1888.] 81 [Cope. 
ing movement to cut a resisting ligament or tendon. The different me- 
chanical movements in the two cases were due to the manipulation of its 
lower jaw by the animals, just as we may see them to-day endeavoring to 
masticate substances in accordance with their hardness, form, ete, It 
would appear in the case of the tritubercular superior molar, that the im- 
pact during the effort to masticate hard and tough substances, as vegetable 
tissues, and seeds, has had its usual effect to stimulate deposit of material. 
The shearing movement has had an opposite effect, viz., that of wearing 
away the surface subjected to it, and the flattening of the sheared face. 
That the development of the grinding mastication should take place in un- 
gulate Mammalia is entirely appropriate to the structure of their digits ; 
the hoofed structure unfitting them for the seizure of living prey. 
In the Amblypoda, however, we have a hoofed order in which the prim- 
itive tritubercular superior and tuberculo-sectorial inferior molar re- 
mained. Of the three families, the latest, the Uintatheriide, display the 
greatest anomaly, while the earliest, the Pantolambdidx (of the Puerco) 
give the simplest known type. It is to the intermediate family, the 
Coryphodontide of intermediate age (the Wasatch Eocene) that we must 
first look for the explanation of the peculiar characters of the order. 
Before doing so I give an explanation of the various mechanical types 
of mastication : 
I. Part or all of inferior molars work between superior molars. Amo- 
bodect mastication. 
1. The inferior molar shears forwards on the superior molar. Proterotome 
mastication ; Creodonta, Carnivora. 
. The inferior molars shear posteriorly against the superior molars. 
Opisthotome mastication ; Coryphodontide, Uintatheriide. 
IL. Molar teeth of both jaws oppose each other. Antiodect mastication. 
3. The movement of the lower jaw is vertical. Orthal mastication; Suo- 
idea, Tapiride. 
4, The movement of the lower jaw is from without inwards. Ectal mas- 
tication ; many Perissodactyla. 
5. The movement of the lower jaw is from within outwards. Ental mas- 
tication ; most Artiodactyla ; some Perissodactyla. 
6. The movement of the lower jaw is from before backwards. Proal; 
most Rodentia. 
7. The movement of the lower jaw is from behind forwards. Palinal ; 
Proboscidia (Ryder). : 
The methods of mastication of Division I may be also defined by the 
terms of Diy. II. Thus the proterotomes are all orthal, and I will show 
that the opisthotomes are also ectal. Some of the orthals are opisthotome, 
as the Tapiride. 
The peculiarities of the Pantodont and Dinoceratous dentition may be 
now taken up in order, and their mechanical causes assigned so far as pos- 
sible. In lminel take the position that the mastication of the Ambly- 
poda was accomplished by the transverse movement of the lower jaw 
PROC. AMER. PHILOS. 800. xxv. 127. K. PRINTED APRIL 4, 1888. 
~*~ 
