SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 339 



Two families are represented among the Santa Cruz marsupial carni- 

 vores. The smaller forms, comprising the genus Microbiotlicrititn, arc 

 opossums comparable in size and, to a certain extent, in dental structure 

 to existing South American representatives of the Didelphyidae, to which, 

 however, they are not ancestral. They have been grouped by Ameghino 

 in a separate family, the Microbiotheridae, but this is hardly warranted in 

 view of their very apparent didelphid affinities, a discussion of which will 

 be found on a later page. 



The larger forms have long been known to resemble in dental struct- 

 ure the Tasmanian genus Thylctcymts, and various hypotheses have been 

 formulated to account for the observed similarity. A comparison of the 

 dentition, skull and skeleton of Tliylacynus and the Santa Cruz genera 

 shows a much closer relationship between the Tasmanian and South 

 American forms than has previously been supposed to exist, so much so 

 that the propriety of referring them to the same family seems beyond 

 question. 



A number of families for the reception of these South American genera 

 have been proposed, among which may be mentioned the Borhya?nidae, 

 Acyonidae, Amphiproviverridas, Hathlyacynidae, Prothylacynidae and Spar- 

 assodontidse. In the present paper the existing Tasmanian and extinct 

 Santa Cruz forms are referred to the family Thylacynidae (Bonaparte, 

 1838). The following is a tabulation of the characters on which this 

 classification is based : 



Family:^ THYLACYNIM:. Incisor formula A~l ; protocone of upper molars variable, external 

 styloid cusps vestigial ; premolar dentition unreduced, posterior premolar well developed ; 

 metaconid absent ; hallux opposable (arboreal adaptation), reduced or absent (cursorial 

 modifications) ; non-syndactylous. 

 A. Skull dolichocephalic. 



(a) Alisphenoid bulla present. 



1. Dental formula , \, f, |. Protocone well developed on all the upper molars. 



M with small but distinct metacone. Posterior premolar exceeding in size the 

 anterior and median premolars in both series. Talonid of M f supporting a 

 single blunt cuspule. Palate perforate. Mandibular symphysis ligamentous. 

 Hallux absent. Terminal phalanges blunt with slight clefts'. Thylacynus. 



(Recent T. cynocephalus, Tasmania. Pleistocene, T. spelceus, Queensland, New 

 South Wales.) 



2. Dental formula \, -\, f , \. Protocone well developed on Ml"-*. M with small, 



conical protocone, large paracone and antero-external style ; metacone reduced 

 to the merest vestige, or absent. Premolars increasing regularly in size poste- 



