348 PATAGONIAN EXPEDITIONS : PAL/EONTOLOGY. 



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 Borhycena 

 " 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. i; 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 fonvard. 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 



