208 EDWARD MCCRADY, JR. 



the hind foot is opposable, an arboreal adaptation. This is in line 

 with Huxley's theory that one of the distinctive features of the mar- 

 supials when they first emerged from the primitive Metatheria was 

 the assumption of an arboreal habit. So this evidence points to the 

 opossum, an aplacental marsupial, as near the bottom of the evolu- 

 tionary tree, and Perameles, with its enlarged fourth digit, reduced 

 hallux, and syndactylous second and third digits, as one of the more 

 specialized forms. 



Bensley showed that the teeth of the American Didelphyidae are 

 nearer the original stem than are those of any other living marsupials ; 

 and the ' ' The molar tooth patterns of the stem form are almost exactly 

 reproduced in those of the Oligocene Opossums (Peratherium) " 

 ( '01 b, p. 249). Both in number of teeth and in elaboration of the 

 molars, Perameles is more specialized than is the opossum. 



Brass (1880) showed that the most primitive form of the vaginae 

 is found in the Didelphyidae and that every gradation can be found 

 between this form and the peculiar median vagina in the specialized 

 Macropodidae. 



Wallace (1876) pointed out that during the Tertiary period in 

 Europe there were no other marsupials than the Didelphyidae, and 

 Lydekker (1896) considered the most primitive Australian mar- 

 supials, the Dasyuridae, to be descended from the Oligocene Didel- 

 phyidae of the Northern Hemisphere. 



All these lines of evidence converge to indicate that the aplacental 

 opossums are the most primitive living marsupials, and that the nicely 

 graded series which Hill (1897) pointed out between the allanto- 

 placenta of Perameles, the allantois in contact with the serosal mem- 

 brane, but not forming a placenta (Phascolarctus and Aepyprymnus), 

 and the small allantois enclosed in the yolk sac (Didelphys), probably 

 indicates an evolutionary line in the opposite direction from that 

 inferred by him. That the non-placental allantois is among mar- 

 supials the more primitive, is also suggested by its widespread distri- 

 bution (Didelphyidae, Phalangeridae, and Macropodidae). In the 

 light of the development of the respiratory allantois in the Sauropsida, 

 there can be no doubt about the fact that the opossum's allantois is 

 degenerate; but the evidence indicates that it has degenerated from 

 the sauropsidan condition, not from the eutherian. The early birth 

 and precocious functioning of the lungs in marsupials removes the 

 need for a respiratory allantois. The opossum's lung begins to func- 

 tion at a much earlier stage of general body development than does 

 that of the chick (see chapter IX, p. 90). With its respiratory func- 

 tion usurped by the precocious lung, it is not surprising that the 

 allantois of most marsupials is to some extent degenerate. 



