VERTEBRATA. 285 



merely clothing tlie roots of the tooth, but in the 

 molars of ruminants may cover the sides of the tooth, 

 and, until it has been worn off by use, the crown also. 

 The part of a tooth which lies above the gum is 

 called the crown, the part below the gum is the root 

 or fang ; the centre is occupied by the pulp cavity, 

 which affords passage for the nerves and blood- 

 vessels. 



iMg, 103. — Succeseion of teeth in mammalia. .Jaws of an Ape (Ceftws) while 

 changing its teeth, showing permanent teeth (marlced with capitals), under the 

 mUk teeth (marked with small letters). (From Glaus and Sedgwick, after 

 Owen.) 



Some kinds of Mammalia never have but one set 

 of teeth : these are called monophyodont, i.e. pro'ducing 

 one set of teeth only. It is a rare arrangement, found 

 among the sloths and in some of the whales. Most 

 mammalia are diphyodont, that is to say, they produce 

 two sets of teeth, the first, or milk teeth, giving, way 

 to a second set, as the animal attains its adult age. 

 This second set are usually more numerous, the, in- 

 crease taking place, of course, at the outside of the 



