No. 2.] THE HARD PARTS OF THE MAMMALIA. 251 
kangaroos, phalangers, and tapirs. In Fig. da theoretical inter- 
mediate movement is represented, such as Ryder supposed to 
have characterized the Anchitherium. In e the usual move- 
ment among ruminants is depicted, as is seen in the deer, etc. 
SUE ge 
a Hal \ 
abe d 
f 
Figure 75.— Diagram of excursions of lower jaw in mastication; from Ryder. 
In f the wider excursion of the jaw is that seen in the giraffe, 
camel, and ox. In these movements from @ to f, the lower jaw 
is moved transversely across the upper jaw from one side to the 
other.t Some of the Diplarthra masticate on one side of the 
jaw when performing this movement, and some on the other. 
That is, in passing the lower 
jaw across the face of the 
upper, some masticate the 
food on the side where the 
external face of the lower 
jaw crosses the upper jaw 
from within outwards. (en- 
tal) ; while in other types the 
food is masticated on the side 
where the lower jaw passes 
the external edge of the up- @*---------..7_MWA 
per jaw from without inwards 
(ectal). While masticating figure 76. — Cervus, molars: a, superior, 
: : ‘ external view; 4, do. inferior view; c, infe- 
with one side of the LENS the rior molars, superior view; from Ryder. 
opposing dental series of the 
other side are not in contact. All mutual effect of the teeth 
of one jaw on the other could therefore appear on the side 
1 Ryder was at first of the opinion that the grinding process was produced by the 
movement from within outwards in all cases; Zc, p.65. He afterwards observed 
that the movement was the reverse in the rhinoceros, or from without inwards, and 
he then changed his former opinion and regarded the latter movement as universal. 
I have shown that both Ryder’s observations are correct, and applicable to different 
groups. 
