BY SYDNEY B. J. SKERTCHLY. 71 
56. The essential feature of the Marsupialia is, of 
course, their aplacental development, and this clearly can- 
not be determined from fossil remains. Hence the mar- 
supial affinities of fossil forms are inferred from certain 
peculiarities of the hard parts, which are alone preserved. 
Of these the most important are (1) a peculiar inflection 
of the jaw; (2) the character of the teeth; and (3) the 
presence of the so-called pubic bones. As a matter of fact 
the remains of pubic bones have never been found fossil 
in Mesozoic forms, and as this is the only certain proof of 
marsupial character, some doubt must always remain. 
Still, it may be admitted without damage to my argument 
that the Mesozoic forms were truly aplacental. 
57. The modern Marsupialia are generally placed in two 
divisions—the Diprotodonta, with two prominent front teeth, 
and the Polyprotodonta, with many front teeth. The 
diprotodont forms are peculiar to Australia, none living 
elsewhere, and none having been found fossil outside Aus- 
tralia. True, an interesting series of forms have been 
recently disinterred from the Patagonian Tertiaries which 
were looked upon as possible ancestral forms of the Aus- 
tralian Marsupialia. But they turn out not to be Marsupial 
at all, their teeth being quite different in structure, and no 
pubic bones having been found, in spite of the fairly com- 
plete skeletons exhumed, they may be left out of our 
consideration. They seem rather to be Creodonts. 
58. Outside the Australian Region the only living 
marsupials are the Opossums (Didelphys) of S. and N. 
America. They belong to the Polyprotodont division 
and their fossil remains are found in the Lower Tertiaries 
(Eocene) of Europe and N. America, but only in late Plio- 
cene in 8S. America. None occur in the rich deposits of the 
Himalayas or Egypt. They certainly did not reach America 
via Australia, but most probably via northern Europe. 
They have no Australian representatives, the so-called 
Australian opossums being diprotodont. In Australia the 
Polyprotodonts are represented by several genera and 
species, but the mass of our Marsupials belong to the dip- 
rotodont division. 
59. Now comes the important, and most singular fact 
in the distribution of the Diprotodont Marsupials. They 
