No. 2.] THE HARD PARTS OF THE MAMMALIA. 257 
true genus Rhinocerus, after passing through intermediate 
stages. In the first of these it extends in a nearly straight line 
anteriorly, as in Aphelops, etc. In later genera its extremity 
joins a short internal process of the external wall, ¢,g. in Rhi- 
nocerus, but it is evident that the crescent thus formed has not 
the character of that which is developed in the genus Equus. 
The essential peculiarity of the superior molars of the Rhi- 
nocerontine line is, that the external wall of the crown does 
not develop V’s. This peculiarity is seen in some of the Lophio- 
dontine ancestry, especially in the genus Hyrachyus (Leidy). 
Ryder has already shown that in Rhinocerus the mastication is ° 
ectal, but he did not apply this fact to the explanation of the 
figure 80.— Hyrachyus agrestis Leidy, superior molar teeth (ectal type) from 
below; Eocene of Wyoming; from Leidy. 
absence of V’s in the superior molars in this line. I suspect 
that the ectal mastication commenced in some of the Lophio- 
dontide. The appearance of the anterior external cusp in 
Lophiodon and Hyrachyus renders this probable. In these gen- 
era the anteroposterior edges of that cusp are drawn zzwards, 
as would result from the ectal mastication, and not outwards, as 
results from the ental (Fig. 80). It presents a narrow V, with 
the apex outwards. The posterior part of the external wall of 
the molar is, however, somewhat flexed inwards, and in some 
types gives the appearance of an open V directed in the oppo- 
site direction from the anterior V. This at once suggests 
ental mastication, and introduces the question whether the ex- 
planation offered for the origin of the anterior V is correct. 
Examination shows that this posterior V is not entirely homol- 
ogous with the’ posterior V of the ental dentition, but is ho- 
mologous with the entering angle between the external cusps, 
directed inwards instead of outwards. This is confirmed by the 
fact that its apex, in the Eocene genera mentioned, the point 
