VERTEBRATA. 



2S9 



they chiefly live on ; these ridges are formed by an 

 alternating arrangement of the substances which com- 

 pose the crown of a tooth, the polished outside sub- 

 stance, enamel, the dentine, and the cement. The 

 canines are frequently absent, and whether they are 

 absent or not, there is a wide gap between the incisors 

 and premolars. The Marsupial herbivora resemble the 

 true herbivora in the presence of this gap, as well as 

 in the possession of ridged molars (figs. 110, 111, 117). 

 Many ruminants, sheep and cows for example, have 

 no incisors in the upper jaw. 



!Fi^. 111. — Herbivorous dentition of a Marsupial, tlie Great Kangaroo, 

 Jlfacropws giganteus. A, upper and lower jaws j JS, upper, and C, lower molars, 



with ridged crown. Dental formula, t-qJ 2' 



Carnivorous Dentition. — In the Carnivora, 

 however, the molar teeth being formed to tear the 

 tough material of animal flesh, the cusps are very 

 sharp, and those of opposite teeth interlock, instead 

 of opposing one another. There is usually one molar 



