500 CHORD ATA. 



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 

 is 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 cutting 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 Eutheria^ 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 syndactylism. It is not 

 a true metatherian character, as it is only found in the 

 Diprotodontia and one family of the Polyprotodontia 



