94 DR. B. A. BENSLEY ON THE EVOLUTION 



that the only important change from the condition in Sm'mtliojisis leuoopns relates to 

 increase in size *. That this character is an essential feature of tlie carnivorous 

 evolution is seen from the fact that it is only after passing through the successively larger 

 species of Phascogale into the genus Basyurus that we meet with animals of sufficient 

 size to present carnivorous characters. Cluetooercus cristicauda, Dasyuroides Byriiei, 

 and Dasyurns hallucatus show no departure from the conditions in Phascogcde; but, 

 beginning with the still larger forms D. vloerrliim and D. Genffroyl, and passing 

 throu^-h D. maciilatus to Sarcophilus, we fiud the molar characters changing as 

 follows : — Taking the second upper tooth as a basis of comparisou of the vipper molars 

 {cf. PI. 5. figs. 3 & 4, Dasyurns maculatus and Sarcophilus urslims), there is («) 

 reduction of the protocone ; {h) enlargement of the metacone, with lateral compression 

 of its tip to form a trenchant blade, and encroachment of its base on the pi'otocone- 

 root ; ((?) elongation of the trenchant metacone-spur and rotation inwards of its distal 

 extremity, so that it comes to shear inwards rather than backw^ards, as in Smiidhopsls; 

 {d) lateral compression of the whole tooth, so that the external styles become 

 approximated to their respective cusps. 



Throughout the series the remaining upper molars differ from the second in exactly 

 the same way as those of Smliithopsis leucopus. The first tooth always presents a 

 condition of greater lateral compression. In Sarcophilus the external styles, which 

 even in the second tooth are so closely approximated to their respective cusj)s that the 

 tooth practically possesses a double cutting-edge, are in the first only separated from the 

 latter by a narrow groove, style ab being almost indistinguishable from the paracone. 

 The third tooth always presents a condition of less transverse compression, and style c, 

 which was seen to be reduced in Sminthopsis, becomes quickly obliterated. The fourth 

 molar becomes further reduced by the reduction of the protocone. 



The extent of rotation of the metacone-spur, which forms such a conspicuous feature 

 of the change, will be more apparent from the following data: — A line passing through 

 the tin of the metacone and the distal extremity of its spur, or, in other words, along 

 the shearing-edge, will pass through a tooth of the opposite side of the jaw — a posterior 

 one in the case of the smaller primitive forms, and an anterior one in the case of the 

 larger carnivorous ones. Such a line, drawn through the metacone-spur of the third 

 molar of one side, will, in the case of Smliithopsis leucopus, Fhascogale TVallacei, or 

 Dasyurns hallucatus, pass through the second molar of the opposite side. In D. vicer- 

 rluus it will pass between the first and second molars ; in D. maculatus through the 

 anterior part of the first molar, or between this tooth and the last (morphologically 

 median) premolar. In Sarcophilus it will pass through the posterior portion of the canine. 



Taking the third tooth as a basis of comparison, we find the following changes in 

 the lower molars {of. PI. 6. figs. 3, 4, & 5, Dasyurns vlverrimis, D. maculatus, and 

 Sarcophilus urslmis) : — There is (a) enlargement and lateral compression of the trigonid ; 

 (6) conversion of the tips of the paraconid and protoconid into trenchant blades ; 

 (<?) rotation outwards of the shearing-edge formed by the adjacent edges of these cusps; 

 {d) reduction of the metaconid ; {e) reduction of the talonid. 



* Cf. general sequence in length of the tooth-rows given on p. OS. 



