70 MEMOIRS OF THE CARNEGIE MUSEUM 
according as the beds in question are of marine or fresh-water origin. While marine 
invertebrates and most terrestrial and aquatic vertebrates are as a rule safe guides 
for purposes of correlation and second only in value to direct stratigraphic evidence, 
fresh-water invertebrates, plants and certain vertebrates as for instance turtles, croco- 
diles and some fishes are as a rule much less reliable guides. 
Stratigraphic Position of the Atlantosawrus Beds.—As originally applied the term 
Atlantosaurus beds refers to that series of sandstones and shales, some 450 feet in 
thickness and containing the remains of dinosaurs, small mammals, ete., lying 
between the red Triassic? sandstones below and the Dakota sandstones above on 
either side of the cafion of Four Mile Creek (Oil Creek) near Canyon City, Colorado. 
The dinosaur remains upon which Professor Marsh relied for the determination of 
the age of these deposits at this locality all came from the lowermost 150 feet of the 
series and it may therefore eventually prove advisable to limit the use of the term 
to the lower one third of the series. Farther north in Wyoming and about the 
Black Hillsin South Dakota similar dinosaur beds are separated from the Red Beds 
by a series of marine shales and limestones named by Marsh the Baptanodon beds. 
These latter beds are rich in the remains of marine vertebrates and invertebrates 
and are universally regarded as of Middle or Upper Jurassic age, while the over- 
lying dinosaur beds have as universally been referred to the Atlantosawrus beds 
usually considered, as noted above, as of Upper Jurassic age. The marine Baptan- 
odon beds throughout Wyoming and South Dakota are everywhere found accom- 
panying and underlying the fresh-water Atlantosawrus beds though thinning out 
toward the south and entirely disappearing as we approach the Wyoming and Colo- 
rado state line. As already noticed they are entirely absent in the locality near 
Canyon City, Colorado, the Atlantosawrus beds there resting directly upon the Red 
beds and with at least apparent conformity. Nor does there appear to be any mate- 
rial break in the conditions of sedimentation in this region from the base of the 
Atlantosawrus beds to the summit of the Dakota. If this be true it would appear 
that at Canyon City the lower members of the Atlantosawrus beds, those worked by 
Marsh and by Mr. Utterback, are the fresh-water equivalents of the marine Baptan- 
odon beds farther north, while the upper beds or those worked by Cope would 
become the equivalents of the Atlantosawrus beds at Morrison, Colorado, and at 
various localities in Wyoming and South Dakota. Such evidences of stratigraphy 
as there are prove conclusively that the Atlantosawrus beds at Canyon City overlie 
the Triassic and underlie the Dakota and that they are intermediate in age between 
the two and are therefore of either Jurassic or Lower Cretaceous age or that they 
represent, either wholly or in part, both those horizons. The latter seems to me the 
