OF THE AUSTEALIAX MAESUPIALIA. 127 



in shape and quadrituhcTculatc, only the protoconid, metaconid, hypoconid, and entoconid 

 being present, as in Tlujlacomys. The protoconid and hypoconid are not yet completely 

 bunoid, but tend to retain the triangular section characteristic of more primitive forms. 

 As in the latter also the two cusps are separated externally by an angular notch. At 

 the anterior and posterior margins of each tooth a minute ledge is seen to be connected 

 with a fine ridge running over the tips of the outer ciisps. Tiiis structure is not very pro- 

 nounced in the present form, but becomes A^ery conspicuous in the more advanced species. 



The first lower molar of Uistcechitrus (PL 6. fig. 20) shows an insectivorous specialization, 

 the protoconid being elongated and curved after tlie manner of a canine, while tbe 

 metaconid is reduced so as to form a small tubercle attached to the inner side. This 

 curious development also appears in a reduced state in more advanced forms. 



The molars of Acrohates are very like those of Distcecluirus. The upper teeth are, 

 however, more definitely quadrate, in which character they are more specialized, but they 

 also decrease only moderately in size backwards, in which respect they are more primitive. 

 In the first lower tooth the protoconid has not been elevated to the same extent, 

 so that the disproportion in size between it and the metaconid is not so marked. 



The anterior upper molars of Dromicia resemble those of Acrohates in their pro- 

 portions and the comjileteness of the hyjiocone, but this is more nearly true of J5. lepkla 

 waA I), nana thsiw oi D. cone i una, which tends to resemble Distcechurus. In the third 

 tipper molar the liypocone is absent, so that the tooth is triangular and trituberculate. 

 In the first lower molar the anterior cusp is elongated only in J), concinna. The 

 metaconid is fairly separate in D. lepida, as in Acrohates, but is absent in D. concinna 

 and D. nana. 



Passing fiom Acrohates, Distcechurus, and Dromicia, it is probable that the next 

 higher member of the series is G ijmnoheHdexis. This genus is, however, not represented 

 in the collection. Its volant form, Fetanrus, which is said by Thomas to be identical 

 with it in dentition, presents the following features. The molar formula is unreduced, 

 and the teeth decrease gradually in size from before backwards. The upper molar 

 patterns almost repeat those of the preceding species, this being especially true of 

 P. Irevlceps and P. sciureus. It is interesting to note in this connection that 

 P. breticeps makes the closest approach in size to Dromicia, P. sciureus lieing inter- 

 mediate in size between P. hreviceps and P. australis. The upper molars of the last- 

 nam.ed species begin to show advanced characters. In those of P. sciureus (PI. 5. fig. 15) 

 a thin ridge is seen to pass over the protocone and hypocone, and to terminate anteriorly 

 and posteriorly in narrow ledges. This structure is vei'y faintly indicated in P. hreviceps 

 as in Bista'churus, but is more definite in P. australis. The internal sides of the 

 paracone and metacone show slight indications of transverse ridging. The lower teeth 

 differ from those of Distoecharns in having the protoconid and hypoconid more completely 

 bunoid, and in having the external band slightly more pronounced. The anterior lobe 

 of the first molar shows the same modification as in Distcechurus, except that the 

 protoconid is not so conspicuously elongated, and the metaconid is absent. It now 

 becomes apparent that this tooth has been formerly modified as a piercing-organ and is 

 undergoing reduction. 



