4IO PATAGONIAN EXPEDITIONS I PALEONTOLOGY. 



inent heels. The latter tooth is the largest of the inferior premolar series 

 and exceeded the molars in degree of elevation of the crown. In all the 

 specimens retaining this tooth, the tip of the crown is abraded to the gen- 

 eral level of the molars. The anterior molars are an almost exact dupli- 

 cation of the corresponding teeth in Didelphys. The first and second are 

 of about the same size, the third is a little narrower than the preceding 

 teeth, and the fourth is considerably reduced. On all, the three cusps of 

 the trigonid are well developed. The talonid is identical in pattern with 

 that of the lower molars of Didelphys in all except M T , in which it is 

 narrower transversely than in the anterior teeth. Unfortunately, the heel 

 of M T is somewhat broken in the only specimen retaining this tooth (No. 

 15,698) and its pattern cannot be fully determined. It appears to have 

 been similar to the teeth preceding it, but with the cuspules less distinct. 

 All the lower molars are double-rooted. A narrow antero-external 

 cingulum is present on the first and second molars of M. tehuelchum, but 

 is wanting in the other species. 



Skull (PI. LXII, fig. i and text-fig. 6). The skull is remarkable for 

 the great length of the premaxillae and the extreme posterior position of 

 the canine. Anterior to the alveoli of the median incisors the premaxillae 

 develop a shelf-like extension. The orbits are large, with elevated super- 

 ciliary borders and prominent postorbital processes, from which the tem- 

 poral ridges converge to the sagittal crest. In No. 15,698 the greater part 

 of the brain-case is wanting. Its narrowest part lies immediately back of 

 the postorbital processes. 



The auditory bulla and the glenoid portion of the squamosal are asso- 

 ciated with a mandible of M. tehuelchum (No. 15,038). This specimen 

 indicates that the inferior bar of the jugal extended to the anterior border 

 of the glenoid fossa. The bulla is large, elliptical in outline, with the 

 alisphenoid and petrous portions equally inflated and articulating in open 

 suture, as in Dasyimis viverrinus (PI. LXII, fig. 7). 



The palate is well preserved in the La Plata Museum specimen (text- 

 fig. 6). It is perforated posteriorly by two large vacuities. In No. 

 15,698, it is so badly crushed that the nature of the perforations cannot 

 be ascertained. Both specimens show the thickening of the posterior 

 palatal border and its extension beyond the last molar, as in Didelphys. 



The infraorbital foramen is situated above the posterior half of the last 

 premolar and outer anterior root of the first molar. 



