348 PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
pearing entirely or almost entirely on worn teeth, although the inner root 
is still proportionately as robust as in Thylacynus, where the protocone is 
unreduced. The metacone spur is much less rotated outwardly than in 
the latter genus. The last molar shows greater reduction than in any 
other of the Santa Cruz genera. Its crown is composed of two cusps, the 
paracone and antero-external style, separated from each other by a sharp 
notch, forming a transversely placed shear. The anterior surface is 
greatly abraded by cutting against the postero-external margin of the pro- 
toconid of the last lower molar. M- is frequently single-rooted. 
In the inferior dentition (Pis. XL ; XLV, fig. 3), the incisors are closely 
crowded and the root of the second is displaced posteriorly with reference 
to the median and lateral teeth, as in Thylacynus and the Santa Cruz 
genera in general. The canine is large, with swollen root and slightly re- 
curved blunt crown, bearing a broad groove on its inner side. As in the 
superior series, the premolars are closely crowded and the anterior tooth, 
situated in contact with the canine, is placed obliquely to the long axis of 
the tooth row. The anterior and median premolars are like those of the 
superior series. The posterior premolar is similarly enlarged, but has a 
much smaller heel. The lower molars are double-rooted and increase in 
size posteriorly. In the heels of the first, second and third molars the 
hypoconid is reduced, while the hypoconulid and entoconid are represented 
by a single cusp. The heel of the last molar carries a single conical cusp. 
The cusps of the trigonid are high and sharply separated by deep notches. 
Milk Dentition . — Ameghino (1894, p. 109) states that in Borliycena 
“ the milk dentition consists of a canine and a molar ; the latter has the 
form of a true molar and is replaced by the third tooth of the permanent 
dentition which follows behind the canine.” The individuals in the Prince- 
ton collection are not sufficiently immature to permit of the confirmation 
’of these important details. 
Skull (Pis. XL-XLIV ; XLV, fig. 1; XLVI, fig. 4).— The skull is 
broad and depressed, with powerful, widely expanded arches and mode- 
rately elevated sagittal and lambdoidal crests. The upper border of the 
facial profile has but slight inclination forward. The cranium is depressed 
in the parietal region and is proportionately less constricted postorbitally 
than in the other Santa Cruz marsupial carnivores. The brain cavity is 
smaller than in Thylacynus and the cerebral hemispheres less convoluted, 
judging from their impression on the cranial walls (PI. XLII). The fossae 
