SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 
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and limb-bones, to which attention will be called in the succeeding 
pages. Of these differences, perhaps the most important are to be found 
in the calcaneum and astragalus, which, though of the same type, are yet 
quite distinct. 
Dentition (Pis. XVII, figs. 4, 5, 10; XVIII, fig. 5 ; XX, fig. 2; XXVII, 
fig. 5). — The dental formula is the same as in Nesodon , If, Cf, P i, Ml, 
and the individual teeth very closely resemble the corresponding ones of 
the latter, though with certain small and constant differences, which are 
not altogether without importance. 
A. Upper Jaw. — The median incisor is broad, heavy and chisel-shaped, 
contracting much to the root and therefore diminishing in size with age 
and the progress of abrasion, while i- is a large, trihedral and acutely 
pointed tusk, growing from a persistent pulp. Both teeth have the same 
form as in Nesodon and pass through the same remarkable series of 
changes in appearance as in that genus. (See p. 1 1 9). Ameghino states 
(’94 / ' ) 227) that in the fully adult animal the posterior part of this tooth 
becomes narrowed, ceases to form enamel and gradually develops a long, 
cylindrical root, but I have seen no example of this. The third incisor 
and the canine are very small, simple teeth, relatively even smaller and 
of less functional importance than in Nesodon. There is some individual, 
or possibly specific, difference with regard to the position of the canine ; 
it may either be almost in contact with the first premolar, or separated 
from it by a very short diastema, or, more rarely, by a considerable 
diastema. 
The second and third premolars are somewhat less complex than the 
corresponding teeth in Nesodon , though the difference is not shown in 
worn teeth, but only in those which are quite freshly erupted ; it consists 
in the much smaller size of the posterior valley in the teeth of Adino- 
therium and in the reduced number, or even absence, of the pits in the 
floor of that valley. In the worn state, the only noteworthy difference 
from the premolars of Nesodon is in the external wall of the crown, which 
in p- is less distinctly divided into two lobes, but much more so in p- 
and In Nesodon , it will be remembered, this division is almost obso- 
lete in p-. The molars are so precisely like those of Nesodon , save only 
in size, as to require no particular description. 
B. Lower Jaw . — The incisors and canine are almost exactly like those 
of Nesodon , but Ameghino states (loc. cit.) that the tusk-like i-3 eventually 
