684 MAMMALIA. 



Strong spines occur thickly in Echidna, more sparsely in Proechidna 

 among the hairs. The snout is prolonged into a slender tube. The 

 limbs bear five toes, two of which in Proechidna are often without claws 

 and somewhat rudimentary. In Echidna^ the eggs seem to be hatched 

 in a temporarily developed pouch. 



Sub-class Metatheria or Didelphia, — Order Marsupialia. 



With the exception of the N. American opossums, all 

 the Marsupials now alive are natives of Australasia. But 

 fossil remains found in Europe and America show that they 

 once had a wide range. As there are no higher mammals 

 indisputably indigenous to Australasia, it seems as if the 

 insulation of that region had occurred after the Marsupials 

 had gained possession, but before higher mammalian com- 

 petitors had arrived. Thus saved and insulated, the 



Fig. 247. — Lower jaw of Kangaroo. 

 a., Inflected angle ; /., single incisor. 



Marsupials have developed in many different directions, 

 the families included in this order being very diverse. 



General Characters of Marsupials. 



The brain is less developed than in placental Mammals, 

 for the convolutions are simple or absent, the anterior com- 

 missure is large, the corpus callosum is small or even absent. 

 In the skeleton there are several peculiarities ; thus the 

 angle of the lower jaw is more or less inflected, except in 

 the genus Tarsipes ; there are generally two epipubic or 

 " marsupial " bones in front of the pubic symphysis ; there 

 are more incisors above than below (except in the wombat), 

 and the number of incisors sometimes exceeds three on 

 each side. 



