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PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
ever, these complications disappear ; the outer portion of the valley, the 
oblique ridge, spurs and fossae are no longer visible, the external wall is 
much thicker and the valley is a mere oblique slit, which eventually is 
converted into a closed lake. The external wall has the same ridges and 
sinuosities as in m 1 , but extends farther behind the posterior crest, en- 
closing a larger fossa. In the unworn condition this wall has a tren- 
chant edge and behind the metaloph curves upward abruptly to the base 
of the crown. When abrasion is well advanced, hardly any difference 
from nr 1 remains, except the greater antero-posterior elongation of the 
crown. 
When fully protruded, m- is still more elongate antero-posteriorly than 
m- and is of quite a different shape, being less quadrate and more tri- 
angular, somewhat as in the rhinoceroses, but the external wall is not fused 
with the metaloph, as it is in the latter. In the freshly erupted tooth the 
crown-pattern differs in several details from that of m-. The anterior 
crest is well developed and at its inner end, behind the protocone, a con- 
spicuous conical cusp is added, no trace of which is visible in m 1 or 
the anterior crest is thus L-shaped, and its transverse and antero-posterior 
limbs are of nearly equal length. The notch between the anterior crest 
and the outer wall is much shallower than in m-, nothing more than a 
slight depression of the free margin. The external wall has a trenchant 
edge, where the thick enamel ends abruptly, and, seen from the side, the 
tooth has somewhat the appearance of a carnivorous sectorial ; the upward 
curve of the trenchant edge begins a little behind the anterior transverse 
crest. The sinuosity of the outer wall is very much as in m-, but the 
anterior vertical ridge is even less distinct. 
The posterior transverse crest is greatly reduced and is very short and 
very low ; to this reduction is chiefly due the triangular shape of the crown ; 
the crest is widely separated from the external wall, a striking difference 
from m 1 and -, but it is, nevertheless, a crest and not a conical cusp. 
The oblique ridge which in m- passes forward from the isolated apex of 
the posterior crest, in m- originates in the middle of the crest and is 
shorter, lower and less distinct than in m-, not dividing the valley into 
internal and external portions and extending only to the third of the four 
spurs which project from the outer wall. These spurs are shorter, thinner 
and more closely crowded together than the two which are found in m-; 
a small fossa is enclosed between the third and fourth spurs and the 
