OF THE AUSTEALIAX MAESUPIALIA. 117 



of much the same kind. It is an interesting and suggestive fact, howevei', that tlie 

 modifications of the median premolars always represent a more primitive stage of the 

 same modifications seen in the posterior premolars of the same forms. Thus the median 

 upper premolar of B. penicillata (PI. 5. fig. 32 a) is much simpler than tlie posterior 

 tooth, being shorter and bearing a smaller number of grooves. It will be apparent that 

 if the present series is a natural one then tlie posterior premolars of the more primitive 

 forms should resemble the median premolars of more advanced forms. The species of 

 Bettongla and ^Jipi/pri/mmis are scarcely removed to a sufficient extent from one another 

 to present sucli relations ; but between B. penicillata and Hypsiprymaodon, two forms 

 which, except for the dental resemblances already described, we would scarcely suspect 

 to be related as successor and ancestor, from a comparison of other chai*acters, the 

 correspondence is exact and unmistakable, the median premolar of the former (fig. 32 a) 

 almost repeating the characters of the posterior premolar of the latter (fig. 31). 



A further application of this principle to the median premolars of Ilypsiprj/mnoclon, 

 the existence of which is, however, only presumed at present, might throw some light 

 on the characters of the posterior teeth in its phalangerine ancestors, and also facilitate 

 the comparison of the animal with the peculiar form Burramys (Broom, 1895), in which 

 the posterior premolars are modified, as in the present series, while the median teeth, 

 although placed far back in the jaw, are not modified and not replaced in the tooth 

 change. 



POTOROIN.E. 



The chief interest of the two genera Potorous and Caloprymnus, composing the 

 present division, lies in their general prototypal relations with the Macropodinse. It will 

 be seen on comparison of the present series with the Bettongiinpe that while both groups 

 present primitive phalangerine characters in the general dentition, the Potoroins alone 

 present premolar characters which correspond to those found in the more jirimitive 

 forms of the Macropodinae ; none of the members of the two latter groups shoAV the finely 

 grooved and rotated condition of the sectorials found in the Bettongiinte. 



The species of Potorous form a natural series, of w hich the incipient or most primitive 

 member is P. platyops, a form which shows an interesting approximation in many of its 

 dental and also of its cranial characters to Petaurus, suggesting an afhuity with Gynino- 

 helideus, the non-volant form of the latter. The remaining members of the series are, in 

 order, P. Gilberti and P. tridactyliis, the typical Tasmanian form of the latter (P. apkalits) 

 being more advanced than the New South Wales form. It is interesting to notice that 

 the evolution cf this series is associated with an elongation of the muzzle, a modification 

 Avhich is exactly the reverse of that characterizing the Bettongiinaj. Caloprymnioi does 

 not continue the evolution of Potorous, but probably rejirLScnts a side development from 

 a primitive species of the latter. It combines nearly all of the ancestral i)halangerine 

 characters seen in Potorous with certain special modifications pointing towards tho 

 Macropodinae. Unlike Potorous, it shows no tendency towards an elongation of the 

 muzzle. 



SECOND SERIES. — ZOOLOGY, VOL IX. 21 



