivipa- 

 rus gillianus, see Paleontology of the Upper Missouri, pp. 115, 116, pi. v. 



