MARSUPIALS 837 



and covered with naked skin, which forms a soft, sensitive collar around 

 the region where the bill joins the rest of the skull. The eyes are very 

 small. There is a well-developed but inconspicuous pinna ; the nostrils 

 lie near the end of the upper part of the bill. The tail is flat. 



True teeth, three on each jaw above and below, are calcified, last for 

 about a year, and are then lost, being replaced by horny plates, two on 

 each jaw, above and below. The spur borne on the heel seems to be 

 sometimes used as a weapon, and as it persists only in the males, is 

 perhaps useful in contests between rivals. 



Echidna and Proechidna live in rocky regions, are mainly nocturnal 

 in habit, and burrow rapidly, legs foremost. They feed on ants, which 

 are caught on the rapidly mobile, slender, viscid tongue. No traces of 

 teeth have been seen. 



Strong spines occur thickly in Echidna, more sparsely in Proechidna 

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

 is a distinct pinna about an inch long. The limbs bear five toes, two 

 of which in Proechidna are often without claws and somewhat rudi- 

 mentary. In Echidna the eggs seem to be hatched in a temporarily 

 developed pouch, which is apparently comparable to a much-expanded 

 mamma of the type seen in the cow. 



The Allotheria or Multituberculata include small extinct Mammals 

 (from Triassic to Eocene) with multituberculate molars, e.g. Plagiaulax, 

 Microlesf.es, Tritylodon. They are often classed with the Marsupials. 



Sub-Class Metatheria, Didelphia, or Marsupialia 



With the exception of the American opossums, and 

 a Httle-known mouse-hke animal {Ccenolestes) from S. 

 America, all the Marsupials now alive are natives of 

 Australasia. But fossil remains found in Europe and 

 America show that they once had a wdde 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 competitors had arrived. Thus 

 saved and insulated, the Marsupials have evolved in many 

 different directions. 



The brain is less developed than in Eutherian Mammals, 

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

 missure is large, the corpus callosum is practically 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 ; the jugal reaches far back to share in 

 making the glenoid cavity ; there is practically only one set 

 of teeth ; there are more incisors above than below (except 



