4 IQ 
PATAGONIAN EXPEDITIONS ! 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. 1 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 Dasyurus 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. 
