632 CHORDATA. 



ECHIDNID.E. The spiny ant-eaters have the body covered with bristles, 

 snout with a worm-shaped tongue used in catching insects; Echidna 

 aculeata of Australia, feet five-toed, with digging claws ; Proechidna 

 (Acanthoglossus) of New Guinea, three-toed. ORNITHORHYNCHID.E. The 

 duckbills are toothless, close-haired animals with horny jaws which 

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

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



Fio. 656. Ornithorhynchus 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- 

 BERCULATA (Allotheria), which is but imperfectly known (Tritylodon, 

 MicrolesteSj Plagiaulax). Their multitubercular teeth resemble the tempo- 

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

 coid existed as a distinct bone. Less certain are the PROTODONTA (Droma- 

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

 jaws are known. 



Sub Class II. Marsupialia (Didelphia). 



These, like the remaining mammals, are viviparous. They 

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

 and develop in the maternal uterus, being nourished by a secre- 

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

 Perameles, is allantoic in origin, in Dasyurus viverrinus possibly 

 also from the yolk sac. In most species there is no placenta. 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 marsupium, a pouch formed by a fold of skin 

 on the posterior ventral surface, into which the nipples 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. 657, a) and the rudimentary replace- 

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

 as a whole retained, only premolar 3 being replaced by another 



