No: 22] THE HARD PARTS OF THE MAMMALIA. 253 
result of the lateral movement in mastication may be under- 
stood by reference to the accompanying cut. The external 
crescents (c) of the inferior molars (4) are seen to pass between 
the internal crescents (@) of the superior molars (a). The mu- 
tual interaction and effect : 
on the form of the cres- 
cents may be readily un- 
derstood. In Fig. 77 the 
successive stages of this 
effect on one or two cusps 
may be seen, beginning 
with a cone (a) and termi- 
: : Figure 78. — Two true molars of both jaws 
nating with crescents of a ruminant: @, superior molars; 4, their inner 
(ef). Thus is the origin crescents; 4, inferior molars; c, their external 
of the selenodont denti- crescents; ef, directions of motion of jaws in 
fioeon thc highest eee mastication; from Ryder. 
odactyle explained by Ryder, and, I believe, correctly. 
The problem of the origin of the Perissodactyle types of molar 
dentition is somewhat different.1. The peculiarity of the supe- 
rior molars of Perissodactyla, as compared with Artiodactyla, is 
that the interior tubercles are of less elevation and complexity 
than the exterior. Of less importance is the fact that in the 
majority of forms, in the lower jaw, the internal cusps alternate 
‘with the external cusps instead of standing opposite to them as 
is the case in Artiodactyla. With the exception of the Tapiridz 
and some Lophiodontidz, the Perissodactyle inferior molars are 
amoebodont. The two arrangements are produced by different 
types of superior molars, but the modifications have been 
brought about in the same way in both antiodonts and amcebo- 
donts. , 
In the Hyracotheriina, the ancestral type of all Perissodac- 
tyla, the mandibular condyles are strongly convex, transversely 
as well as anteroposteriorly. The effect of this structure on 
mastication is to allow the ramus of the mandible to twist as it 
rises at the end of its transverse excursion. In other words, the 
contractions of the masseter and temporal muscles throw the 
1 Ryder has supposed, after examining the superior molars of Symborodon, that 
the modifying force has acted from without inwards, and has applied this view to all 
Diplarthra. I believe this to have been impossible, and that his first view is the cor- 
rect one, 
