No. 412.] THE AUSTRALIAN MARSUPIALIA. 247 
composite structure such as might be expected if they repre- 
sented migrated portions of the fauna of another continent, or, 
again, if they were of polyphyletic origin. Except in a very 
few instances, they show no signs of degeneration; as a group 
they are, like the placentals, progressive throughout. In short, 
the only satisfactory explanation that can be given for the 
homogeneous character of the group and their adaptive resem- 
blances to placentals seems to be that they have undergone an 
entirely similar radiation under similar conditions, that it has 
proceeded from a single sharply marked ancestral type, and that 
the center of radiation has been the Australian region itself. 
Assuming that a similar radiation has taken place in mar- 
supials and placentals, it will be apparent that a plan of evolu- 
tion may be constructed for the former by inference from that 
of the latter. Thus, by noting the sequence of events in the 
development of the adaptive modifications of the teeth and feet 
in placentals during the Tertiary period, we may infer a similar 
sequence in those of marsupials, and in this way determine 
their probable course of evolution in these structures, and also 
the characters of their stem form. 
THE PROBABLE CHARACTERS OF THE MARSUPIAL STEM Form. 
Dentition. — In the case of the placental mammals, progres- 
sive evolution in dentition is accompanied by a reduction of 
certain of the teeth and by an elaboration of certain others, 
notably the molars. Unreduced dentitions with slightly elabo- 
rated teeth are therefore primitive. But just asin formulating 
such a law it is necessary to exclude those forms whose denti- 
tion, while being of primitive aspect, is not typically primitive, 
but primitive through degenerate specialization (Delphinide, 
Otariidze), it is also necessary in applying it to the case of 
the marsupials to exclude similarly conditioned forms of that 
series, 
The only marsupials which call for consideration in this 
respect are the forms Myrmecobius and Notoryctes.! There 
1 Tarsipes appears to be degenerate in dentition, but the teeth are not primi- 
tive in aspect. 
