I 12 
PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
with enamel, which forms broad vertical bands. In the post-Santa Cruzian 
genera complete hypsodontism is attained. 
Except in Phobereotherium , the median upper incisor (i A ) is broad, 
antero-posteriorly compressed and somewhat chisel-shaped ; it early de- 
velops a root, toward which the crown contracts and is thus steadily 
reduced in size by abrasion. The second upper incisor (i-) is the tusk 
and is placed in the same transverse line as i-; it is of D-shaped cross- 
section, terminating in a sharp point, and somewhat resembles the bayonet- 
like canine tusk of the peccaries, but is abraded on the posterior face. The 
third upper incisor (i-) is very small, hardly more than vestigial, and can 
have had little functional importance. The same is true of the canines 
in both jaws, which may even be absent, though probably only as an indi- 
vidual abnormality. The two median lower incisors (iy and ?) are broad, 
chisel-shaped and rooted and diminish in size with age and wear. The 
inferior tusk (iy) is laterally compressed and inclined forward, more or 
less procumbent, and is obliquely truncated by the abrasion of the upper 
tusk. In the Pampean Toxodon all the teeth are hypsodont and the lower 
incisors are all strongly procumbent, so that the appearance of the anterior 
teeth is quite different from that seen in the Santa Cruz toxodonts. 
The premolars are all of simpler pattern than the molars and, though 
having very high crowns, they early form roots in the Santa Cruz repre- 
sentatives of the suborder, which distinguishes them from those of the 
contemporary Typotheria. In toxodonts from the Pampean beds the 
premolars are completely hypsodont. The upper molars are all strongly 
curved, with the convexity outward, those of the opposite sides almost 
meeting in the middle line of the palate ; they also are partially hypso- 
dont in the Santa Cruz genera, completely so in the later forms. As 
Sinclair has pointed out (p. 8), the molars are constructed on the same 
plan as those of the Typotheria and have some resemblance, though not 
a close one, to the rhinocerotic pattern, with broad external wall and two 
oblique transverse crests. The grinding surface is much complicated by 
spurs and crotchets given off from the outer wall and transverse crests, 
though in the genera of the post-Santa-Cruzian formations the molar- 
pattern is greatly simplified. 
The lower molars are composed of two crescents, each with an internal 
pillar. The posterior pillar appears to be common to all of the groups of 
the Santa Cruz ungulates, though in some of the genera of the Litopterna 
