SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. 
129 
being a cylindrical, quill-like tooth. During my visit to La Plata, how- 
ever, I was convinced by the study of the materials in Dr. Ameghino’s 
collection that, so far at least as the mandibular incisors and canines are 
concerned, there were actually functional teeth of the pre-lacteal series in 
Nesodon. One mandible (PI. XIX, fig. 6), in particular, was especially 
instructive, as it showed the milk-incisors and canines in process of erup- 
tion and entirely unworn, with the roots of the pre-lacteal series still in 
place. Unfortunately, the photographs which I made of this and other 
specimens in a corresponding stage of development are not satisfactory for 
reproduction, but apparently the same individual is figured by Ameghino 
(op. cit., fig. 1, p. 12). 
This remarkable phenomenon loses something of its entirely exceptional 
character through Ameghino’s recently published observation of a pre- 
lacteal premolar in the tapir. 
The succession of the teeth in Nesodon may be followed out in much 
detail. The first of the permanent teeth to appear is mi, in both jaws, 
which becomes functional before any of the milk-teeth are shed. This is 
followed by pi, dpi being shed so early that it is rare to find an individual 
retaining it and, in some cases at least, the replacement takes place before 
mi comes into use. Next follows m2 and then dii is shed and replaced 
by ii, while di2 still continues in function. The second and third incisors, 
the canines, second and third premolars are replaced in rapid succession 
and before the last molar is erupted ; m3 then comes into use, and, last of 
all, dp4 is replaced. 
This account, which is drawn chiefly from the material in the Princeton 
collection, does not exactly agree with the description given by Ameghino 
(’94^, 233) according to which pi is not erupted until after m3 has ap- 
peared. No doubt, there is some individual variability in the order of 
succession. 
Ameghino (’91, 357 ff.; ’94^, 231-234) has summed up the individual 
changes in the dentition of Nesodon , dividing them into twelve stages, 
which are here reproduced in abbreviated form, with the addition of a 
stage for the prelacteal dentition. 
Stage A. — Prelacteal dentition of three incisors, a canine and three 
premolars, i- small, simple and styliform. 
