214 The American Naturalist. [March, 
DIVISION OF THE TITANOTHERIUM Bens Intro Lower, MIDDLE 
AND UPPER BEDS. 
During the seasons of 1886, 1887, and 1888, the writer spent ` 
fifteen months in the White River Miocene beds of S. Dakota — 
and Nebraska, collecting material for Professor Marsh’s Mon- 
ograph on the Titanotheride (Brontotheriidx). Among the 
material collected was 105 nearly complete Titanotherium 
skulls, and many portions of skeletons and disarticulated 
bones; besides the remains of many other associated animals. 
Early in the season of 1886 it became apparent that certain 
forms of skulls were characteristic of certain horizons in the 
beds. This fact showed the importance of keeping, so far as 
possible, an exact record of the horizon from which each skull 
or skeleton wastaken. From actual measurement the vertical — 
range of the Titanotheriide was found to be about 180 feet. 
For convenience in keeping a record of horizons the beds were 
divided into three divisions of 60 feet each, and each of these 
three divisions was subdivided into three divisions of 20 feet 
each. The different skulls and skeletons, when dug out, were — 
each given a separate letter or number, and this letter or num — 
ber was placed in that subdivision of the beds from which the 
skull or skeleton was taken. : 
At present about 60 of these skulls and several more or less 
complete skeletons have been freed from their matrix. When 
studied in connection with the horizons from which they were 
taken, these remains show that a regular and systematic devel- 
opment took place in these animals from the base to the top 
of the beds. The most noticeable change which took place 10 
the Titanotheriidæ was a gradual and decided increase in their 
size from the lowest to the uppermost beds, as is shown by the — 
increase in the size of the skulls, fore and hind limbs, 
other portions of the skeleton. Individuals found near 
bottom of the beds are little, if any, larger than the liv 
rhinoceros. From this they gradually increase in size 88 ew 
go up until at the top we find a type described by Professor 
Marsh as Titanops, rivaling the modern elephant in size. 
This increase in size from the base to the summit of the beds 
