MAMMALS. 23 



rest while walking only on the tips of the toes, which 

 are sheathed in hoofs (unguligrade). 



The teeth of mammals are wedged into special 

 sockets in the jaw-bones. The structure of a mam- 

 malian tooth is made clear by Fig. 12. We first 

 distinguish a pulp-cavity (p), which in the living 

 animal is fiUed with a substance supplied by a blood- 

 vessel and nerve. This cavity is surrounded by the 

 dentine {d), a hard substance which makes up the 

 greater part of the tooth. Hard enamel (s) covers 

 the whole of the crown in man and many animals, 

 while in certain other forms it is found only on part 

 of the crown. The root of the tooth is covered with 

 cement (z), a bone-like substance. 



All teeth in which the entire surface of the crown is 

 covered by enamel only are known as simple teeth, 

 while those into which the enamel only penetrates in 

 more or less deep folds, leaving the rest of the crown 

 uncovered, are known as compound teeth (Fig. 13). 

 The structure of the teeth is related to the nature of 

 the food. We distinguish three kinds of teeth in the 

 same animal, which, however, are not all present in 

 every species ; these are the incisors, canines, and 

 grinders. The first two kinds are changed ; but only 

 the anterior grinders, known as the premolars, are 

 changed, while the hinder ones, the true molars, do 

 not first appear as " milk " teeth, but rather later on 

 with the other " permanent " teeth. 



The following orders of Mammals are distinguished : 

 I. Bimana (Man), II. Quadrumana (Apes), III. Gar- 

 nivora (Beasts of prey), IV. Insectivora (Insect- 

 eaters), V. Cheiroptera (Bats), VI. Kodentia (Gnawers), 

 VII. Ruminantia, VIII. Solidungula, IX.. Pachyder- 

 mata, X. Cetacea, XI. Edentata, XII. Marsupialia 

 (Pouched animals), XIII. Monotremata. 



I shall deal here only with those orders which are of 

 agricultural importance. 



