460 JNCISOR AND MOLAR TEETH CHAP. 
grow to a very great length, extending back within the jaw to 
near the hinder part of the skull. These teeth are reinforced in 
the upper jaw by a small second pair in the Lagomorpha only. 
The incisors are chisel-shaped, and often brown or yellow upon 
the outer face, as is the case also with some Insectivores. This 
peculiar shape, and their strength, renders them especially capable 
of the gnawing action which characterises the Rodents. It has 
been pointed out that where the incisors are wider than thick, 
the gnawing powers are feebly developed; and that on the con- 
trary, where these teeth are thicker than wide, the animals are 
good gnawers. The incisors have often an anterior groove, or it 
may be grooves. 
Fro. 232.—Molar teeth of Rodents. A, of Capybara (ydrochoerus) ; B, of Squirrel 
(Sciurus) ; C, of Ctenodactylus. (After Tullberg. ) 
The cheek teeth vary in nwnber from two (Hydromys) to six 
(Rabbit) on each side of the two jaws. Four is the prevailing 
number outside the large division of the Rat-like Rodents. 
They are often set at an angle to the horizontal plane of the jaw, 
looking outwards and obliquely to its longitudinal axis; the in- 
dividual teeth too are not unfrequently bowed in form, remind- 
ing us of those of Z'owodon. This of course only occurs in those 
genera which have hypselodont teeth. The pattern of the teeth 
varies much, and the different forms recall the teeth of more 
than one other group of mammals. They are either bunodont 
or lophodont. In many cases the tooth is encircled with a ridge 
of enamel, which is either almost simple or has a more com- 
pleated contour; such teeth are by no means unsuggestive of the 
Toxodonts. Some of the lophodont molars are by no means 
unlike those of the Proboscidea. In Sciurus vulgaris the 
