THE I'LACENTATION OF PERAMELES. 435 



iudeed, the evidence on all hands goes to show that the Dij)ro- 

 todontia represent a comparatively recent offshoot from a 

 primitive polyprotodont stem. And we are entirely unable to 

 accept the derivation of the Eutheria from a Perameles type 

 through a Phascolarctus type^ as suggested by Semon (21, 

 p. 310). For it must ever be borne in mind that on the 

 strength of the evidence derived from a study of dentition 

 the whole marsupial order constitutes a well-marked natural 

 group^ exhibiting like characteristics of degeneracy from the 

 typical and original mammalian condition, And^ in this 

 group, Phascolarctus is distinguished not by less, but by an 

 even greater degree of retrogressive dental modification thau 

 Perameles. It therefore seems unlikely that the former should 

 have retained unmodified more primitive embryonal nutritive 

 arrangements than the latter. So far, indeed, as the decision 

 of this problem can be shown to depend upon the question of 

 the primitiveness of the general structural organisation ex- 

 hibited by Phascolarctus and Perameles respectively, it can 

 hardly be denied that the evidence at our disposal is strongly 

 in favour of the latter to be regarded as representing a more 

 archaic marsupial type. 



It is, of course, possible that in the remote past the imme- 

 diate promammalian (?) ancestors of the protoplacental stock 

 may have exhibited a condition of the fcetal membranes some- 

 what resembling that of Phascolarctus; but we are unable to 

 avoid the conviction that as in dentition, so in its embryonic 

 appendages, Phascolarctus has shared largely in the general 

 marsupial decadence. And the fact that in all non-placental 

 marsupials, with the single exception of Phascolarctus, so far 

 as is known, the allantois never reaches the cliorion, but 

 remains buried in the extra-embryonic splanchuocoele as a 

 rudimentary structure, with no respiratory function, we con- 

 sider as indirect evidence in favour of our view. For, as 

 Seleuka and Semon have pointed out, this condition is cer- 

 tainly to be regarded as a secondary one ; and if this be so, 

 then the admitted existence of such a process of late secondary 

 reduction renders our view of still earlier ccEuogenetic sim- 



