WHITK.J ANNOTATED CATALOGUE, 429 



On the other hand, if those two Devonian species are rejected as not 

 being members of the UnionidaB, the earliest species of Anodonta that 

 are yet known in North American strata have been obtained from the 

 Laramie Group, although, as we have seen, diverse and characteristic 

 forms of true Unio existed as early at least as the Jurassic period $ and 

 Margaritana appears also to have existed in the earliest epoch of the 

 known North American Cretaceous. 



One of the two species of Anodonta, which have been discovered m the 

 Laramie Group, namely, A. propatoris White, from the Judith Eiver beds 

 of the Upper Missouri Kiver region,* is represented on Plate 19. In 

 form and general aspect it is exceedingly like certain species which 

 are now living in North American waters. The example represented by 

 Figs. 7 arid 8, on Plate 19, is not of fully adult size, as is shown by 

 an accompanying figure of another, but less perfect example ; but it 

 serves to illustrate the form of the species with considerable accuracy. 

 Fragments found associated with them show the characteristic edentu- 

 lous hinge of Anodonta, one of which is represented on tbat plate. In- 

 deed there can be no reasonable doubt that both Unio and Anodonta 

 have come down from at least the close of Mesozoic time, wholly un- 

 changed, not in generic characters only, but in those characteristics also 

 which separate subordinate types within those genera from each other. 



The other Laramie species of Anodonta, namely, A.parallela White,t 

 was obtained from the valley of Crow Creek, Northern Colorado ; but 

 only fragments of the shell have yet been discovered, Fig. 5, on Plate 

 19, being a restoration of the form, which has been prepared by aid 

 of those fragments. It is an unusually elongate form, but it is appar- 

 ently a true Anodonta. 



At the present time lacustrine waters appear to form a more COD ge- 

 nial habitat for Anodonta than fluvatile waters do, although various 

 species of that genus occur in both ; but notwithstanding this fact, no 

 specimens of Anodonta have been discovered in any of the great lacus- 

 triue deposits of Tertiary age which succeeded those of the Laramie Sea 

 in Western North America, although several species of true Unio, as 

 well as other fresh-water inolluscan forms, are frequently found in those 

 deposits. Notwithstanding the fact that so few of the remains of Ano- 

 donta have been discovered, it cannot be doubted that it was continu- 

 ously represented by different species from at least as early a period as 

 the Laramie down to the present time. { 



Ee turning again to the genus Unio, we find it remarkably well repre- 

 sented in the strata of the Laramie Group and those of the immediately 

 succeeding fresh-water Eocene Tertiary groups. 



That division of the Laramie Group which is known as the Bear Eiver 



*An. Rep. U. S. Geol. Sur. Terr, for 1878, Part I, p. 62, pi. 24, fig. 2. 

 t An. Rep. U. S. Geol. Sur. Terr, for 1878, Part I, p. 62, pi. 24, fig. 3. 

 jSee reference to Anodonta decurtata, Conrad, under the head of spurious and doubt- 

 ful species. 



