ARGYROHYRAX 83 
two premolars, and by the increase of the bifurcated in- 
ternal fold. Three species have been described, A. pro- 
avus, the type species, A. acuticostatus, of a little smaller 
size, and A. proavunculus of still smaller size. 
Argyrohyrax proavus Ameghino 
A. proavus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436. 
This species occurs three times in the Amherst collec- 
tion. The best specimen has pm. I, 3, 4, and m. I and 2 
of the upper jaw. The species is characterized by a nar- 
row furrow near the anterior external margin of the pre- 
molars and molars; and by pm. 3 and 4 and the molars 
Fig. 50. Right upper dentition, the outline teeth after Ameghino— 
natural size. 
having a deep internal bifurcated inflexion, which tends to 
make these teeth three-lobed. 
The genus is known only by the upper dentition, and 
while I did not find any associated lower teeth, I believe 
that some one of the genera known only by the lower den- 
tition, like Plagiarthrus, is that lower dentition. 
MEASUREMENTS 
Upper dentition, length inc. 1 to m. 3 (@ Ameghino) 71 mm. 
premolar 1, length 7 mm., width 4 mm. 
premolar 3, length 7 mm., width 6 mm. 
premolar 4, length 8 mm., width 6 mm. 
molar 1, length 8 mm., width 6; mm. 
Argyrohyrax proavunculus Ameghino 
A. proavunculus Amegh., 1897, Bol. Inst. Geog. Argen., t. 18, p. 436. 
The species is simply said to be much smaller than the 
preceding, which, by the measurements, would figure out 
about 27%. The measurements given are pm. I to m. 3, 
33 mm. 
