80 FOSSIL JAWS OF MACROPODID.K, 



With one exception the whole of the fossils haAe Ijeen collected 

 at various points on the Darling Downs. 



On the ground that "the characters 1)}' which Kangaroos and 

 Wallabies are separated from each other are neither sufficient!}^ 

 constant nor important to found generic distinction upon," we are 

 invited by Mr. Thomas to forego the admitted Ijenefit of keeping 

 them apart. The ease and certainty with which the unlearned 

 l)ushman distinguishes betw^een Wallabies and Kangaroos by their 

 build, gait, and hal)its, are derived from a kind of evidence to 

 which we are not accustomed to pay much heed, ))ut — that apart 

 — it appears to the writer that in the Vjehaviour of the premolar 

 we have a distinguishing character of sufficient constancy and 

 importance for our purpose. It is rare to meet wdth an aged 

 wallaby'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 su])stance of tlie 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 wdth far less force against 

 the immo\able 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 deca}' in .situ. The comparative unimportance 

 of the premolar function in Jfacropus, expi-essed in the feeloleness 

 and short duration of these teeth, especially of the so-called per- 

 manent tooth, and its high functional value in Ilalmaturits, 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 l^etween 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. 



