108 DK. B. A. BENSLEY ON THE EVOLUTION 



It is only the fact that all of the remaining members of the Dasyuridse as at present 

 defined, with the exception of Miirmecohius, present a uniform dental evolution which is 

 thus the predominant and characteristic one for the Australian radiation, whicli accen- 

 tuates the resemblances of such an isolated form as Tlujlactnus to the Sparassodonta, 

 whose type of evolution isjust as characteristically South American, and lends probability 

 to the view that its origin is to be sought in the latter group. 



Lenqth of Tootli-rows. — On comparing the tooth-rows of Thylaciims with those of 

 Sarcophihis or the more advanced species of Dasyimts, we notice a conspicuous diiference 

 in their relative lengths. In the latter, notwithstanding the lateral compression of the 

 molars, there has been a shortening of the Avhule dental series, while in the former an 

 elongated condition has been retained and possibly increased. It is interesting to note 

 that similar differences characterize the dental evolution of the placental Creodonta and 

 Carnivora, although in the latter Orders they are closely associated with the elaboration 

 of posterior or more anterior cheek-teeth as sectorials. The elongated condition in 

 Thijlacimis is repeated in the South-American group. 



Molar Patterns. — The characters of the upper molars are represented in PI. 5. figs. 6 

 & 7, of Thylacinus cynncephalus (m. 3) and T. spelceus {m. 2). The protocone is well 

 developed, and is supported on a separate root. In this character Thylacinus differs 

 from the carnivorous members of the Dasyurinse, and points back to the insectivorous 

 forms. The crown-surface of the protocone in the adult presents a curious spur-like 

 appearance. Its posterior border tends to be slightly trenchant. The metacone is much 

 enlarged and trenchant. As in Sarcophilus, its tip is lanceolate from a filling out of the 

 concavity originally present on its outer side. The metacone-spur is well developed and 

 trenchant, but its distal extremity has not been rotated inv^ards to the same extent as 

 in Sarcophilus. The paracone is small and comparatively undifFerentiated. Unlike 

 those of the Dasyurinye, the external styles are vestigial or wholly absent. Style ?«i 

 is apparently always present in the first, second, and fourth molars. Style c is not 

 represented, but a small element probably equivalent to style c, {cf. PL 5. fig. 1 b, 

 JPeratheriiim) is apparently always present in the first molar, variable in the second, and 

 scarcely distinguishable in the third. The vestigial nature of the external styles gives 

 the teeth a very different appearance from that seen in Sarcophilus, where their great 

 development and approximation to their respective cusps result in the production of a 

 double cutting-edge. The fourth molar is small, and of much the same character as 

 that of the Dasyurinaj. The cusps represented are the protocone, paracone, and style ab, 

 the metacone being barely indicated by a small posterior protuberance. 



The patterns of the lower molars are represented in PL 6. figs. 10 & 11, of the third 

 and first teeth of T. cynocephalns. As in Sarcophilus, the paraconid and jirotoconid arc; 

 modified to form trenchant blades. The metaconid is wholly absent, as in Frothy lacimis 

 and Jmphiproviverra. The anterior portion of the base of the trigonid in the first and 

 second molars bears a small protuberance representing an antero-external shelf. Except 

 in the fourtli tooth the talonid is well developed, and its crown-surface is fiat and shelf- 



