OF THE AUSTEALIAX MARSUPIALIA. 139 



is the case ; (c) that the molar patterns of each group have been independently derived 

 from a secodont type, like that seen in Perameles iJoreyana. 



On comparing the molars of Fliascolarctiis -with those of the polyprotodonts, it will be 

 seen that there is ample evidence in favour of Winge's view that certain characters 

 of the former are directly derivable from those of the latter. ]''or example, the selenoid 

 modifications of the protocone, paracone, metacone, liypoconid, and protoconid arc 

 already present in the polyprotodont forms, and are only more fully elaborated in the 

 Phascolarctiute. The liypocone is crescentic in the Peramelidse. It may also be shown 

 that the selenodont molars could not have originated from a buuodont type : in the first 

 place, on account of the fact that the external styles are absent in the latter ; in the 

 second place, that the outer and. inner cusps in the lower molars of the Phascolarctinfe 

 present a differentiated condition which would in all probability not be indicated if all of 

 the cusps had been derived from a common bunoid type. In the polyprotodonts the 

 outer cusps are quite selenoid, with their concavities external as in the Phascolarctinoe. 

 The inner cusps, and especially the motaconid, also tend to be selenoid, but their con- 

 cavities are on their external sides, or, in other words, oa tlie wrong side, with reference 

 to the Phascolarctinae. The blade-like character of the inner cusps in the latter seem 

 therefore to indicate an intermediate stage in the conversion of semiselenoid cusps with 

 external concavities into selenoid cusps with internal concavities. The condition found 

 in Fscndochirus is just on the line between the two phases, and that of Pliascolarctus 

 just beyond. 



There are, however, no reasons for concluding from this that the selenodont molars of 

 the Phascolarctinse are ancestral to the bunodont patterns of the Phalangerinie, while to 

 regard the dental characters of Pliascolarctus as ancestral to those of Pseuclochiriis is 

 simply to reverse the order of evolution. As already mentioned above, the various 

 members of the Phalangerinae show a close general sequence of dental modifications, the 

 most primitive conditions being found in Acrobates, Dromlciu, and Distoecharns. The 

 ancestors of the Phalangerinie were minute insectivorous forms, which, apart from the 

 diprotodont modification of the antemolar teeth, possessed a full antemolar formula. 

 Their secodont molars began to be modified in the same way as those of Caluromys are 

 being changed at the present time. They could not have possessed any of the special 

 characters which now distinguish the Phascolarctinse, with the exception of the 

 crescentic chax"acter of certain of the cusps and tlie possesuon of external styles. The 

 Phascolarctinse are comparatively large specialized forms, which are well advanced in an 

 herbivorous evolution, the latter being here, as in the Macrojjodidce, Phascolomyidae, 

 Diprotodontidae, and specialized Phalangerinae, secondary, and succeeding, not preceding, 

 the insectivorous evolution as exemplified by the primitive Phalangerinie. The fact that 

 the Phascolarctime show resemblance to the polyprotodonts in the character of certain 

 cusps means notliing more than that in the formation of selenodont molars from others 

 of secodont type these cusps have been able to undergo a complete change of function 

 with little alteration of their original character. Tlie molars of the Phascolarctinte 

 and Phalangerinai liave been derived by a divergent evolution from a common 

 insectivorous secodont type. It is possible that, as in the primitive Pej-ameliuie, this 



SECOND SERIES. — ZOOLOGY, VOL. IX. 20 



