4o6 



MAMMALS. rUYllM i. H O K P A T A 



resemble the living sqiiirrols. Soiuo marsupials have four or five 

 incisors in each quarter of the jaw. and others are notable for 

 * svndactvlv *. a condition in wluch the second and third digits of 





Fig. 3S8. — Dentary of kang.n 00, inner surface, showing the uitlected angle, 



the hind-limb are small and bound toi^ether in a comnion sheath. 

 In the opossums {Didelphis], which are arboreal, this appears 

 to be because the hallux, which is opposed to digits four and 

 five so that the foot can grasp a bough, has squeezed them 

 nearly out of existence. The presence of syndactyly in ground 

 forms like the kaui^aroos. which have no hallux, is held to sui^gest 

 that thev had arboreal ancestors. 



MONOTREMATA 



There remain the Prototheria or Monotremata, of which there 

 are only two genera. EchiJnii, the spiny ant-eater, and Oruitho- 



FiG. 3S9. — The Duckmole {OrmthorhyHck\4s). — From Thomson. 



rhynchus, the duck-billed platypus (Fig. 389), both confined to 

 Australasia. They have the characteristic mammalian features of 

 hair, diaphragm, and the rearrangement of the lower jaw bones. 



