Dlcerathen'um Proavitum. — Hatcher. 315 
Hatcher has recently shown that this species is followed by 
another, related to the John Day genus Diceratherium Marsh. 
The distinctive features of the skull are the high, narrow 
occiput and powerful sagittal crest, etc." 
From the above it will be seen that the type of A. tridactti- 
Ui,m belongs to the series, with skulls characterized by Prof. 
Osborn as high, long and narrow and in which the "nasals 
are perfectly smooth " ; while the type of D. proavitum per- 
tains to the other series with skulls characterized by Prof. 
Osborn as shorter, lower and broader and with prominent 
rugosities upon the nasals. 
As shown by the above quotation Prof. Osb(»rn has stated 
that these two varieties are not due to age, but that the}^ ma}' 
be partly sexual. In view of the fact that we do not find in 
the skulls of recent rhinoceroses or other perissodactyls simi- 
lar sexual variations, I think it fair to conclude that these 
variations are not sexual, but of generic and specific import- 
ance. I therefore retain Diceratherium proavitum as distinct 
both generically and specifically from Aceratherium tridaefy- 
lum Osborn; unless we abolish altogether the genus Dice- 
ratherium and with it Aceratherium and call them all Rhin- 
oceros as has been done by Flower and Lydekker,* but which 
does not seem advisable. 
Prof. Osborn's objection to considering the rugosities on 
the nasals of D. proavitum, as indicative of horns because he 
finds similar rugosities on the zygomata and over the orbits 
can scarcely be considered tenable, since the latter rugosities 
are not in a position at all relative to that known to have 
been occupied by horns in later forms from the John Day 
beds, and very similar rugosities may be seen on the zygoma- 
ta and over the orbits in the recent rhinoceroses which are 
known to bear horns on the nasals. 
Aside from the rugosities, there are other evidences even 
more in favor of considering them as having borne horns ; 
such as the great thickening of the nasals (shown in fig. la) 
in order to give them the necessary strength to support the 
horns, and the low, short and broad sagittal crest as shown 
in fig. 1,. Furthermore, the geological horizon (Protoceras 
beds) in which the type was found is just that in which we 
*See Mammals Living and Extinct, pp. 410-411. 
