THE EVOLUTION OF THE MAMMALIA. 23 1 
nasal horns begin to be apparent, while in the upper Uinta beds 
we meet with a new genus of the series {Diplacodon), which ex- 
hibits notable progress. In this genus the stature is much 
increased, and the horns have become quite conspicuous ; the 
canine teeth are reduced in size and the premolars are more com- 
plex in pattern. The series attains its final stage in the wonder- 
ful animals of the genus Titanotherium, found in the lower divis- 
ion of the White River, to which it gives the name Titanother- 
ium Beds, and to which it is strictly confined. Even within the 
limits of these beds, which, being only about one hundred and 
fifty feet thick, cannot represent any very great lapse of timei 
measured by geological standards, these animals underwent many 
and striking changes, and indicate several species. 
In essentials, Titanotherium differs but little from its Eocene 
predecessors, but its appearance is very greatly changed. Most 
obvious is the great increase in the height and bulk of these 
animals, of which the larger species may fairly be called elephantine, 
and all parts of the skeleton show an elephant-like massiveness 
of structure. The head is enormous and the lateral arches are 
immensely heavy ; the brain, however, has not kept pace with 
the enlargement of the skull and is quite absurdly small, 
being, proportionately, distinctly smaller than in the Eocene ances- 
tors of the series. This is a reversal of the ordinary rule, 
according to which the brain is larger in the descendant than in 
the ancestral type, and it is doubtless the reason why these great 
and powerful animals had such a brief career. The increased 
size of the skull is due principally to the growth of the facial 
region and to the development of air sinuses in the cranial bones 
which form the brain case. The horns, also, are greatly enlarged, 
and in the males of some of the species, they have a grotesque 
size and shape. The neck is very heavy and the spines of the 
anterior trunk vertebrae are immensely elongated for the attach- 
ment of the muscles which support the huge head ; these spines 
must have produced a conspicuous hump at the shoulders. The 
trunk is very large, and the hip bones are expanded, as in the 
elephants. The limbs are extremely massive and of proportions 
