814 The American Geologist. November, 1897 
is rather strong and well developed, the succeeding premolars 
increase in size from in front backward. The dorsum of the 
molars and premolars is very flat with no suggestion of a 
median costa. There is a basal ridge on the dorsum of the 
true molars but not on the premolars. The median sinus is 
shallow, especially in the premolars and the first molar; it is 
obstructed by only faint rudiments of the crochet and ante- 
crochet. The anterior and posterior valla are shallow. There 
is a cingulura on the inner border of the premolars but none 
on the true molars. The principal measurements of the type 
are as follows : 
mm. 
Length of skull from extremity of nasals to occipital crest 475 
Breadth across frontals at post-orbital processes 165 
Hight of occipital crest aVjove foramen magnum 115 
Width of nasals at rugosities , 102 
Width of nasals just in front of rugosities at see. shown in fig. la. 63 
Thickness of nasals on median line in front of rugosities 24 
Length of premolar — molar series 195 
Length of premolar series 97 
Prof, Osborn* has considered D. proavitum as a synonym 
of his Aceratherium tridactylum. In his latest communica- 
tion on this subject he says: "The large number of skulls 
in the collection belonging to Aceratherium tridactylum 
demonstrates that the species ran to two extremes, a high, 
long and narrow type, and a shorter, lower and broader type. 
The latter exhibit very prominent rugosities upon the nasals, 
which we might, with Hatcher, interpret as prophetic of 
Dicer atherium were it not for the fact that equally rugose 
areas are found above the orbits and upon the zygomatic 
arches." 
"These two varieties of A. tridactylum are not due to age, 
but may be partl}^ sexual. The molar structure has no con- 
stant dilferences." 
Prof. Osbornf in his original description of A. tridactylum 
says: "The occiput is high and rather narrow" and again 
in a more recent description he says:;]; "In the type the 
nasals are perfectly smooth, (italics mine), but in another 
skull (No. 541) the nasals exhibit a pair of rugosities which 
at once suggest the possession of a pair of horns, and Mr, 
*See Bull, Am, Mus, Nat. Hist., vol. vii, 1895, p. 373. 
tSee Bull. Am. Mus. Nat. Hist. vol. v, 1893, p. 86. 
jSee Bull. Am, Mus, Nat. Hist. vol. vi, 1894, p. 207. 
