34 NON-MARINE FOSSIL MOLLUSCA. 



have been found in the EoceDe fresh -water deposits of Southern Wyo- 

 ming, which are, like those of the Laramie Group, so similar to certain 

 living species as to call for no distinguishing remarks. 



We yet know nothing of the existence of Sphwrium in North America 

 between the close of the Eocene epoch and the Post-Tertiary except the 

 two species which were respectively described by Meek under the names 

 Sphwrium rugosum and S. idahoense, both of which are represented on 

 Plate 32 by copies of his original figHres. The types of these two 

 species were obtained from the fresh-water deposit of the Kawsoh Mount- 

 ains of Northern Nevada, which is regarded as of Miocene age by par- 

 ties connected with the United States Geological Survey of the 40th 

 parallel.* We do not yet know even so much as this of the history of 

 the Unionidae between the close of the Eocene epoch and the beginning 

 of the Post-Tertiary ; f yet we cannot doubt that both Unto and Sphw- 

 rium flourished together somewhere during all that time. 



PISIDIIDiE. 



Of the genus Pisidium, only one fossil species, namely P. saginatum 

 White, has yet been discovered.! Its form is represented by figures 14 

 and 15, on Plate 20, and it is by its outward features alone that it is 

 referred to Pisidium, the interior markings of the shell having never 

 been ascertained. It was obtained from the series of coal-bearing strata 

 near Evanston, Wyoming, which are at present understood as belonging 

 to the upper part of the Laramie Group. 



While it seems to be unquestionable that the living Unionidse of the 

 Mississippi drainage system are generically descended from those spe- 

 cies which, as we have seen, existed during the Laramie period, and 

 from their associates which yet remain to be discovered, no descend- 

 ants of the Cyrenidse which then existed seem to have survived the 

 close of the Laramie period, except those of the genera Sphwrium and 

 Pisidium. In fact, all three of the sections of the genus Corbicula. which 

 have been mentioned as having lived during the Laramie period, seem 

 to have required the same conditions of habitat that the contempora- 

 neous Ostrea and Anomia did. When, therefore, at the close of the Lar- 

 amie period, the waters of the interior region of North America became 

 entirely fresh, all the forms of Corbicula which had flourished through- 

 out that great region ceased to exist; while Sphwrium and Pisidium, whose 

 habitat is in fresh water only, survived to the present time, doubtless in 

 company with representatives of the Unionidae, and with many fresh- 

 water gasteropods. This opinion of course implies not only the belief 

 that while the living Uniones of the Mississippi drainage system are, 

 either wholly or in part, directly descended from those whose remains 



*U. S. Geol. Sur. 40th parallel, vol. iv, pp. 182, "183, pi. xvi, Jigs. 1 audi!. 

 t That is, if we omit all those which are mentioned on following pages mulerthe head 

 of spurious and doubtful species. * 



t Powell's Report Geology of the Uinta Mountains, p. 128. 



