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 Microbiotherium, are 
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 Tltylacynus , and various hypotheses have been 
formulated to account for the observed similarity. A comparison of the 
dentition, skull and skeleton of Tltylacynus 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 Borhyaenidae, 
Acyonidae, Amphiproviverridae, Hathlyacynidae, Prothylacynidae and Spar- 
assodontidae. 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 : Thylacynidae. Incisor formula ; 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 -i, |, Protocone well developed on all the upper molars. 
MA with small but distinct metacone. Posterior premolar exceeding in size the 
anterior and median premolars in both series. Talonid of 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. spel<zus t Queensland, New 
South Wales.) 
2. Dental formula |^, 1 , Protocone well developed on with small, 
conical protocone, large paracone and antero-external style ; metacone reduced 
to the merest vestige, or absent. Premolars increasing regularly in size poste- 
