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PATAGONIAN EXPEDITIONS : PALAEONTOLOGY. 
pit in the floor of the posterior valley. At a certain stage in the abrasion 
of each molar, but not more than one at the same time, there are three 
enamel-lakes in the masticating surface ; of these, the anterior one is 
formed by the isolation of the slit between the posterior horn of the an- 
terior crescent and the spur of the posterior valley, the second by the 
fossa in that spur itself, and the third by the pit in the floor of the pos- 
terior valley. The small differences of structure between the molars and 
premolars thus become clear, only when several stages of growth and 
abrasion are available for comparison. The last molar (mj) is remarkable 
for the great antero-posterior extension and transverse narrowness of the 
hinder lobe. As the posterior border of the tooth slopes strongly back- 
ward and downward, the antero-posterior length of the crown increases 
as the tooth is worn down and begins to decrease only in old individuals. 
Milk Dentition (Pis. XVII, fig. i ; XVIII, fig. 6 ; XIX, figs. 1-3, 7-8 ; 
XXI, figs. 1-2). — This temporary series of teeth is complete, all of the 
antemolars having predecessors in it, and there are some striking differ- 
ences between the corresponding teeth of the two series, especially among 
the incisors, where the absence of tusks from the milk-teeth makes a great 
difference in the appearance of the skull and jaws, a difference which has 
led to the creation of many synonyms. 
A. Upper Jaw . — The first incisor (di 1 ) is much the largest of the series 
and, while still quite unworn, considerably resembles the permanent i 1 , but 
is, of course, actually smaller and proportionally thinner antero-posteriorly. 
When the root has been formed and the crown considerably worn, the 
appearance of the tooth is much changed, as it then has a low and very 
broad crown, contracting abruptly into the long and slender root. The 
second incisor (di-) is not placed in the same transverse line with di— , as 
are i- and i-, but behind it and in line with the cheek-teeth. It is at first 
much smaller than di 1 and of quite a different shape, being trihedral with 
rounded angles, except the postero-external one, which is a sharp ridge. 
In the more advanced and rooted stage the crown has become much 
broader, resembling that of di 1 , but smaller. The third incisor (di— ) is the 
smallest of the series and has a simple, laterally compressed crown, the 
principal diameter of which is antero-posterior. The canine resembles 
di- in form, but is somewhat larger ; the temporary incisors and canine 
all have a shallow enamel-pit, or mark, on the cutting surface, but this 
is soon obliterated by wear. 
