i6 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
tion of the second external ridge ; the latter is intensified by wear and 
becomes the third external serration mentioned above. The prominent 
antero-external serration exhibited in worn teeth is developed from the 
elongated antero-external angle and is not indicated in unworn molars. 
A thin layer of cement is quite generally present on the premolars and 
molars. The crowns of these teeth are characterized by a strong inward 
curvature, as in all the Santa Cruz Typotheria. 
The median lower incisors (Pis. IV, fig. 15 ; V, figs. 13, 14, 22) are 
inclined forward obliquely, continuing the slope of the inner surface of the 
symphysis. The crowns of the first and second are cylindrical and symmet- 
rically divided longitudinally by a deep cleft, producing a structure re- 
sembling the tines of a fork. A similar subdivision into three tines is 
observable in the lower incisors of Procavia. As the tooth wears, the cleft 
disappears completely. Apparently the incisors continue to elongate until 
the adult condition is reached, or even later, as the length of these teeth 
below the alveolar border is great, but the tooth does not grow persistently, 
becoming greatly constricted transversely toward the extremity of the root. 
The third incisor and the canine have broad crowns convex externally, but 
excavated internally by a broad groove, producing a serrate effect on the 
cutting edge. The bifurcation of the inferior incisors has been used as a 
character to separate the genus Patriarchus from Protypotherium, but as 
the bifurcation is present in all unworn teeth and absent in well worn 
specimens, with all transitional stages between the two extremes, and as 
there are no other characters of generic importance to separate individuals 
with this peculiarity well developed from those without it, Patriarchus can- 
not be regarded as a valid genus and is here made synonymous with Pro- 
typotherium. The third incisor overlaps internally on the second, the 
canine on the third incisor and the first premolar on the canine. The two 
teeth last mentioned are indistinguishable from each other structurally 
and in many specimens are of approximately the same size. The second 
premolar and the teeth succeeding are also inclined to the long axis of 
the tooth-row, but in a direction opposite to the incisors, canine and first 
premolar, the anterior margin of each overlapping externally the tooth 
preceding, instead of overlapping internally, as in the case of the teeth 
first mentioned. The second, third and fourth premolars increase regularly 
in size posteriorly, but none is completely molariform. Externally, they 
are rendered bilobate by a deep groove. Of the two lobes thus produced 
