68 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
of the crowns of these teeth cannot be ascertained, as the enamel-covered 
portions have been worn away, leaving merely cylinders of dentine. In 
all the specimens examined the teeth are well worn, but the patterns de- 
veloped by attrition in the molars and premolars and the outlines of the 
crowns are characteristic, and cannot be confused with those of any other 
genus. The first premolar is more or less cylindrical in shape, depending 
on the amount of wear to which it has been subjected. The crown is 
slightly curved, with the convexity of the curve directed anteriorly; the 
buccal surface is convex ; the lingual surface is sometimes slightly 
grooved. The worn area on the occlusal surface varies in different indi- 
viduals from a deep notch to a slight, postero-internally directed concavity. 
The second premolar is triangular, with the base of the triangle directed 
postero-internally. Externally, the anterior portion of the crown is chan- 
neled by a groove between two ridges (PI. I, fig. i). The third and 
fourth premolars are molariform and may be described together with the 
molars. Externally, the crowns are convex, the convexity flattening out 
anteriorly and posteriorly. An antero-external groove is present in each, 
but its depth and distinctness decrease with the age of the individual 
and the amount of wear to which any particular tooth has been subjected. 
Internally, the crowns are convex except in the third molar, which is 
slightly grooved on the inner side posteriorly, the depth of the groove 
decreasing as the tooth wears down. The antero-external angle of the 
molars and molariform premolars is slightly elongated. Posteriorly, these 
teeth are rectangular in outline except in the third molar, where the pos- 
tero-external angle is considerably elongated. The degree of elongation 
of this portion of the third molar varies somewhat in different individuals. 
The triturating surface of the crown of each tooth (P--M-) is traversed 
by two ridges of dentine trending inward from two serrate cusps in the 
enamel of the buccal face. These serrations interlock with the external 
crescents of the lower molars. The molars and molariform premolars are 
invested externally with a thin layer of cement. All are hypsodont. The 
first molar is the largest, the remaining members of the series decreasing 
anteriorly and posteriorly. The primary crown pattern must have been 
quite superficial, as the surfaces developed by attrition show no traces of 
enamel lakes or other indications of a deeply indented crown. From P- 
to M- all the teeth are implanted obliquely, the antero-external portion 
of each projecting beyond the postero-external angle of the tooth next 
