308 HOMOLOGIES OF THE TEETH. 



of the deciduous series unusually long retained, or the 

 unusually small and speedily lost successor {p 1) of an 

 abortive {d 1). 



The law of development, so beautiful for its instructive- 

 ness and constancy in the placental diphyodonts^ is well 

 illustrated in the little hyrax, in which the c? 1 is normally 

 developed and succeeded by a permanent, p 1, differing 

 from the rest only by a graduated inferiority of size, 

 which, in regard to the last premolar, ceases to be a dis- 

 tinction between it and the first true molar. 



The elephant, which by its digital characters belongs to 

 the odd-toed, or perissodactyle, group of pachyderms, also 

 resembles them in the close agreement in form and struc- 

 ture of the grinding teeth representing the premolars, with 

 those that answer to the true molars of the hyrax, tapir, 

 and rhinoceros. The gigantic proboscidian pachyderms 

 of Asia and Africa present, however, so many peculiari- 

 ties of structure as to have led to their being located in a 

 particular family in the Systematic Mammalogies. And 

 this seems to be justified by no character more than by 

 the singular seeming exception which they present to the 

 diphyodont rule which governs the dentition of other 

 hoofed quadrupeds. In fact, the elephant, like the du- 

 gong, sheds and replaces vertically only its incisors, 

 which are also two in number, very long, and of constant 

 growth, forming tusks, with an analogous sexual differ- 

 ence in this respect in the female of the Asiatic species. 

 The molars, also, are successively lost, are not vertically 

 replaced, and are reduced finally to one on each side of 

 both jaws, which is larger than any of its predecessors. 

 These analo2;ies are interesting^ and suo-o-estive in connec- 



O O CO 



tion with the other approximations in the " Sirenia" to 

 the pachy dermal type. 



