SINCLAIR: MARSUPIALIA OF THE SANTA CRUZ BEDS. 
407 
third.” (Bensley, 1903, p. 108.) This style is well shown in figure 1 a 
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- 
viverra , Prothylacynus, nor Cladosictis, is the metacone on M- 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 Thylacynus , 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 Thylacynus 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 Prothylacynus. 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 
Tasmanian thylacynes from a common ancestor, it may be interesting to 
notice that certain large carnivorous marsupials from the Pyrotherium 
beds (Amegh., 1897, PP- 97 - 100 ) named by Ameghino Proborhycena and 
P harsophorus retain the metaconid in the lower molars, while the pre- 
molar formula is unreduced. The loss of the metaconid in the Thylacyn- 
idas 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 Thylacynidse 
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 
