white.] ANNOTATED CATALOGUE. 29 



of transversely oval outline, and having the anterior portion rather short 

 in front of the beaks.* 



The remaining three species of TJnio yet to be noticed are all of the 

 simple elongate, or transversely oval form, so common among living 

 representatives of the genus. 



The first is JJ. shoshonensis White,t which is figured on Plate 28. 

 Specimens of it have been found at various localities in Southern Wy- 

 oming and the adjacent parts of Colorado and Utah, in the Wahsatch 

 Group of fresh-water Eocene strata there. 



The next is JJ. washalciensis Meelc,t which is figured on Plate 28, and 

 which has a similar geographical range and geological position. It 

 seems also to range upward into the Bridger Group. 



The third and last is JJ. haydeni Meek,§ which is represented on 

 Plate 28. This species closely resembles JJ. shoshonensis in external 

 form, but it is a thinner and more delicate shell, with a more slender 

 hinge. It also comes from a different group of strata. It is not improb- 

 able that JJ. liaydeni is identical with the species that was described 

 by Hall under the name of Mya tellinoides.\\ For purposes of compar- 

 ison Professor Hall's principal figure of that form is copied on Plate 

 28. 



Although there are fresh-water deposits of considerable extent in 

 Western North America of later date than the Eocene Tertiary epoch 

 which have furnished numerous vertebrate, and a few molluscan remains, 

 no Uniones, and only a few other bivalve species, which are referred to 

 Spliwrium, have been found in any of them. In a few instances, some 

 Unioues have been discovered in certain Post-Tertiary deposits, but as 

 they have all been referred to living species, they do not come within 

 the scope of this article.fi 



CYREXID.E. 



The geological history of the Cyrenida? of North America, as it is at 

 present known, begins with the earliest epoch of the Cretaceous; but it 

 was no doubt actually introduced much earlier. 



The family has apparently never formed a very prominent feature of 

 any molluscan fauna, either marine or non-marine, in any of the geo- 

 logical periods, except that of the Laramie, since its introduction. In 

 this period there was so extraordinary a development of the genus Cor- 



*This species is described, and a cast of one valve figured in An. Rep. U. S. Geol. 

 Sur. Terr., for 1878, Part I, p. 43, pi. 19, fig. 1. Owing to the imperfection of all the 

 specimens of this species that have yet been discovered, no figure of it is given in 

 this article. 



t An. Rep. U. S. Geol. Sur. Terr., for 1878, Part I, p. 41, pi. 19, fig. 2. 



lAn. Rep. U. S. Geol. Sur. Terr., for 1878, Part I, p. 42, pi. 19, fig. 3. 



§ Simpson's Report Great Basin of Utah, p. 364, pi. 5, fig. 11. 



|| Fremont's Rep. Oregon and N. California, p. 307, pi. iii, figs. 1 and 2. 



1f For an interesting discovery of this kind, by Prof. John Collett, in Indiana, see 

 7th An. Report Geol. Sur. Indiana, p. 246. See also, on a following page, remarks on 

 spurious and doubtful species. 



