328 O. C. Marsh—American Jurassic Mammals. 
In connection with this work, the writer has also examined 
the more important specimens from the Jurassic of Europe, 
and, likewise, the few specimens known from the Trias, in 
both Europe and this country. 
The American Jurassic Mammals hitherto found are all 
from essentially the same geological horizon, in the Atlanto- 
saurus beds, of the Upper Jurassic. The principal! locality is 
in Wyoming, on the western slope of the Rocky Mountains, 
and remains of two or three hundred individuals have been 
obtained at this place alone. At other points in the same re- 
gion, a few remains have been found. A second locality of 
importance is in Colorado, about three hundred miles south of 
the most northern known limit of these remains. 
The other vertebrate fossils from this horizon are mainly 
dinosaurs, many of them of gigantic size, but some scarcely 
larger than the mammals. Crocodiles, turtles, small lizards, 
and fishes, are also well represented. A single bird (Laop- 
teryx), and one small pterodactyl, have likewise been recog- 
nized from these deposits. More recently, various bones of 
small, anourous amphibians (Hobatrachus agilis) have been 
found, the first detected in any Mesozoic formation.* The 
deposits are lacustrine, as shown by the fresh-water shells they 
contain. 
In investigating these American Jurassic Mammals, it was 
necessary to compare them, first of all, with those from the 
same formation in Europe. On this subject, the elaborate 
memoir of Owen, on British Mesozoic Mammals, was taken as 
the main authority.t 
The first specimens discovered in this country proved to be 
very near allies of European forms, and most of those since 
found show a remarkable resemblance to others described by 
Owen. Some fragmentary specimens cannot indeed be dis- 
tinguished from the English fossils, but where the remains are 
more complete, various differences are seen, which appear to 
be distinctive. A few, well-marked, American genera have no 
known representatives in Europe, while some forms found 
there are unknown here. : 
One difficulty in the investigation of the remains from the 
two widely-separated regions arises from the necessity of rely- 
ing mainly upon figures for comparison. Again, these minute, 
delicate fossils are often embedded in a matrix from which 
they cannot be removed without great danger of injury or 
destruction. Hence, the jaws and teeth in many cases must 
be examined and described from the single side exposed. If 
* Proceedings British Association, Aberdeen Meeting, p. 1033, 1885. 
+ Monograph of the Fossil Mammalia of the Mesozoic Formations. Paleeonto- 
graphical Society, vol. xxiv, London, 1871. 
