252 COPE. IVGLe It. 
temporarily used for mastication only. Among recent Ungu- 
lata the ruminants present the ental mastication ; the rhinoce- 
ros and horses, the ectal ; and rodents and proboscidians, the 
proal and palinal respectively. 
When the crests of the inferior molars were developed, their 
relation to the crests of the superior molars was always anterior 
in mastication. That is, the inferior crest, in the closing of the 
jaw, collides with the crest of the upper molar, with its posterior 
edge against the anterior edge of the latter. This is because: 
first, as to position, the two anterior cusps of the lower molar 
are the remains of the anterior triangle which fit originally be- 
ine 
a 
v 4 \ 
Figure 7’7.— Cusps of superior premolars and molars: @, external cusp of molar of 
Sarcothraustes; 4, of Phenacodus; c, of Anthracotherium; ¢, of Oreodon; e, half of 
inferior molar of Cervus; /, superior premolar of Coryphodon; from Ryder. 
tween two superior molars, and because, in the closing of the 
jaw, these cusps continue to hold that position ; and second, as 
to function, because the canine in the ungulate series diminishes 
in size, and does not, therefore, draw the inferior molars forwards 
as in the Carnivora, but allows free scope to the posterior trac- 
tion of the temporal muscle in its exercise on the lower jaw. 
In those forms which masticate from the inside outwards, the 
cusps of the inferior molars, passing between those of the supe- 
rior molars, would tend to flatten the sides on which they exerted 
friction, and to extend those sides outwards beyond the median 
apex of the cusp. The result would be, and taking into view the 
yielding of the tissue to such strain, has been, to modify the 
shape of the cusp by pushing its side walls, so that a horizontal 
section of it would become successively more and more cres- 
centic. The effect on the inferior teeth would be to produce 
the same result in their external cusps, but in the opposite direc- 
tion. The sides of the cusps would be pushed inwards, past 
the apex, giving a crescentic section more or less perfect, as the 
operation of the cause had been of long or short duration. The 
