Sinclair: typotheria of the santa cruz beds. 69 
preceding. As in all the Santa Cruz Typotheria, the crowns of the upper 
molars and premolars curve inward. 
In the inferior series (PI. I, fig. 5), the median and second pair of in- 
cisors are greatly enlarged, with the enamel confined to the outer surface. 
These teeth are inclined forward obliquely, both shearing against the tips 
of the median upper incisors, and like the latter growing from persistent 
pulps. The third incisor and the canine are cylindrical in shape and are 
directed forward. The first premolar is also cylindrical, is slightly larger 
than the teeth preceding it and differs from them in its erect position. As 
in the case of the vestigial members of the superior series, the enamel- 
covered portions of the crowns have been entirely worn away, leaving 
cylinders of dentine. The third incisor, canine and first premolar are 
separated from each other and from the second incisor and second pre- 
molar by short diastemata. In some specimens in which the vestigial 
teeth are less worn, the third incisor is close to the second, and owing to 
its oblique position, overhangs the posterior border of the latter. The 
absence of the lower canine has been used by Ameghino to separate the 
genus Selatherium from Hegetotheriwn. All the specimens on which this 
determination is based are more or less broken in the region of the lower 
canine, and even admitting the correctness of this diagnosis, it is a ques- 
tion whether the loss of such a minute vestigial tooth would justify a 
generic distinction. In the opinion of the writer, Selathermm is invalid 
and should be united with Hegetotheriwn. As in the upper series, the 
premolars become increasingly molariform posteriorly. The shape of the 
second premolar varies with the amount of wear to which it has been sub- 
jected. In less worn teeth, it is triangular with the apex of the triangle 
directed forward. Externally, the crown is impressed with a narrow groove. 
Internally, it is broadly grooved. As the tooth wears down, the outer 
groove disappears and the angularity of the crown decreases, until finally 
it assumes a cordate outline in cross-section (PI. I, figs. $a, 5^). The 
third and fourth premolars are fully molariform and may be described with 
the first and second molars. Externally, the crown is divided by a deep 
V-shaped notch into two crescents, while internally the tooth wall is 
smooth and broadly convex, differing markedly in this respect from Pro- 
typotherium. In the fourth premolar and first and second molars the 
postero-internal corner is elongated. The third molar is trilobate exter- 
nally, the second groove being much shallower than the first. Internally, 
