250 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
description. The external face of the crown, in the unworn or moderately 
abraded condition, is short antero-posteriorly, and quite high dorso- 
ventrally, but their appearance changes much with advancing abrasion, 
which steadily reduces the height of the teeth. This outer face seems to 
be convex, but there is a well-defined vertical ridge near the anterior 
border and another, much less distinct, near the posterior border, with a 
very shallow concavity between the two ridges. Two short, transverse 
crests are given off from the anterior and posterior borders of the external 
wall and curve towards each other so as to meet internally. The two 
internal cusps (deutero- and tetartocones) with which the transverse crests 
are indistinguishably fused, are very incompletely separated by a shallow 
vertical groove and only in perfectly unworn teeth is there an internal 
opening of the valley between the apices of the inner cusps, the valley 
being almost immediately converted into a closed enamel-lake. I have 
actually observed this internal opening of the valley only in p A , no 
perfectly unworn examples of p- and - being available. If present in 
them at all, the opening must be exceedingly shallow. Flower states 
that p-, “in addition to the principal oval fossa [/. e., valley], has a second 
smaller one behind and to the outer side of it” (p. 177). This is very 
early removed by attrition ; it appears in one of the skulls of the La Plata 
Museum collection, but not in any other individual which I have examined. 
The molars have considerable resemblance to those of the rhinoceroses, 
but the likeness is seen to be a superficial one, when the freshly erupted, 
unabraded teeth are examined. While all of the upper molars are of the 
same essential pattern, they differ appreciably from one another in con- 
struction. Here again, it is necessary to compare unworn teeth, for the 
differences are much less clear after abrasion has made much progress. 
The molars are notably larger than the premolars, the change from p- to 
m A in this respect being quite striking, and not only is there a difference 
of size, but also one of proportions. In the premolars the transverse 
diameter of the crown distinctly exceeds the antero-posterior, while in the 
molars the two diameters are nearly equal, or the antero-posterior is the 
greater one. The fore and aft dimension increases posteriorly and is 
greatest in m-, though the width of m- makes that tooth the largest of the 
series. All the molars are so closely appressed that the antero-external 
angle of each tooth overlaps the one in front of it, another difference from 
the premolars, which is also found in the Toxodonta. 
