The genus was founded on a lower jaw with premolar 3 to molar 3 in position. We found beside the above an upper jaw with premolar 3, and molars 2 and 3 complete while premolar 4 and molar 1 are more or less fragmentary; from which the following generic characters may be made out. The incisors are tiny; the canine very large, equal to that of _Borhyaena_; the upper and lower premolars progressively smaller from front to back. Upper premolar 3 is a simple two-rooted tooth, the crown consisting of a single blunt central cusp. On the upper molars the protocone is not developed as a cusp, though the third inner root is present and carries a rounded shelf. The paracone is the chief cusp, and is developed as a high central pointed denticle. The metacone is not developed as a cusp, but is represented by a long slanting ridge to the rear, the apex of which has been fused to the paracone. The last upper molar is better developed than in most Santa Cruz genera, consisting of a high median cusp, the paracone; a small anterior cusp, the anterior external style; and a shelf-like posterior cusp, the protocone. Lower premolars 1-3 are simple two-rooted teeth, each carrying a single cusp on the crown. The fourth premolar carries a well marked paraconid in front, a large median protoconid on the rear of which is a tiny metaconulid; and a tiny talonid or heel which is without a basin and consists of a single tiny cusp. The molars are all of the same character as the last premolar. The lower jaws are united by a ligamentous symphysis. Ameghino distinguished four species, _P. lacerans, P. tenax, P. mitis_, and _P. tenuis_, in the order of their size. The last two are but little known but are quite certainly another genus. =Pharsophorus lacerans= Ameghino P. lacerans Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 503. [Illustration: Fig. 139. Left mandible—½ natural size, after Ameghino.] The species was founded on a lower jaw with the roots of the incisors, canine, and first two premolars, and with the remaining teeth intact. We did not find the species, so I have reproduced Ameghino’s figure and give his measurements. Lower dentition, length incisor 1 to molar 3 114 mm. Lower dentition, length premolar 1 to molar 3 90 mm. Lower dentition, height of mandible under pm. 4 38 mm. =Pharsophorus tenax= Ameghino P. tenax Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 504. [Illustration: Fig. 140. Left upper jaw—½ natural size; _A_, molars 3 and 4 from above; _B_, molars from external side.]