g LLOYD’S NATURAL HISTORY. 
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 
all, 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 
