DEVELOPMENT AND SUCCESSION OF TEETH IN PERAMELES. 449 
confirmation (in Perameles) of the view that Thomas’s 
“pm. 2” is present in the developing marsupial jaw. 
We must therefore hold with Leche that embryological in- 
vestigation yields no support to the theory of the presence of 
this hypothetical element of the marsupial dentition; and 
though its absence may not be conclusive against that theory, 
it forms an additional difficulty in the way of entertaining a 
system of homologies based upon the ideal existence of such 
an element. : 
Only the third consideration quoted above in support of 
Thomas’s nomenclature remains for consideration, viz. that 
derived from the dental formula of Triconodon. This would 
certainly seem to render it highly probable that the ancestors 
of Marsupials were possessed of four premolars. Thomas’s 
argument rests upon the usually accepted close phyletic or 
even phylogenetic relationship between Triconodon and 
modern Marsupials, but it also requires the identification of 
the last premolar in the latter with “pm. 4” in the former. 
Now both of these propositions may doubtless be justified as 
highly probable, but neither, surely, can so far be regarded as 
scientifically certain. And, in particular, we hold that the 
evidence so far brought forward is insufficient to determine 
which premolar, if any, has disappeared in the course of 
evolution. 
On the whole we believe it to be safer in the meantime to 
designate the premolars simply numerically in the order of 
their occurrence in the modern marsupial type, instead of 
founding a system of nomenclature upon the condition of 
Mesozoic forms whose precise zoological relationships can only 
be inadequately determined. 
And, in adopting in this paper the older method of numera- 
tion, we do so the more readily that Thomas’s system has not 
as yet gained general acceptance among Continental writers. 
