OF THE AUSTEALIAN MARSUPIALIA. 



129 



of the hypocone are seen in the Peramelidge, whose molars are undouhtedly of tritubcr- 

 culate origin {cf. PL 5. figs. 8, 9, & 10). Secondly, while in the majority of tlie existing- 

 Didelphyidae we meet with molar patterns of the same insectivorous type as those seen 

 in the Dasyiirinse, in one form, Calaromijs, we find indications of a hunodout omnivorous 

 development, which is now in its very incipient stages, but which if continued would 

 probably give rise to exactly the same conditions as are seen in Distoeohurus and its 

 allies. Comparing the upper molars of Calnromtjs (PL 5. fig. 27) with those of a normal 

 didelphyid form (fig. 26), we notice the following modifications: — The cusps are lower 

 and their triangular section not so sharply indicated. The protocoue is conspicuously 

 broadened. The disproportion in size between the paracone and metacone is not so 

 obvious, the metacone-spur being reduced. The external styles are reduced, and the 

 ridge bearing them is thin and usually crenulated. In all of these characters the pattern 

 of Caluromijs is prophetic of that of Bistcechmnis (PL 5. fig. 13), and if the protocoue 

 were still further broadened to a stage of differentiation of a hypocone, and the 

 triangular section of the outer cusps slightly more reduced, the result would be much as 

 in the latter genus. 



Confirmatory evidence may be obtained from a consideration of Coenolestes. In this 

 form {cf. Thomas, 1895 ^> pl- 1- fig- 6) the fii'st and second upper molars show a bunodont 

 quadrituberculate condition, Avhich is very similar to that seen in Petaurus, while the 

 third shows a trituberculate condition, which is intermediate between that in Caluromi/.s- 

 and that in Bistcechurus, the outer cusps being bunoid and the external styles absent, as 

 in the latter. A somewhat similar relation may, in fact, be seen between the second and 

 third molars of the three-molared Phalangerinse themselves. 



The derivation of the lower molars of the Phalangerinae presents no great difficulties. 

 The quadrituberculate pattern is the same as that seen in Thijlaoomys, the latter being- 

 shown by analogy with the Peramelinoe to be of tubereulo-sectorial origin. A prototypal 

 condition is again seen in CulnromnH, where there is a tendency towards the reduction 

 of the paraconid and hypoconulid, and a rounding off of the remaining cusps. As 

 already mentioned, the molars of Distoeohurus show a tendency to retain the original 

 angular character of the protoconid and hypoconid. 





Incisors. — The general characters of these teeth have already been referred to in 

 connection Avith the origin of the diprotodont modification, so that only the sequence of 

 their special modifications calls for consideration. 



As in tlie case of tiie molars, the most primitive conditions are to be found in the three 

 genera Acrobales, DistoBchurus, and Dromicia. In all of tliese the median incisors are 

 of much the same procumbent piercing type as those of the DasyuriuLV, the only note- 

 worthy ditference being seen in a slight tendency towards the lateral compression of the 

 tips. The upper lateral teeth {cf. PL 5. fijr. j]9) are relatively small, this being especially 

 true of tlie anterior one (<". 2). In the unworn condition they are spatulate, as in the 

 Dasyurinse, but their edges are now slightly turned inwards so that they stop the tips ol' 

 the lower teeth instead of letting them pass inside. The latter are recurved and sharply 

 pointed. 



