48 
PATAGONIAN EXPEDITIONS : PAL/EONTOLOGY. 
size, while in others it is quite small, present on one side only or entirely 
absent. This character, which does not seem to be due to difference in 
age, has been applied to the separation of two of the species (/. robustum 
and /. extensuni). The second and third incisors are usually separated 
by a slight diastema. Both I- and the canine have laterally compressed 
pointed crowns. A supernumerary canine is sometimes present (PI. 
VIII, fig. 26) or the canine may be entirely wanting (PI. VIII, fig. 
16. Age character). Between I- and the canine a diastema is always 
present, and usually also between the latter and P-. The first upper 
premolar is a simple-crowned, cylindrical, laterally compressed tooth, 
strongly curving inward and backward. The remaining premolars and 
molars may be readily distinguished from those of Protypotherium by the 
great elongation of the antero-external angle and the strong development 
of the outer ridges and the intervening groove. The second premolar 
is incompletely molariform, while the third and fourth resemble the molars. 
This is a distinct advance over Protypotherium , in which none of the pre- 
molars have attained the molariform condition. In the second premolar 
of Interatherium the crown is deeply grooved internally, producing a pair 
of lobes, of which the anterior is the smaller. Antero-externally a deep 
groove is lodged between two ridges, as in Protypotherium , but the groove 
is proportionately much deeper and the ridges higher. The same arrange- 
ment of inner lobes and outer ridges, is observable in the third and fourth 
premolars and in the molars, but in the latter the inner lobes are equal in 
size. The antero-external portion of the crown is greatly elongated. As 
the crown wears down, the external groove with its bounding ridges dis- 
appears and there remains only the elongated antero-external angle (PI. 
VIII, fig. 18). The triturating surfaces of the inner lobes are deeply 
cupped. Plate VIII, figure 20 shows the pattern of the unworn second 
and third premolars. The molar-pattern appears to be very similar to 
these, but no specimen in the collection shows an unworn tooth of this 
series. The crown pattern consists essentially of two internal crescents 
separated by a deep groove, and an ectoloph concave anteriorly and con- 
vex posteriorly. The antero-external groove lies between the anterior 
horn of the first inner crescent and the ectoloph. The crescentic lobes 
are cuspidate internally, but the cusps soon wear down. A cement layer 
1 is present as in Protypotherium. 
In the inferior series, the incisors are pronate with the crowns of the 
