3i8 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



IV. THE MECHANICAL ORIGIN OF DENTAL TYPES. 



In investigating the origin of dental types it is 

 necessary to become acquainted with the nature of the 

 mutual movements of the series of the opposing jaws. 

 I have classified them as follows : x 



I. Inferior molars work within superior molars, but not between 

 them. Psalidodect mastication. 



1. The inferior molars shear on the interior side of the su- 

 perior : Triconodontidcz. 



II. Part or all of inferior molars work alternately to and between 

 superior molars. Amosbodect mastication. 



2. The inferior molar shears forwards on the superior mo- 



lar. Proterotome mastication : Creodonta; Carnivora. 



3. The inferior molars shear posteriorly against the superior 



molars. Opisthotome mastication : 



Coryphodontidce, UintatheriidcE. 



III. Molar teeth of both jaws oppose each other. Antiodect mas- 

 tication. 



4. The movement of the lower jaw is vertical. Orthal mas- 

 tication : Suo'idea, Tapiridce. 



5. The movement of the lower jaw is from without inwards. 

 Ectal mastication : many Perissodactyla. 



6. The movement of the lower jaw is from within outwards. 

 Ental mastication : 



most Artiodactyla; some Perrissodactyla. 



7. The movement of the lower jaw is from before backwards. 

 Proal : 



some Monotremata Multituberculata and most Glires. 



8. The movement of the lower jaw is from behind forwards. 

 Palinal : Proboscidea (Ryder). 



The distinction of teeth into incisors, canines, and 

 molars appears independently at various points in the 

 line of Vertebrata. Incisors and molars are distin- 



IMechan. Origin Hard Parts of Mammalia, 1889, p. 226. 



