632 CHORD AT A. 



EcHiDNiD^, The spiny aut-eaters have the body covered with bristles, 

 snout with a wonn-shaped tongue used iii catching insects; Echidna 

 acideala of Australia, feet five-toed, with digging claws ; ProecJiidua 

 (Acantlioglossus) of New Guinea, three-toed. Ornithorhynchid^. The 

 duckbills are toothless, close-haired animals witli lioruy jaws which 

 resemble those of a duck; the flve-toed feet with a swimming web especially 

 well developed on the fore feet. Ornithorhynelius paradoxus of Australia. 



m- 



Fig. 6.56. — Ornithorhynelius paradoxus^ duckbill- (From Schmarda.) 



The male has a spine with a gland on the hind feet which fits in a corre- 

 sponding pit on the thigh of the female and apparently plays a role in 

 copulation. 



The oldest fossil mammals are possibly to be regarded as belonging to 

 the monotremes. These appear in the trias and form a group, MULTITU- 

 BEECULATA (Allotheria), which is but imperfectly known {Trityhxion, 

 Microlestes, Plagiaulax). Their multitubercular teeth resemble the tempo- 

 rary ones of OrnithorhyivJuts, while there are indications that the cora- 

 coid existed as a distinct bone. Less certain are the Protodonta (Broma- 

 therium, Microconodon) of the American Jurassic, of which only the lower 

 jaws are known. 



Sui Class IT. JfarsKpialia {DideJphin). 



These, like the remaining mammals, are viviparous. They 

 have small eggs which undergo a total segmentation in most species, 

 and developi in the maternal uterus, being nottrished by a secre- 

 tion from its walls. In a few species there is a placenta which, in 

 Peranu'lcs, is allantoic in origin, in l>asi/nri(S I'ivcrri/uis possibly 

 also from the yolk sac. In most species there is no jilacenta. In 

 all there is insufficient nourishment and the young are born in a very 

 immature condition. They are therefore carried a long time by 

 the mother in the marsuiiium, a j^ouch formed by a fold of skin 

 on the posterior ventral surface, into which the nipi)les open. 

 The ventral surface is supported by the marsupial bones, slender 

 rods articulated, right and left, at the pubic symphysis. Other 

 characteristics of the marsupial skeleton are the inflected posterior 

 angle of the lower jaw (fig. (357, a) and the rudimentary replace- 

 ment of teeth. The milk teeth and molars (first dentition) are 

 as a whole retained, only premolar :i being replaced by another 



