266 ANNALS NEW YORK ACADEMY OF SCIENCES 



placental Carnivora. On the other hand, Thijlacinus has retained cer- 

 tain didelphyid characters which are already lost hy the most primitive 

 of the BorhyaenidjE (palatal vacuities, posterior position of the orbits/® 

 an external lachrymal duct, double perforation of the basisphenoid), 

 while in other features (brain development, cursorial specialization, etc.) 

 it is more progressive. The Borhyaanida? are more progressive in the 

 reduction of the last molar, in the dilferenti^ion of enamel from dentine, 

 less so in the cursorial adaptation of the limbs and feet. 



Descent from a common ancestral type is undoubtedly shown, but some 

 at least of the above differences point back to Didelphyida3 as this common 

 type. The characters which Sinclair uses to separate the thylacines are 

 the reduced number of incisors, the carnassial specialization of the molars 

 and especially the loss of the metaconid. Every one of these features, 

 besides numerous other common characters which he docs not specify, 

 may be paralleled in two or more distinct lines of Carnivora whose com- 

 mon ancestors are not more predaceously specialized than Didelphys. 

 The loss of the metaconid occurs in Cijori, Iscliyrocyon, Simocyon and 

 Enliydrocyon among the Canidas, in all the post-Oligocene Felidas, in 

 Gulo, Megalictis, Mustela, etc., among the Mustelidae, in the later Hyse- 

 nidse, in Hycenodon and Pterodon among the Hya^nodontidfe, in Palrio- 

 felis among the Oxyanidse, in all the later Mesonychida?. Each one of 

 these genera is independently descended from genera in which tlie 

 metaconid is well developed. In every case, it is simply a stage in 

 predaceous adaptation of the molars, nor can it be assigned any other 

 significance in the marsupial carnivores. There is, in short, no evidence 

 for assuming a closer affinity between thylacines and borhysenids than 

 common descent from didelphyid ancestors, and^there is strong evidence 

 against such an assumption. But if this be true, these animals afford 

 no evidence for Antarctic connections between the southern continents; 

 for we have seen that Didelphyid marsupials were certainly present in 

 the Mesozoic and early Tertiary of Holarctica and of South America, 

 and we have no reason to believe that they would have had greater diffi- 

 culty in reaching Australia in the Mesozoic or early Tertiary than the 

 murine rodents found at a later date. 



The supposed presence of Diprotodont marsupials in the South Ameri- 

 can Tertiary and in modern Australia has also been used in support of 

 Antarctic connections between the two continents. The recent mor- 

 phologic studies of Dederer'^" and Broom®° have shown that Ccenohstes 



'8 Interpreted by Sinclair as a progressive character in Thijlacinus, but certainly the 

 reverse in analou'ous placental adaptations. 



'" Pauline H. Dedkrkk : Amer. Nat, vol. xliil, p. 614. lOOn. 

 WR. Broom: Proc. Linn. Soc. N. S. W., vol. xxxvi, p. .S15. 1911. 



