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PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
the rounded anterior border. Enamel is not present on the posterior face, 
which is bevelled by the abrasion of h, but covers the front and sides in 
a thick layer. Apparently, there is a sexual difference to be noted in the 
development of the tusk-like i-, which is heavier and more robust in some 
individuals than in others of similar age, and there is nothing in the 
skull-structure to indicate that this difference is specific rather than 
sexual. 
The first and second incisors are implanted in the same transverse line 
and form the front of the broad muzzle, but i- is inserted in the same fore- 
and-aft line as the premolars and is quite invisible from the front. This 
tooth is very much smaller than i 1 or i- and of simple, irregularly style- 
like form, but, in the unworn condition, with enamel-pit on the cutting 
surface; it cannot have had much functional importance. To a varying 
degree, the enamel is absent from more or less of the inner face. A short 
diastema in front of and behind it gives to this small tooth an isolated 
position. 
The canine is slightly larger than i-, which it resembles in form and in 
the presence of an enamel pit on the masticating surface, while still un- 
worn ; it stands quite near p 1 - and, at first sight, appears to be one of the 
premolar series. This tooth also must have been functionally insignificant. 
The seven cheek-teeth (Pis. XIII, XV, XVII, figs 1-5) form a contin- 
uous series, progressively increasing in size from p L to m-, though all the 
premolars are of simpler pattern than the molars. These teeth are all 
strongly curved, with the convexity turned outward, as in Toxodon , and 
almost meet those of the opposite side in the median line of the palate. 
When first erupted, they have open bases and no roots, which are devel- 
oped later, first in the premolars and then successively in the molars, m- 
remaining rootless till a very advanced period. In freshly erupted and 
unabraded condition, the premolars all have a very deep central pit, or 
valley, completely enclosed by the external wall and a continuous ridge, 
which joins the outer wall in front and behind and bounds the valley 
anteriorly, internally and posteriorly. This valley is much complicated 
by spurs and pillars given off from the enclosing wall, their number and 
prominence differing in the successive teeth. In the first premolar, p 1 , 
which is the smallest and simplest of the series and is implanted by a 
single root, there is no spur, but the valley is in two portions, an anterior 
and much deeper part and a posterior, shallower part. When the tooth 
