442 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
and pointed, as in the Didelphyidae. Unfortunately, nothing is known of 
the upper dentition. Garzonia is more specialized than the other mem- 
bers of the Casnolestinae, having the last lower molar single-rooted. 
Some account has already been given of the lower molars in the Palaeo- 
thentinae and an attempt has been made to show that Palceothentes , Callo- 
menus and Decastis are members of a closely related series characterized by 
a progressive reduction in the number of antemolar teeth, reduction in the 
size of the posterior premolar and increasing perfection in the adaptation 
of the first molar to a sectorial function (see pp. 427-436). It remains 
to point out the striking similarity in upper molar patterns exhibited by 
Palceothentes and certain of the phalangers. Reference to the accompany- 
ing plates will at once make this clear (see Pis. LXIII, fig. 7 ; LXIV, 
fig. 1 ; LXV, fig, 4). Indeed, it is possible to trace in the Palaeothentinae 
the constructive stages in the evolution of the bunodont type of molar 
characteristic of the more primitive of the existing phalangers. The devel- 
opment of the hypocone in the upper molars of Palceothentes is less com- 
plete than in these phalangers, only the first molar being quadrangular 
and fully quadritubercular, the second retaining a triangular outline, with 
incipient hypocone. In the higher phalangers ( Phalanger and Tricho- 
surus , PI. LXV, figs. 3, 3#), all the molars are quadritubercular, inter- 
mediate stages in molar complication occurring in Petanrus , with three 
quadritubercular molars, and Dromicia with two. In Ccenolestes there exists 
the apparent anomaly that a genus more primitive than Palceothentes 
should have the second upper molar more complicated. This complica- 
tion, however, may be a measure of the extent of dental evolution in the 
Caenolestinae in post-Santa Cruz time. The condition in the Santa Cruz 
representatives of the family is not known, but presumably the upper 
molars were less advanced than in Palceothentes. In the highly special- 
ized Abderitinae, judging from Ameghino’s figure of A. meridionalis 
(Amegh., 1898, p. 184, fig. 49, II ; 1903, p. 142, fig. 64, p. 178, fig. 107), 
the second upper molar is fully quadritubercular. Palceothentes , then, 
represents an early constructive stage of a progressive complication of the 
upper teeth, which began with M- and proceeded backward. 
Abdentes is near the end, if not the terminal member, of a highly 
specialized line, the intermediate stages of which have not been found in 
the Santa Cruz beds. Affinities with the Plagiaulacidae have been com- 
monly assumed from the striking resemblance between the first lower 
