80 
FOSSIL JAWS OF MACROPODID^K, 
With one exception the whole of the fossils have been collected 
at various points on the Darling Downs. 
On the ground that "the characters by which Kangaroos and 
Wallabies are separated from each other are neither sufficient!} 7 
constant nor important to found generic distinction upon," we are 
invited by Mr. Thomas to forego the admitted benefit of keeping 
them apart. The ease and certainty with which the unlearned 
bushman distinguishes between Wallabies and Kangaroos by their 
build, gait, and habits, are derived from a kind of evidence to 
which we are not accustomed to pay much heed, but — that apart 
— it appears to the writer that in the behaviour of the premolar 
we have a distinguishing character of sufficient constancy and 
importance for our purpose. It is rare to meet with an aged 
w r allaby's jaw with fewer than the whole five cheek teeth in place 
at once. It is equally rare to find even a recently adult kangaroo 
jaw with all the cheek teeth together in place. In the one a 
strong progressive movement of the substance of the jaw carries 
forward all the teeth, and, unhindered by any fixed impediment 
on the brink of the diastemal declivity, hurries them over it: in 
the other the hinder teeth, propelled w T ith far less force against 
the immovable barrier set up by the premolar, are kept on duty 
throughout life, or, if an anterior molar ever be lost, it is so by 
lateral out-thrust or decay in .situ. The comparative unimportance 
of the premolar function in JIac?'opus, expressed in the feebleness 
and short duration of these teeth, especially of the so-called per- 
manent tooth, and its high functional value in flalmaturus, in 
which the latter is better developed than the deciduous tooth and 
is to old age one of the best preserved of the grinders, point to 
physiological differences between the two groups important 
enough to render the constant transiency or permanency of the 
premolars a good diagnostic character. 
Allowing then the practical convenience of recognising the 
genus Halmaturus to outweigh a theoretical reason which seems to 
him to lack foundation, the writer proposes to retain that genus 
for the present. 
