THE TEETH OF MAMMALS 



133 



varieties of the pattern of the grinding surface. 

 Sometimes they have long roots, again short (Fig. 

 43), or may be without roots, these differences 

 being due to differences of diet. Those "which 

 subsist on a mixed diet or have a carnivorous 

 tendency (as the true Eats), or eat only soft vege- 

 table substances, as the oily kernels of nuts (as 

 the squirrels), have the molar teeth less compli- 

 cated in pattern and with small tubercles and less 



Fig. 43. — Roentgenogram of jaw of "Woodchuck (Arctomi/s monax), 

 showing long incisor and short-rooted molar. (By Dr. E. H. Skinner.) 



firmly implanted. But those which subsist on 

 hard substances, as the bark or branches or roots 

 of trees, have molars with an arrangement of the 

 tissues like the herbivorous Lngulata, in which 

 hard wear produces a constantly rough surface. 

 Such molars are of more or less continuous 

 growth. In some rodents there are parallel 

 plates of enamel placed transversely of the crown 

 like the molars of the elephant, and like them 



