
(B.)\—Report on the Vertebrate Fossils from the Fort Union Group of 
Milk River. 
By E. D. Cope. 
The fossils submitted to my examination by Mr. Dawson, are generally in a 
fragmentary condition, and belong to a number of distinct individuals. In but few 
instances are the fragments so characteristic as to admit of specific diagnosis, but 
they leave no doubt as to the ordinal relations of the animals to which they be- 
longed. These are exclusively Dinosauria, tortoises, and gar-fishes, no Mammalia or 
marine vertebrata being indicated. 
The collection is of especial interest, as furnishing one more station whence a 
glimpse of the life of the “transition period” of the history of the western part of 
America may be obtained. And the resulting information is similar to that already 
derived from studies of the paleontology of the same horizon by Prof. Leidy and 
myself in Dakota, Wyoming, Colorado, etc. The species are evidently the latest 
examples of the terrestrial life which characterized the Mesozoic ages, and which 
preceded the advent of the mammals in the northern hemisphere, The formations 
in which the fossils occur consist of greenish-brown arenaceous clays of various 
degrees of hardness, frequently including small gravel-stones, and sometimes 
forming a hard cement between them. The fossils were found near the base of 
thr formation} and “not more than one or two hundred feet above yellow 
arenaceous beds, which I conceive represent Cretaceous No. 5, and which are 
rapidly followed in descending by well-marked No. 4 with characteristic fossils.” 
(Extracted from letter of Mr. Dawson.) 
The species are the following: Clastes, sp.; Compsemys ogmius; C. ? victus ; 
Plastomenus costatus ;* P. coalescens ; Trionyx ? vagans; Trionyx ? sp.; ? Hadrosaurus 
2 sp.; Cionodon stenopsis, 
The dinosaurian remains are quite abundant, and indicate several species, but 
are mostly so fragmentary as to be unfit for determination. The diagnostic gen- 
era of this list are Compsemys, Plastomenus, and Cionodon; the species referred to 
Hadrosaurus being represented by caudal vertebre only. The first-named genus is 
characteristic of the Fort Union epoch only; the fragment referred to C. victus, 
the only species of the list previously known, is too small for final specific 
reference. The Plastomenus coalescens is represented by a more perfect specimen 
than any other species referred to this genus from the Fort Union Beds, but is 
not sufficiently complete to render the reference to this Eocene genus final. It 
is, in any case, not a member of any other known genus. One species of Trionyx 
is represented by a hyosternal bone, and is not definable; while the fragment 
* Two species are provisionally referred to the Tertiary genus Plastomenus, but are too fragmentary 
for final determination, 
