148 DR. B. A. BENSLEY ON THE EVOLUTION 



The evolution of Potorous presents a further instance of the principle of increase in 

 size of the body already noticed in tlie case of the Dasyuridse and Phalangeridse. An 

 interesting exception is, however, found in the final forms, where of the two Tasmanian 

 derivatives of the New South Wales species, P. triclactylns, one is a more progressive and 

 larger form, while the other {P. rufus) is a dwarf [cf. Thomas, 1888, p. 120). 



Sequence of Molar Patterns. — In Potorous platyops the molars are comparatively 

 small and lightly built, as in Petaurus. The tooth-rows are very slightly bent inwards 

 posteiiorly. In the upper jaw the first molars are equal in size to the second, from 

 which point they decrease gently in size backwards. The same conditions obtain in the 

 lower jaw, except that the first teeth are slightly smaller than the second. It will be 

 observed that we have here the same primitive size-relations common to the incipient 

 members of the Bettongiinte and to the PhalangerinsB. Both upper and lower teeth are 

 of the usual qiiadrituberculate bunodont tyj)e. The protocone and hypocone in the 

 iipper molars are placed on a level with the outer cusps, and the latter, as well as the 

 inner cusps of the lower teeth, show a tendency towards the formation of transverse 

 ridges. The fourth upper teeth are almost quadrate ; they have evidently been 

 triangular at an earlier stage, but now show a division of the posterior lobe into two 

 cusps. The patterns of the first lower molars are doubtful, these teeth being much 

 worn in the specimens examined. All of the molars are longer in the crown than those 

 of Petaurus, but would nevertheless be described as In-achyodont. 



Passing through the I'eraainiug species of Potorous we find the molars showing but 

 little modification. They become slightly more robust and more hypsodont. Their 

 proportions change, so that the three anterior teeth above and below become equal in size. 

 In P. Gilberti and P. apicalis the anterior lobe of the first lower molar bears an accessory 

 outer cusp. As in the Bettongiina3, this tooth is probably changing from the insecti- 

 vorous modification characteristic of the Phalangerinaj to the quadrituberculate one seen 

 in normal molars. 



In Caloprpnnus the general characters of the molars are as in Potorous. The upper 

 teeth decrease gently in size backwards, while the lower increase in size to the third. 

 The tendency towards hypsodontism is almost more pronounced than in the final form 

 P. tridactylus {apicalis) of Potorous. The patterns of the molars are as in Potorous, 

 except that the ridging of the cusps is, if anything, more pronounced. On the whole, 

 tliis form points towards the Macropodinae. 



The molar patterns of Caloprymnus are represented in PL 5. fig. 19, and PL 6, fig. 25, 

 for comparison with those of the phalangerine genus Trichosurns (PL 5. fig. 16, PL 6. 

 fig. 19), on the one hand, and with those of the Macropodinae (PL 5. figs. 20-23, PL 6. 

 figs. 26-29), of which they may be taken as rejjresenting the ancestral type. The chief 

 progressive changes relate to the development of the ti-ansverse ridges. As already 

 described, in the case of the Phalangerinae, Bettongiina3, and Potoroinae, with the 

 exception of Calopri/mnun, to a certain extent also of JEpj/prymnus, what little ridging 

 is indicated is confined to the outer cusps of the upper molars and the inner cusj)s of 

 the lower ones. In Caloprymnus the ridges in both uj)per and lower teeth wnll be seen 



