204 Professor Marsh’s Monograph of the Dinocerata. 
“The cause of the successive disappearance of each Bes | of 
these large Tertiary mammals is not difficult to find. The 
small brain, highly specialized characters, and huge bulk, ren- 
dered them incapable of adapting themselves to new condi- 
tions, and a change of surroundings brought extinction. The 
existing Proboscidians must soon disappear, for similar reasons. 
Smaller mammals, with larger brains, and more plastic struc- 
ture, readily adapt themselves to their environment, and sur- 
vive, or even send off new and vigorous lines. ; 
“The Dinocerata, with their very diminutive brain, fixed 
characters, and massive frames, flourished as long as the condi- 
tions were especially favorable, but, with the first geological 
change, they perished, and left no descendants. 
x * ~ % * s ¥ 
CLASSIFICATION. 
“The Dinocerata now known may be placed in three genera: 
Dinoceras, Tinoceras, and Uintatherium. These may be sepa 
rated by characters of the skull, vertebrae, and feet. There 
are also indications of several intermediate forms, which may, 
perhaps, be found to represent sub-genera, when additional 
specimens in good preservation are secured for come a 
wenty-nine species may be distinguished, mainly by t 
alone, which, at present, offers the best distinctive characters. 
Sub-order DINOCERATA, Marsh. 
Family Tinoceratin#, Marsh. 
Uintatherium, Leidy. | Dinoceras, Marsh. Tinoceras, Marsh. 
Teeth, thirty-six. Teeth, thirty-four. Teeth, thirty-four. 
Lower premolars, four. | Lowerpremolars,three. Lower premolars, three. 
nine tusk, ine tusk, | Base of canine tusk 
n cal. nearly vertical. horizontal. 
Parietal protuberance, Parietal protuberance, | Parietal protuberanc™ 
above post-glenoid above post-glenoid behind post-gleno! 
rocess. process. process. 
sas Hahei of Cervical vertebrz, less — vertebre, 
moderate length. elongate. short. : : 
Lunar, articulating Liar prticniatiag Lunar, not articulating 
with trapezoid ? with trapezoid. with trapezoid. 
“ These three genera clearly represent three stages of develop 
ment of the Dinocerata, and these stages correspond to t r 
successive horizons of the middle Eocene in which the remal™ 
of these animals were entombed. Uintatheriwm, the ve 
neralized type, is found at the lowest level; Door a 
om a somewhat higher stratum; and Zinoceras, the ™ 
specialized of all, occurs in the latest deposits.” er 
In the Synopsis which follows this chapter, a kage she 
of all the species of the Dinocerata is given in detail, aD aoe 
B. 
volume closes with a Bibliography of the important er 
+ F 
