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PATAGONIAN EXPEDITIONS : PALEONTOLOGY. 
lower incisors are flattened and proclivous, the third is triangular and 
raised above the level of the other two. All three are however worn to 
nearly the same transverse line by the upper teeth and their alveoli are 
nearly in the plane of the inferior border of the jaw” (’93, 13, 14). 
While in general character the dentition of Toxodon is thus of the same 
type as that of Nesodon , there are several significant differences which 
should be emphasized. (1) The teeth are all conspicuously hypsodont 
and grow throughout life. (2) I 1 is thin antero-posteriorly, broad and 
scalpriform, while i- is more trihedral and tusk-like. There is a marked 
difference in the relative size of these teeth in the different species ; in T. 
ftlatensis, for example, i- is much longer and more tusk-like than i but 
in T. burmeisteri these proportions are reversed. (3) The third upper 
incisor and the canine have disappeared, and there is a long diastema 
between the incisors and the cheek-teeth. (4) The lower incisors are 
much more strongly procumbent than in Nesodon , pointing almost directly 
forward and with their inferior faces in nearly the same plane as the ven- 
tral side of the symphysis. It and 2 are much broader and more flattened 
than in Nesodon , but i-3 retains something of its trihedral and tusk-like 
form, though worn to the same transverse line as the others. (5) The 
premolars are reduced in number and in relative size and importance, and 
are even less molariform than in Nesodon. (6) The upper molars are 
more triangular and have a much simpler pattern than those of Nesodon; 
the posterior valley has disappeared and only the principal spur, which 
projects into the main valley from the external wall, is retained. In the 
genus Xotodon even this spur has been lost. 
The Skull of Nesodon (Pis. XIII-XV; XVIII, fig. 1) is remarkable 
in many ways and strikingly different from the typical ungulate skull, but 
closely similar to that of Toxodon ; its most characteristic peculiarity is in 
the remarkable structure of the auditory region, which Roth (’03) was the 
first to point out and has so strongly emphasized. The upper profile of 
the skull may be nearly straight from the occiput to the end of the nasals, 
or may rise quite steeply from the forehead to the occiput, a difference 
which does not appear to be of specific value. The orbits are well forward, 
the anterior rim being in advance of the middle of the skull, making the 
cranial region considerably longer than the facial, but the brain-case 
proper is short, narrow and of small capacity. The great development 
and spread of the zygomatic arches give to the cranium a width which 
