112 BE. B. A. BENSLET ON THE EVOLUTION 



Ihe Dasyui'inse {cf. figs. 2 & 3, Sminthopsis leucojms, Dasyurus viverrinus). Three 

 points of departure are, liowever, indicated. Tliere is, first, a slight reduction of tlie 

 paraconid ; secondly, a partial elevation of the crown-surface of the talonid ; and, lastly, 

 a slight reduction of the hypoconulid. These remarks apply to all three anterior 

 molars, hut not wholly to the fourth, the latter having the talonid reduced, as in the 

 DasyurinsB {cf. fig. 16, P. Boiigaincillei). 



Proceeding from P. Doreyanu and P. Raffrayana we find the following changes in the 

 molar patterns. In the upper teeth (PI. 5. figs. 9 & 10, P. Bouga'mvillei, P. obesula) the 

 general shape from being triangular becomes quadrate, the spaces originally existing 

 between the internal apices of the teeth being gradually filled partly by the growth of 

 the hypocone and partly by a very slight rotation of the body of the metacone. The 

 metacone-spur becomes partially reduced, and the former disproportion in size between 

 the paracone and metacone becomes less obvious. In the lower teeth (PI. 6. fig. 13, 

 P. obesvla) there is a further reduction of the paraconid and an elevation of the talonid, 

 the result of which is the production of a practically quadritubei'cular pattern. It is 

 interesting to compare the tooth of P. obesula here figured with that of Tliylacomys 

 Irucura (fig. 14), which shows a more advanced stage of the same evolution. In this 

 form the paraconid has completely disappeared, and the remaining cusps of the trigonid 

 have now the same characters as those of the talonid. 



Viewing the molar changes from an adaptive standpoint it is seen that the teeth 

 of P. Doreyana and P. Majfrayana perform the same piercing, crushing, and cutting 

 functions as those of the smaller Dasyurinse. In the insectivorous stage the general 

 movement of the teeth is vertical. In the omnivorous stage the teeth assume a trans- 

 verse grinding action, and the cusps become arranged ia such a way as to present a 

 uniform crown-surface, and at the same time to obliterate all of the interspaces. Tlie 

 crown-surface of the talonid, originally low, is raised to the level of that of the trigonid. 

 The protocone and hypocone are similarly raised. The change in the latter cusps is, how- 

 ever, not so essential as that in the talonid, since the movement of the teeth is necessarily 

 vertical and internal as well as transverse, and is accordingly slower. It is, in fact, 

 only completed in the more advanced Phalangeridae {cf. PI. 5. figs. 13 & 16, Distoechurus 

 peniratus, Trichosurus vuljjecula) . The triangular spaces between the internal apices of 

 the upper molars become obliterated by the growth of the hypocone, and the somewhat 

 similar spaces between the external apices of the trigonids and talonids in adjacent 

 loAver teetli become obliterated by the reduction of the paraconid and hypoconulid. 



JTypsodontism of fhc llolar Crowns. — The molars of the various members of the Pera- 

 melinte furnish an interesting example of the perfect correlation of two adaptive 

 changes, which are due to different causes, in the association of a gradually increasing 

 hypsodontism of the molar crowns M'ith the omnivorous elaboration of the patterns. 

 The hypsodont development is scarcely perceptible in the teeth of the primitive forms 

 P. Doreyana and P. Bafrayana, but becomes more and more obvious as we pass to the 

 final forms P. obesula and P. macrura. Unlike the hypsodontism which is found in the 

 Macropodidse, that of the Peramelidae aff'ects the bodies of the teeth rather than the 



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