194 DR. B. A. BENSLEY ON THE EVOLUTION 



their ori'^in from a diminutive form of PJiascogale. Antechinomys is a derivative of 

 Sminthopsis, since it sIioavs the same dental characters as the latter genus, and a more 

 advanced stage of the same digitigrade modification of the pes characterizing that genus. 

 Chcetocercus cristicauda represents a terrestrial modification of Fhascogale, which 

 parallels Sminthopsis, but is distinguished by its dental characters and size as a 

 derivative of a larger species. Basijnroides represents a more advanced member of the 

 same terrestrial line leading to CUa:tocercus. Within the genus Basyurus we find 

 I), halhicatus directly continuing the evolution of Fhascogale in size, dental characters, 

 and foot-structure. The advanced carnivorous form D. maculatus represents a further 

 continuation of the same line and also its culminating point as far as the arboreal 

 evolution is concerned. D. viverrinus and 1). Geoffroyl, which are intermediate between 

 tlie two last-named species in dentition, represent terrestrial derivatives in respect to 

 foot-structure. Of the two species L. viverrinus is the more advanced, the hallux having 

 entirely disappeared. Sarcophilus, which, in dentition, represents the culminating 

 stao-e of the carnivorous evolution of the Dasyurinae, is a terrestrial plantigrade 

 derivative of Basyurus. It has no relation with the remaining specialized carnivorous 

 form Thylaciniis. 



The foot-structure of Myrmecohms cannot be relied on for evidence of the special 

 afB.nity of the animal. A close study of the dentition shows a balance of evidence in 

 favour of derivation by retrogression from Fhascogale rather than any direct relation 

 with the Mesozoic Mammalia. 



ThylaciniiS, so far as one may judge from the dentition and foot-structure, may be a 

 derivative of Fhascogale ; if this is the case, it must be derived from a very primitive 

 form of the latter genus, since it retains the original proportions of the posterior pre- 

 molars. The evidence of the dentition, however, favours a relation with the Sparasso- 

 donta of the South-American Miocene, the most important considerations being, first, 

 that the dentition is well advanced in the carnivorous evolution, but its characters 

 are in many respects different from those gradually developed in the Dasyurinae and 

 ])erfected in Sarcophilus, so that Thylaciniis comes to occupy an isolated position in the 

 Australian family, and, secondly, that the dentition resembles that of the carnivorous 

 Sparassodonta in exactly those characters in which it differs from that of Sarcophilus. 

 This may not be regarded as indicating that the Uasyuridse are the descendants of 

 Sparassodonta through forms like Thylacinus, but that Thylacinus is a foreign element 

 in the Australian series. 



PERAMELID.^. 



The line of evolution leading to the Dasyuridte has already been referred to as one of 

 two primary lines along which the Australian radiation has proceeded. The fundamental 

 characters of the second line include an insectivorous-omnivorous progression in denti- 

 tion, coupled with a continuation of the arboreal evolution of the stem-form in foot- 

 structure, up to what has been designated as the second arboreal phase or that in which 

 syndactylism appears. The arboreal evolution of the pes culminates in the Phalangerida^ 



