QUADRUPEDS. 169 



Miccessive renewals take place at intervals of several 

 years. 



The teeth are usually divided into three kinds, from the 

 position which they occupy in the jaw. The front teeth, 

 or Incisor es, are four in number in each jaw, in Man ; and, 

 in the lower animals, however numerous, they are distin- 

 guished by their insertion in the intermaxillary bone in the 

 upper jaw ; or, in the lower, by their opposition to the in- 

 termaxillary teeth or bone. The Tusks are situated im- 

 mediately behind the incisors, and, in man, are four in 

 number, or two in each jaw. They are likewise termed 

 Denies canini, or cuspidati. The grinders, or molares^ oc- 

 cupy the back part of each jaw. These different kinds of 

 teeth usually exhibit well-marked peculiarities. The inci- 

 sors have, in general, an even sharp summit, suited for 

 cutting. In order to preserve their sharpness, the outside 

 only, in some species, as the hare, is covered with enamel, 

 which, being less subject to wear than the inner layer of 

 ivory, is kept constantly fit for use. The tusks are in ge- 

 neral conical. The crown of the grinders is more extend- 

 ed. It is uneven, and wholly covered with enamel, in the 

 carnivorous kinds ; while, in the herbivorous, it is flat, or 

 the enamel partially distributed. 



The mammiferous animals admit of subdivision, from tlie 

 circumstances connected with the condition of the foetus. In 

 one extensive tribe, the foetus is nourished directly by the 

 aerated blood of the mother, received through a placenta ; 

 while, in the other, the foetus derives its nourishment fj-om 

 the absorption of the glairy fluid with which it is surrounded. 



PLACENTARIA. 



Char. — Uterus furnished with a placenta. 



The nature of the foetal connection with the mother has 



