SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 407 



third." (Bensley, 1903, p. 108.) This style is well shown in figure la 

 of Plate LXV. 



2. The last upper molar in the Santa Cruz genera is more reduced than 

 in Thylacynus, reaching an extreme in Borhycena, where but two cusps 

 remain, the paracone and antero-external style. In neither Amphipro- 

 mverra, Frothy lacynus, nor Cladosictis, is the metacone on M A as strongly 

 developed as in Thylacynus. 



3. In the lower dentition of all the Patagonian thylacynes the hypo- 

 conulid is undifferentiated from the entoconid, and the heel of the fourth 

 molar is not only smaller proportionately than in T/iy lacy tins, but has 

 undergone greater reduction, except in Amphiproviverra, in which all the 

 lower molar heels are bifid. 



4. In cranial characters Thylacynus is decidedly progressive, while the 

 Santa Cruz forms are conservative. The elongation of the face and pos- 

 terior shifting of the orbit, the great increase in brain capacity, the acqui- 

 sition of palatal vacuities and the prenatal shedding of the milk teeth in 

 the recent genus are all progressive characters. 



5. The peculiarities in podial structure observable in l^hylacynus are 

 readily understood, if interpreted as adaptive modifications. The foot 

 structure of the common ancestor of the family was probably not unlike 

 that in Amphiproviverra. Adaptation to a cursorial mode of progression 

 resulted in a reduction of the hallux, as in Frothy lacy nus. With increase 

 in speed and the assumption of a digitigrade gait the complete loss of the 

 hallux and the curious shifting of the tarsal elements noticed in Thyla- 

 cynus have been produced. 



In connection with the question of the descent of the Patagonian and 

 Tasrnanian thylacynes from a common ancestor, it may be interesting to 

 notice that certain large carnivorous marsupials from the Pyrotherium 

 beds (Amegh., 1897, PP- 97~ioo) named by Ameghino Proborhycena and 

 Pharsophortis retain the metaconid in the lower molars, while the pre- 

 molar formula is unreduced. The loss of the metaconid in the Thylacyn- 

 idae separates them sharply from all other carnivorous marsupials. It is 

 possible that the two genera mentioned, in which this cusp is retained, will 

 be found to occupy an intermediate position between the Thylacynidae 

 and Dasyuridae, but until they are better known it is unsafe to attempt 

 generalizations of so broad a character. 



Confining the discussion to the mutual relationships of the Santa Cruz 



