442 PATAGONIAN EXPEDITIONS I 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 Caenolestinae, 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. i ; 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*2), all the molars are quadritubercular, inter- 

 mediate stages in molar complication occurring in Petaums, 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 1 and proceeded backward. 



Abderites 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 



