122 
PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
obscurely, a division into two nearly equal lobes by a shallow median 
groove. 
The upper molars are entirely different from the premolars both in shape 
and pattern, being, in the fresh or little worn state, elongate antero-pos- 
teriorly and narrow transversely and having an incompletely lozenge- 
shaped masticating surface. The angle between the anterior and external 
walls is quite acute and overlaps and projects prominently external to the 
tooth in front, giving to the molars the appearance of being set en echelon , 
while the outer faces of the premolars all lie in nearly the same vertical 
plane. The relative proportions of the three molars and the shape of 
their crowns change much with age and wear. When first brought into 
use, m- appears to be much smaller than m-, which, in turn, is consider- 
ably larger than m 1 , but, as m 1 greatly enlarges in antero-posterior diam- 
eter toward the base, this tooth becomes both relatively and absolutely 
Fig. 17. 
Nesodon imbricatus : Last upper molar (m-) 
of the right side, much worn, external view, 
Xi- E mel band white, dentine dark. 
Fig. 18. 
The same, crown view, Xy. Enamel bands 
with double contour. 
larger with advancing age, until it is much the largest of all the grinding 
teeth, though in very old animals, when m 1 begins to develop roots, the 
crown grows smaller toward the base and diminishes in size with abrasion. 
The external wall in the molars shows no division into lobes and is quite 
smooth, but there is reason to think that the posterior lobe is considerably 
larger than the anterior. 
The enamel does not cover the entire crown, but is restricted to vertical 
bands, the extent of which varies in the different teeth and also changes 
