62 NON-MARINE FOSSIL MOLLUSCA. 



never been discovered, all its known characteristics, except tlie one 

 mentioned, agree with Viviparus, from which genus some naturalists do 

 not separate Tulotoma. There appears to be good reason, however, for 

 separating the two groups as distinct genera among living North Amer- 

 ican mollusca; but it is nevertheless reasonable to suppose that they 

 had a common origin in a single generic form in some one of the geo- 

 logical epochs. Such a community of origin, and probable original 

 generic identity of these fossil Viviparine forms, is strongly suggested 

 by comparing some of the specimens of T. thompsoni, upon which the 

 nodules are only faintly developed, with "typical specimens of V. trochi- 

 formis, which have a closely similar form and similar revolving ridges, 

 but no nodules. Indeed, some of the characteristics are so strikingly 

 alike in both these species that one can hardly resist the suggestion that 

 they themselves present the examples of such a community of origin as 

 has been supposed. If this really was the case, and we accept Tulotoma 

 as a genus separate from Viviparus, we have an example of the evolution 

 of a new generic form without a break in a recognizable line of descent. 

 In other words, it is a case in which the generic and specific divergences 

 in the line of descent were coincident, the former being the more dis- 

 tinct of the two, at least in the features presented by the shell. 



From certain strata in Utah, which were referred to the Tertiary, but 

 which probably belong to the Laramie Group, I described a species of 

 Viviparus, under the name of V. ionicus ;* but all the known examples 

 of it are very imperfect. 



In the Eocene fresh-water strata of the West, perhaps the most abun- 

 dant species next to Goniobasis tenera Hall, with which it is constantly 

 associated, is Viviparus paludinceformis Hall.t This species has been 

 found especially plentiful in those strata of Colorado, Wyoming, and 

 Utah. It presents some degree of variation in different districts, and 

 in different layers at one and the same locality ; but it is not nearly so 

 variable a shell as is its associate Goniobasis tenera. Illustrations of V. 

 paludinceformis are given on Plate 30. 



The only remaining species of Vivaparus to be regularly noticed in 

 this article is V. tcyomingensis Meek;f which is figured on Plate 30. 

 It has been found only in the Bridger Group, where it is comparatively 

 rare.§ 



The differences which are recognizable between the shells of living spe- 

 cies of Viviparus and Gampeloma, respectively, are quite as distinctly 

 discernible among the fossil forms which are noticed in this article. 

 Recognizing the two genera as separate and distinct, we find, as one of 

 the earliest known North American species of Gampeloma, a form that 



'U. S. Expl. and Sur. West of the 100th Merid., vol. iv. p. 215, pi. xxi, fig. 6. 

 tFreniont's Rep. Oregon and N. California, p. 309, pi. iii, fig. 13. 

 jProc.Acad. Nat. Sci. Phila., vol. i,(3), 1871, p. 182. 



§See references to V. lyelli Conrad, V. glabra H. C. Lea, and Paludina subglobosa 

 i I'm i - on a subsequent page, under the head of spurious aud doubtful species. 



