16 



species the tooth-matrix does not develope the germ of a 

 second tooth, destined to succeed the one into which the matrix 

 has been converted ; such a tooth, therefore, when completed 

 and worn down, is not replaced. The Sperm Whales, Dol- 

 phins, and Porpoises are limited to this simple provision of 

 teeth. In the Armadillos and Sloths, the want of generative 

 power, as it may be called, in the matrix is compensated by 

 the persistence of the matrix, and by the uninterrupted growth' 

 of the teeth. 



In most other Mammalia, the matrix of the first-developed 

 tooth gives origin to the germ of a second tooth, which some- 

 times displaces the first, sometimes takes its place by the side 

 of the tooth from which it has originated. 



All those teeth which are displaced by their progeny are 

 called 'temporary,' deciduous, or milk-teeth, (figs. 2 and 3, 

 d, 1...4) ; the mode and direction in which they are displaced 

 and succeeded, viz. from above downwards in the upper, from 

 below upwards in the lower, jaw, in both jaws vertically are 

 the same as in the Crocodile; but the process is never re- 

 peated more than once in any mammalian animal. A con- 

 siderable proportion of the dental series is thus changed ; the 

 second or * permanent' teeth having a size and form as suitable 

 to the jaws of the adult, as the ' temporary' teeth were adapted 

 to those of the young animal. 



Those permanent teeth, which assume places not pre- 

 viously occupied by deciduous ones, are always the most pos- 

 terior in their position, and generally the most complex in 

 their form. The term ' molar ' or ' true molar ' is restricted 

 to these teeth (fig. 2 and 3, m). The teeth between them and 

 the canines are called ' premolars,' (ib. p) ; they push out the 

 milk-teeth, (ib. d,) and are usually of smaller size and simpler 

 form than the true molars. 



Thus the class Mammalia, in regard to the times of form- 

 ation and the succession of the teeth, may be divided into 

 two groups, viz. Monophyodonts 1 or those that generate a 



s, once ; 0tfw, I generate ; oSovs, tooth. 



