OF THE AUSTRALIAN MAESUPIALIA. 137 



triangular in shape as in the Plvalangerinae, and it shows three cusps, probably representiog 

 the protocone, paracone, and metaeone. In some species the metacone-like element 

 shows a tendency towards division, and this condition appears to be prophetic of that in 

 Fhascolarctus, where the posterior lobe of the fourth molar bears two distinct cus2)s. 

 The same process of development, resulting in the formation of new cusps, with 

 apparent, but without actual, homologies, is also observable in tlie same teeth of the 

 Macropodidae, and also in the first lower molars of that family. Apart from its special 

 significance, this condition throws an interesting light on the general problem of tlie fate 

 of vestigial members. In the most primitive of the polyprotodont forms we find the 

 fourth upper molars in an advanced condition of reduction, while passing through a 

 series of intermediate forms in the Phalangeridae and lower Macropodidae we find these 

 teeth becoming finally the largest and most perfect elements of the upper molar series. 

 Both in the Macropodidae and the Phascolarctinae the cause of this peculiar development 

 is to be sought in the greater possibilities of service involved in the substitution of a 

 transverse grinding action of the teeth for a vertical cutting and piercing one. Iti 

 primitive polyprotodonts, as already indicated, the action of the metaeone of the anterior 

 upper molars is directed backwards against the trigonid of the succeeding lower teeth. 

 In the foiirth upper molar the metaeone is reduced, the reason being that there is no 

 trigonid working behind it, that of the lower tooth being in relation with the third ixpper 

 one. In the Phascolarctinae, as in the Macropodidae, on the other hand, the action of 

 the metaeone is directed forwards against the hypoconid and entoconid. The fourth 

 tooth may therefore become capable of service by the development of its vestigial 

 metaeone into a functional cusp, and may increase its utility by the addition of a new 

 cusp simulating the hypocone in normal teeth. 



The lower molars of P. peregrinus (PI. 6. fig- 22) are brachyodont, and, like those of 

 Thykicomys and the Phalangerinae, completely quadrituberculate. The selenoid modi- 

 fications are confined for the most part to the outer cusps, the inner cusps being 

 rather blade-like than crescentic. It will be seen that the concavities of the outer cusps 

 occupy their internal faces, this condition being the reverse of that in the upper molars. 

 In the first lower molar the protoconid is greatly reduced and flattened against the side 

 of the metaconid. In P. Albertisi, however, it tends to be set apart, a condition which is 

 found to recur in PhascoI<irctiis. From the presence of tw^o distinct cusps in the latter 

 form, however, it is possible that the element here designated as a metaconid may be 

 in reality a protoconid, the former having disappeared as a result of an insectivorous 

 specialization, as it undoubtedly has in the case of the Phalangerinae. According to this 

 view, the outer element represents a new accessory cusp. Such a development of a 

 new protoconid-like element can be directly traced in the Macropodidae. 



Passing from Fseudochinis to Fhascolarctus avc find a more specialized stage of the 

 same modification found in the former genus. In the upper teeth (PI. 5. fig. 18) only 

 the main cusps are well developed, the intermediate conules being very minute. All of 

 the main cusps are more completely selenoid than in Pseiidochirtts. The protocone is 



