﻿8 Allen's naturalist's library. 



in their development, although the hinder pair are invariably 

 the larger of the two, and form the chief agents in progres- 

 sion ; the greatest development of the hind limbs occurring 

 among the Kangaroos. As a rule, there is a well-developed 

 tail, this appendage being generally long and tapering, in 

 some cases prehensile, and at others forming an additional 

 support to the body. Great variation obtains in regard to the 

 form and number of the teeth, which may be adapted either 

 for a carnivorous, insectivorous, or herbivorous diet, although 

 they are always divided into incisors, canines, premolars, and 

 molars. There is, however, a marked peculiarity in regard to 

 the succession of the teeth, which may be considered as 

 characteristic of the order. Thus, instead of the whole, or 

 nearly the whole of the first set of teeth in advance of the true 

 molars or hindmost cheek-teeth, being replaced by a second set 

 of permanent teeth, only one tooth is thus replaced, and that 

 not invariably so. The tooth thus replaced has been hitherto 

 generally regarded as corresponding to the last or fourth milk- 

 molar of the higher Mammals, while the apparently replacing 

 tooth has consequently been identified with the last or fourth 

 premolar of the same. Recent researches have, however, 

 tended to show that this is not a true case of replacement at 

 ail, and that the tooth which makes its appearance late in life 

 is really a retarded premolar, which will consequently be the 

 fourth of the full series, while the apparently replaced tooth is 

 really the fifth. Be this as it may, the mode of succession is 

 peculiar and unique ; and it may be still convenient to speak 

 of the replacing tooth as the fourth premolar, and the one it 

 displaces as the fourth milk-molar. It is worthy of mention, 

 as an admirable instance of adaptation to peculiar conditions, 

 that during the period while the helpless young are adherent 

 to the teats of their parents their breathing organs are specially 

 modified in order to prevent the danger of choking by the 



