IOO 
PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
P3 is entirely different from the shape seen in all the other species. 
The inner side is almost plane, presenting a notable analogy to that of P. 
obstruction* \ however it displays three faintly marked vertical crests 
which bound two broad but entirely superficial depressions. The outer 
side is distinguished by a broad and shallow perpendicular depression 
which does not form a reentering fold, a depression which replaces the 
deep and narrow groove that the other species have in the hinder part of 
the outer side, forming a sharp entering fold. The perpendicular pos- 
terior face, instead of being narrow and rounded as in the other species, 
is of the same width as the rest of the tooth and grooved perpendicularly 
in the middle. The grinding surface is of regularly elongate or elliptical 
shape, somewhat flattened on the inner side but not divided into unequal 
lobes as in the other species. It measures .0035 by .0025 
P T has an internal vertical groove and another external, opposite, as in 
almost all of the other species, but the two lobes are of nearly equal size, 
the anterior slightly the larger. It measures .004 by .0026. 
The first and second molars are bilobate by opposite vertical grooves 
as in the other species, with the anterior lobe considerably smaller than 
the posterior. On the inner side the former is convex, ending behind in 
a broad and rounded column, but the posterior lobe is depressed and 
somewhat excavated in its anterior part. Each molar measures .0065 by 
.003. Length P3-M2 inclusive .021. Two mental foramina are present, 
a larger one under P ¥ and a smaller one beneath P T . Depth of hori- 
zontal ramus at M T .016 (’89, 480). 
INTERATHERIUM. 
Interatherium trilineatum (Ameghino). 
Icochilus trilineatus Amegh. ; Enum. Synoptique, etc., p. 16, 1894 ; Se- 
gundo Censo, etc., p. 15 1, 1898 (listed). 
This species is of the same size as /. externum, from which it is easily 
distinguished, as well as from the other species, by the form of the upper 
molars and premolars. Each of these teeth has on the external face a 
broad and deep groove which divides it into two lobes and on each lobe 
there is a narrow and deep groove which divides it into two vertical 
columns. On the external face there are thus four columns separated by 
* A Monte Hermoso species. 
