226 COPE. [Vot. III. 
I. Inferior molars work within superior molars, but not between them. 
Psalidodect mastication. 
1. The inferior molars shear on the interior side of the superior. Tricono- 
dontide. 
II. Part or all of inferior molars work between superior molars. Amce- 
bodect mastication. 
2. The inferior molar shears forwards on the superior molar. Proterotome 
mastication . é : . Creodonta; Carnivora. 
3. The inferior molars chest pasteriony against the superior Beene Opis- 
thotome mastication : : . Coryphodontide, Uintatheriide. 
III. Molar teeth of both jaws oppose each other. Antiodect mastication. 
4. The movement of the lower jaw is vertical. Orthal mastication ; 
Suoidea, Tapiride. 
5. The movement of the lower wee is from without inwards. Ectal mastica- 
tion . : : . many Perissodactyla. 
6. The movement of aes jower jaw is porn within outwards. Ental mastica- 
tion . A . most Artiodactyla; some Perissodactyla. 
Vie The movement of the lowers jaw is from before backwards. Proal; 
most Bonewaa: 
8. 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 Division II. Thus the proterotomes are all 
orthal, and the opisthotomes are ectal. Some of the orthals are 
opisthotome, as the Tapiridez. 
4. ORIGIN OF THE TRICONODONT MOLAR. 
The triconodont molar is the earliest example of a process 
which has been frequent in the history of the Mammalia, viz. : 
the appearance.of cusps or folds at the base of the crown of the 
tooth. It is, in the case of the early Jurassic Mammalia which 
exhibit it, the Triconodontidz, intimately connected with the 
final distinction of the roots of the molars. The ancestors of 
this family are probably the Dromotheriidz of the Trias, in 
which the separation of the roots is only indicated by a groove,! 
a state of things already observed in a species of the Permian 
genus of Reptilia, Dimetrodon (Cope). The three or four small 
denticles of Dromotherium, are replaced by one in front of and 
one behind the principal cusp, in Triconodon. No mechanical 
cause can be assigned for the development of these cusps, but 
1 See Osborn, American Naturalist, Dec., 1888. 
