CHAPTER XVIII 



THE MONOTREMES 



Extreme antiquity and specialization of both existing fami- 

 lies — The Spiny Anteater — Food habits and dentition of 

 the Duck-billed Platypus — No evidence of close relation- 

 ship between existing Monotremes and Multituberculata. 



Since one of the existing Monotremes, the Spiny Anteater 

 of Australia and New Guinea is entirely toothless like the 

 placental Anteaters and Pangolins, and since the other, the 

 Duck-billed Platypus of Australia and Tasmania has teeth 

 which are merely transitory, this order is not of great signifi- 

 cance for our immediate purpose. 



Many archaic and reptilian features are retained in the 

 anatomy of these animals but nothing is known of their actual 

 fossil history which must therefore be inferred from the study 

 of modern forms alone. The characters of the shoulder girdle 

 indicate that Monotremes originated from very early Mam- 

 mals not far beyond the reptilian stage and certainly before 

 the diverging marsupio-placental forerunners had begun to 

 adopt an arboreal existence. During the lengthy period since 

 their origin, very marked peculiarities have appeared in both 

 animals which must therefore be considered separately from 

 even the primitive marsupio-placental stock. Certain bodily 

 features however occur in both Spiny Anteater and Duck- 

 bill which in addition to the geographical distribution of the 

 animals indicate a common if very remote ancestry for the 

 Monotremes and Marsupials. Beyond this we cannot proceed 

 until further evidence is available. 



Ignoring then the edentulous Spiny Anteater the skull and 



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