262 DEVELOPMENT OF MAMMALIAN TEETH. 



and in most rodents and pachyderms, cTiaracterizing by 

 their varions forms the genera of the last two orders. 



The teeth of the sloths, armadillos, and sperm-whales 

 have no true enamel. The tusks of the narwhal, walrus, 

 and elephant consist of modified dentine, which in the 

 last great proboscidian animal is properly called "ivory," 

 and is covered by cement. 



The forming-organ of a mammalian tooth consists, as 

 in the lower classes, of a pulp and a capsule. The sub- 

 stance of the pulp is converted into the "dentine;" that 

 of the capsule into the "cement." Where enamel is to 

 be added, a peculiar organ is formed on the inner surface 

 of the capsule, which arranges the hardening material 

 into the form, and of the density, characteristic of enamel. 

 This substance is so hard in the tooth of the hippopota- 

 mus, as to "strike fire" like flint with steel. The whole 

 forming-organ is called " matrix." 



The matrix of certain teeth does not give rise during 

 any period of their formation to the germ of a second 

 tooth, destined to succeed the first ; this tooth, therefore, 

 when completed and worn down, is not replaced. The 

 sperm whales, dolphins, and porpoises are limited to this 

 simple provision of teeth. In the armadillos and sloths, 

 the want of germinative power, as it may be called, in 

 the matrix is compensated by the persistence of the ma- 

 trix, and by the uninterrupted growth of the teeth. 



In most other mammalia, the matrix of the first de- 

 veloped tooth gives origin to the germ of a second tooth, 

 which sometimes 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; the mode and 

 direction in which they are displaced and succeeded, viz : 



