500 CHORDATA. 
replaces its antecedent “milk” tooth, so that only one 
premolar persists. 
Another peculiar feature is that a younger kangaroo with 
the milk premolar not yet replaced also has only five teeth 
in all, because the last molar does not appear till the first 
premolar has dropped out. 
Thus, although the old kangaroo has only five back 
teeth, of which the first is the second premolar and the 
other four are the molars, the dental formula of the species 
1s 3.0.2.4, because another premolar has been lost during 
life in front of the remaining teeth. The lower jaw has 
one long incisor on each side which has a cutting edge 
down each side. The two rami of the mandible are 
bound by ligament only, which permits a movement of 
one ramus upon the other. When the two posterior ends 
of the rami are approximated the incisor teeth diverge 
and cut any substances between them and the upper 
incisors of each side. On divergence of the two pos- 
terior ends the two incisors come together like the blades 
of a pair of scissors and sever any substances lying between 
them. Hence the kangaroo differs from the sheep and 
horse in cu¢ting its forage rather than breaking it. There 
are no canines and the premolars and molars resemble 
those of the upper jaw. The inflected angle is another 
metatherian character. 
In the vertebrae the chief feature to notice is the presence 
of chevron-bones in the tail. These hang down under the 
vertebrae and are usually present only in those mammals 
which have a highly-developed tail. The fore-limb is small 
and has five complete digits with claws. The shoulder- 
girdle is closely similar to that of the Zutheria, the coracoid 
and precoracoid elements being only represented by 
vestiges. The hind-limb usually has only four digits, the 
hallux or big toe being lost. Of the remainder, the fourth 
is very large and strong, with a powerful claw; the fifth is 
smaller and the second and third are reduced to attenu- 
ated remnants. These two are united together in one 
flap of skin from which the two little claws protrude. This 
very peculiar condition is known as syxdactylism. It is not 
a true metatherian character, as it is only found in the 
Diprotodontia and one family of the Polyprotodontia 
