The Perissodactyla, 99I 
main of relatively small dimensions, and often primitive form. In 
: the tapir, where the movement in mastication is almost vertical, 
neither set of tubercles attains a remarkable specialization. 
For the three modes of mastication mentioned the following 
_ terms are proposed: Vertical movement (Carnivora, Bunodonta), 
l; from within outwards, Ental; from without inwards, 
a Ectal (from ahew, to grind). 
_ The three lines which originated from the Lophiodontide are 
_ those which terminated in the Equide, the Tapiride, and the 
. Rhinocerontidæ, They differ in the characters of the superior 
Molar teeth as below pointed out. The mechanical cause of 
these peculiarities is not far to seck. In the Equine line mastica- 
Hon has been effected on that side of the mouth where the lower 
f Jaw, in its transverse motion across the superior molars, was mov- 
7 ing from the inside outwards, as in the selenodont Artiodactyla 
(ruminants). In the Tapiroid line there has been little trans- 
_ “erse movement of the lower jaw, so that Y’s have not been de- 
‘ 
_ Weloped in the molars of either series. In the Rhinocerontine 
a mastication has been performed on that side of the mouth 
s : uthe lower jaw was passing from the outside inwards. 
es is the method of mastication of the Rhinoceros to-day, as 
ve by Professor Ryder: The effects of these distinct 
anes of mastication are seen in different forms of the external 
Walls of the Superior molars. Inthe Equine line (Chalicotheriide, 
snodontide, and Palzotheriide) the edges of the ¢xter 
ig are been drawn externally by the ental movement of the 
4. jaw, thus producing the legs of the two Y's. In the Rhi- 
ine the branches of the single large anterior V have 
Teeedes S. Academy Philadelphia, 1879, p. 49. In this paper Professor Rydet 2 
“Medes from hi f $ 
raa t the apices of the cusps which are engaged in primitive mastication, = a 
And lateral ridges. The former fall between the cusps of the opposite 
