IV. VERTEBRATA: MAMMALIA, MARSUPIALIA 559 



termination of the ureters in the urogenital sinus (fig. 604), the existence 

 of a cloaca in both sexes, and the specifically bird-like character of the 

 female sexual organs, in which the large left ovary is alone functional, 

 and uterus and vagina are not differentiated. But with all this it 

 must not be forgotten that the monotremes have the hair, the skull 

 (ear bones, articulation of jaw), the urogenital sinus of true mammals; 

 and in the presence of marsupial bones connected with the pelvis (fig. 

 606, Om) show a close relationship with the marsupials. The jaws are 

 toothless and enclosed in horny sheaths, yet in the young of Ornilhor- 

 hynchus there are in each jaw three pairs of multitubercular molars, 

 which are later replaced by four broad horny plates. In the embryo 

 Echidna tooth germs are formed and also a dentine egg-tooth for opening 

 the shell. 



ECHIDNID^. The spiny ant-eaters have the body covered with spines, 

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

 feet five-toed, with digging c]s.ws; Proechidna, New Guinea, three-toed. Ornith- 

 ORHYNCHID^. The duckbills are toothless, close-haired animals with horny 

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





Fig. 607. — Ornithorhynchus paradoxus, duckbill (from Schmarda). 



especially well developed on the fore feet. Ornithorhynchus paradoxus of 

 Australia; male with a spine and gland on hind feet which fits in a corresponding 

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



The oldest known fossil mammals are possibly related to the monotremes. 

 They appear in the trias and form an imperfectly known group, MULTITU- 

 BERCtfLATA {Tritylodon, Microhstes, Plagiatilax) . Their multitubercular 

 teeth resemble the temporary ones of Ornithorhynchus, while there are indications 

 that the coracoid existed as a distinct bone. Less certain are the Protodonta 

 {Dromatherium, Microconodon) of the American Jurassic, of which only the 

 lower jaws are known. 



Siib Class II. Marsupialia {Didelphia). 



These, like the remaining mammals, are viviparous. They have 

 small eggs which undergo a total segmentation in most species, and de- 

 velop in the maternal uterus, being nourished by a secretion from its walls. 

 Only in Perameles is there a placenta in which the allantois is so intimately 

 connected with the uterine wall that an exchange of fluids between the two 



