434 JAS. P. HILL. 



within the limits of the maramalian class. Such would, in 

 our opinion, be a most remarkable instance of parallelism. 



It is true that in the existence of a vascular syncytium 

 formed from the uterine epithelium, the placenta of Perameles 

 exhibits a modification of structural arrangement of a kind 

 occurring in no other mammal. But it cannot be held that 

 the existence of even such a unique modification gives support 

 to the view of the genetic independence of the placenta of 

 Perameles, any more than the existence of manifold and even 

 more marked structural difi'erences in the types of placenta- 

 tion occurring among the Eutheria themselves witnesses to 

 their essential morphological diversity. In each of these cases 

 alike we find the real, or at least by far the most probable, 

 explanation in a diff'erentiation of truly homologous parts due 

 primarily to physiological adaptation. 



It is no doubt a tempting and easy solution of the problem 

 to regard the allantoic placenta of Perameles as a direct and 

 natural advance upon such a condition of foetal membranes as 

 occurs in Phascolarctus (alone, so far as is yet known, among 

 marsupials), where, as Caldwell (12) and Semon (8) have 

 shown, the vesicular allantois reaches and fuses with the dis- 

 coidal area of true chorion, develops a rich respiratory surface, 

 but forms no union with the uterine wall. And certainly, 

 if the placenta of Perameles be an independent acquisition 

 within the marsupial order, Phascolarctus would seem to 

 present the more primitive type of arrangement of foetal 

 membranes. As Semon (8) has pointed out, '' wir haben uns 

 nur vorzustellen, dass beim Phascolai'ctus — Typus im Bereich 

 der Athemflache der Allantois eine innige Vereiuigung der 

 Keimblasenwand mit den miitterlichen Geweben eintrat um 

 auf den Urtypus der Eihiillenanordnung der Placentalier zu 

 kommen." At the same time he makes the further important 

 qualification, " Deshalb, weil sich bei Phascolarctus in der 

 Anordnung seiner Eihiillen primitivere Zustande erhalten 

 haben als bei den meisten anderen Marsupialieren, halte ich 

 ihn natiirlich nicht iiberhaupt fiir ein besonders primitives 

 Beutelthier, oder gar fiir den Stammvater der Ubrigen.'^ But, 



