80 MAMMARY APPARATUS OF THE MAMMALIA 



animals. But this similarity rests solely on 

 convergence. 



With the establishment of these facts two 

 old misapprehensions naturally fall to the 

 ground. We have first of all to give up the 

 idea that the marsupium represents a formation 

 inherited from the Monotremes ; and, in the 

 second place, we can no longer hold that all 

 Didelphia originally possessed a marsupium, 

 and that in all those forms where it is now 

 absent the pouch has been secondarily lost. 

 Forms certainly do exist where the marsupium 

 has secondarily disappeared, as, for example, 

 Myrmecobius amongst the Dasyuridag, where, 

 during ontogenesis, the primordia of the mar- 

 supial pockets and of a typical marsupium are 

 to be observed, though they disappear relatively 

 early. Moreover, in spite of this, a sphincter 

 marsupii is formed, and persists in the adult. 

 On the other hand, when we proceed now to 

 examine the pouchless Didelphyidfe, we find in 

 development, so far as known, no rudiments of 

 a pouch, whilst the sphincter marsupii is also 

 absent, even in the adult. We have therefore 

 every reason to suppose that they have never 

 possessed a pouch, and we are justified in 



