254 COPE. gore the 
inferior molars with especial force against the external edge of 
the superior molars in passing or meeting them. In this way 
the external cusps in both jaws sustain the greater part of the 
strain and friction in mastication. Thus it is, I suspect, that 
the external cusps have acquired such pre-eminent dimensions in 
this order. They have also sustained the same lateral shearing 
from within outwards that the internal crests have experienced 
in the selenodont Artiodactyla, and with the same result, their 
gradual modification into crescents. The absence of such shear- 
ing pressure between the internal cusps has resulted in their 
retaining their primitive conical form to a comparatively late 
geological age, as is seen in all forms of the order except the 
genera Hippotherium and Equus. 
The peculiar form of the mandibular condyle of Hyracotherium 
is visible in a less degree in Anchitherium and in Symborodon. 
It is still less visible in Equus, and is quite wanting in Tapirus 
and Rhinocerus. But the latter have retained the dental char- 
acter inherited from the Hyracotheriinz. 
figure 79. — Hyracotherium venticolum Cope; mutual relations of molars in 
mastication. The space bordered by light lines the superior masticating surfaces; 
those by heavy lines, the inferior; C, canine. Lettering as in Fig. 58. 
It has been already pointed out that three lines of descent 
diverge from the Hyracotheriinz, viz.: the Rhinocerotine, the 
Tapirine, and the Equine. The origin of the dental peculiar- 
ities of these may be now considered. 
In the Hyracotheriinz the intermediate tubercles of the supe- 
rior molars, derived from the Condylarthra, remain, and assume 
an important part in the construction of the crown. In the 
Artiodactyla, on the contrary, they disappear. This is possibly 
due to the crowding in a transverse direction which the crowns 
of the molars experience in that suborder. In the Perissodac- 
tyla they may have one of two positions: they are either in the 
transverse line with the other cusps, or they alternate with 
