DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELES. 543 
subsequent evolution and maturation lags behind the neigh- 
bouring and more juvenile teeth, and is finally outstripped by 
them in its developmental progress. But Leche has himself, 
as we have seen, shown the worthlessness of the criterion of 
period of maturity, and we cannot follow him in his virtual 
application of that criterion in the passage above quoted. 
The subsequent behaviour of a developing tooth does not 
cancel, or otherwise alter, the significance of its first appear- 
ance, and we have shown that the latter cannot be disposed of 
in the way Leche suggests. We are far from affirming the all- 
sufficiency in every case of Leche’s test of synchronicity as 
applied to the earliest differentiation of enamel-organs. No 
doubt other considerations, such as those above referred to, 
must in many cases be taken account of. Here, however, we 
believe that we have a case in which the value of the criterion 
of contemporaneousness is clearly illustrated; and it teaches, 
in our view unmistakably, the doctrine that dp. 3 is not 
serially homologous with the other antemolar members of the 
adult dentition. 
This decision is of course the key of the whole position in- 
volved in the question under consideration, viz.:—‘‘ Do the 
existing teeth of Marsupials in front of the last premolar cor- 
respond to those of the milk or of the successional dentition of 
higher mammals ?” 
It can hardly be denied by anyone that the deciduous pre- 
molar of Marsupials is homologous to the milk-teeth of other 
Mammalia.’ A recognition that dp.3 is not homologous with 
the other antemolar teeth, but belongs to an “ earlier” cate- 
gory, practically implies the serial homology of these teeth 
with the successional p.3, and therefore their true homology 
with the successional teeth of higher mammals. 
The view that p.3 is in series with the teeth in front of it 
may at first appear, when viewed in isolation, an improbable 
one. But we think it may be taken as established by the 
facts before us, and at the very least estimate it agrees better 
with the facts in Perameles than the alternative theory. 
1 But see above, p. 441. 
