220 The American Naturalist. (March, 
in the size of the skulls from the base to the summit of the 
beds. They are arranged on the opposite page in the order 
in which they occur in the beds commencing at the top. 
Intermediate forms in the middle of the beds have a much 
greater vertical range than their smaller ancestors in the 
underlying beds or their larger survivors in the overlying 
eds. 
Between the skulls here figured there are many intermediate 
forms, showing every stage of development from a very small 
skull found at the very bottom of the beds, which measures 
only 22 inches from occipital condyles to the extremity of 
nasals, with very rudimentary horncores only 14 inches high 
to forms similar to that represented at the top, which measure 
more than 36 inches from occipital condyles to extremity of 
nasals, with horncores over a foot long and having an expanse 
of two feet or more. 
In the section of the Titanotherium beds accompanying 
this paper the relative thickness of the Lower, Middle, and 
Upper beds are shown, with brief descriptions of forms of 
Titanotheride characteristic of each. a 
Much additional information bearing upon the develop- 
ment which took place in the Titanotheriide will doubtless be 
brought to light, when the large collections either made or 
purchased by the writer for use in the preparation of Professor 
Marsh’s monograph and now numbering nearly 200 complete 
skulls and many more or less complete skeletons shall have 
been worked up. It issafe to say that there is scarcely a single 
foot of sediment within the entire vertical range of the Titano- : 
theridæ that is not represented by material in this collection. 
Only those facts bearing upon the evolution of the Titano 
theriidæ which have presented themselves to the writer m 
connection with his field work are set forth in this paper- 
is to be hoped that the few results here recorded may suggest 
to other collectors the importance of keeping, as nearly as po% 
sible, accurate data of horizons; since a familiarity with the 
These facts were communicated by the writer to Professor Marsh in 168. 
were referred to by the latterin the Am, Jour. Sci., Feb., 1889, p. 163, and the Nun 
Ann. Report U. S; Geol. S., p. 114. fe 
