60 MEMOIRS OF THE CARNEGIE MUSEUM 
ing localities really contain an identical fauna, it may be regarded as at least prob- 
able that they were deposited in one and the same lake. The distance between the 
Colorado and the Wyoming localities indicates that the supposed lake was nearly 
200 miles across; and, if the Black Hills fossils also belonged to the same contem- 
poraneous fauna, the assumed lake was much larger. The existence of a fresh-water 
lake of even the smaller size suggested makes it necessary to infer that there was 
then in that part of the North America of to-day a continental area of considerable 
size, for such a lake could hardly be other than a part of a large drainage system. 
“ But aside from these considerations, the existence of such fresh-water faunas as 
are represented by these collections whether in large or in small bodies of water, 
indicates with hardly less clearness than the proved existence of one great lake 
would do the synchronous existence of a large continental area. Indeed it seems 
necessary to assume that in the fresh waters of a large land area alone, could faunas 
of such a character as those which are represented by these collections be developed 
and perpetuated.” I can fully agree with Dr. White as to the necessity of assuming 
the existence in Jurassic times of a continental land-mass of the dimensions inti- 
mated in his paper. But it does not seem to me at all necessary to presuppose the 
existence of a Jurassic lake of even the smaller or more moderate dimensions 
assigned by him. While I do not wish to be understood as denying the possibility 
of the existence of a great lake in Jurassic times extending as Dr. White has sug- 
gested from the Arkansas River in Colorado to the Black Hills of South Dakota, it 
does appear to me that our present knowledge of the character of the faunas, both 
terrestrial and aquatic (fresh-water) as well as of the lithogic and stratigraphic 
features exhibited by the beds themselves is decidedly against such a presumption. 
If I properly understand Dr. White he finds nothing in the character of the aquatic 
mollusca to preclude the possibility of their having lived and developed in smaller 
lakes. After a personal examination of the localities at Green River, Utah, at Grand 
River in western Colorado, Canyon City and Morrison in eastern Colorado, Como 
and Sheep Creek in southern Wyoming, at the Spanish Mines in eastern Wyoming, 
along the Big Horn Mountains in central Wyoming, about the Black Hills in South 
Dakota and in the country near Billings in southern Montana, in all of which 
localities the Atlantosawrus beds are exposed and exhibit in more or less abundance, 
the remains of those dinosaurs which are characteristic of them, I am convinced 
that neither the character of the vertebrate fauna nor the facts of stratigraphy at 
any one of these places can be taken as affording anything lke conclusive evidence 
of the presence of a great body of water. At several of these localities, however, the 
occurrence at intervals of sandstones showing frequent examples of cross-bedding, 
