ZO) CONDYLARTHRA CHAP. 
here collected it is clear that a direct effect is produced. If we 
are to regard horns as secondary sexual appendages which have 
been subsequently handed on to the female by heredity, we should 
expect to meet with examples of animals now horned in both 
sexes, of which the earlier representatives had the horns confined 
to one sex. This is most interestingly shown by the extinct and 
Miocene Giraffe, Sumotherium, of which the male alone had a pair 
of short horns, while the skull of the female was entirely hornless ; 
the modern Giraffu, as is well known, has horns in both sexes. 
It is interesting to note that the existing Perissodactyles and 
Artiodactyles are to be distinguished by their unpaired or paired 
horns. But while there are no Artiodactyles with unpaired horns 
(save occasional sports) the Perissodactyles have more than once 
tried, so to speak, paired horns, which ultimately proved fatal 
to them. The Rhinoceros Diceratheriwm apparently inherited and 
unproved upon the small paired horns of Aceratherium, but it has 
left no descendant. The paired horned Titanotheria offer another 
instance of the same apparent incompatibility between the Perisso- 
dactyle structure and the persistence of paired horns. 
SUBZORDER Ly CONDYLARTHERA: 
This group is characterised by the following assemblage of 
characters. Extinct, often plantigrade Ungulates, with five-toed 
limbs. Bones of carpus and tarsus not always interlocking, but 
sometimes lying above each other in corresponding positions. 
The humerus has an entepicondylar foramen. Dental formula 
quite complete; the molars brachyodont and bunodont. The 
premolars are simpler than the molars. The canines are small. 
As with other early types, the zygapophyses are flat and do not 
interlock. The astragalus is like that of the Creodonta. This 
group was American and European in range, the remains of its 
rather numerous genera being of Eocene time. The best-known 
genus is Phenacodus, of which some account will be given before 
discussing the, in many cases, more fragmentary remains of 
other allied forms. 
The genus Phenacodus was first described so long ago as 1872, 
from a few scattered teeth. Since then several nearly complete 
skeletons have been obtained, and we are in full possession of 
