86 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
and the second premolar. The remaining upper teeth are in close series. 
All are rootless, with long, inwardly curving crowns and in all the crown 
pattern has been modified by wear, producing a structure closely similar to 
that developed in Hegetotherium. The second premolar is triangular 
in cross-section, decreasing in width anteriorly. Antero-externally, 
there is a shallow groove bounded by ridges and on the lingual side 
of the crown a broad groove situated somewhat farther back than the 
external one. The margin of the ectoloph is serrate, two or three cusps 
being developed, of which the first and second mark the position of the 
external ridges just mentioned, while the third is not located with respect 
to any ridge and corresponds in position with the posterior serration on 
the margin of the ectoloph already described in Protypotherium. The 
third and fourth premolars are molariform and may be described with the 
molars. In these teeth the ectoloph is almost plane externally, except for 
the slight antero-external groove with its bounding ridges. In the molars, 
another slight ridge is present, terminating at the apex of the notch be- 
tween the two most prominent points on the margin of the ectoloph. 
Internally, the crown is evenly convex. The posterior margin is almost 
at right angles to the ectoloph except in M-, while the anterior margin 
meets it at an acute angle. Two cross-crests are developed as a result 
of wear, terminating externally in sharp serrations at the margin of the 
tooth-crown. The first molar is the largest, the series decreasing in size 
posteriorly. M- is narrower posteriorly than anteriorly, with the postero- 
external angle produced as in Hegetotherium , but the degree to which this 
is developed varies somewhat with the state of wear. The premolars and 
molars are arranged in an imbricating series, each overlapping externally 
the tooth just preceding. A layer of cement is usually present. 
In the inferior series (PI. X, fig. 13) the first and second incisors are 
pronate, growing persistently, like those of Hegetotherium , which they 
further resemble in having the enamel limited to the anterior surface. 
The third incisor, canine and first premolar are absent, producing a long 
diastema similar to that in the superior series. The remaining teeth are 
unspaced. P2 is incompletely molariform, P3 and Pt completely so. The 
first mentioned tooth is triangular in cross-section, tapering anteriorly. 
The inner wall is convex posteriorly and broadly grooved anteriorly, 
while externally the crown is divided by a deep groove into two crescentic 
lobes, of which the posterior is the larger. The third and fourth premolars 
