SCOTT: TOXODONTA OF THE SANTA CRUZ BEDS. I 1 3 
it is much reduced or even absent. This pillar divides the valley of the 
posterior crescent into two parts and does not close it internally, as it 
does in the Santa Cruz Typotheria. Like the upper molars, the lower 
ones are incompletely hypsodont and form roots, at a comparatively late 
stage. 
The milk-dentition differs in several respects from the permanent one. 
The incisors, which are rooted before eruption, are all more or less chisel- 
shaped, not forming tusks, and thus the appearance of the young skull 
with milk-teeth is very different from that of the adult and has led to great 
confusion in the nomenclature. The grinding teeth are quite similar to 
their permanent successors, but are more brachyodont and dpq is 
molariform. 
Ameghino has reported the very extraordinary fact that in Nesodon and 
Adinotherium there is a complete and functional pre-lacteal dentition, 
which is not known to be the case in any other mammal, although the same 
author has shown that pre-lacteal grinding teeth are present in the tapir. 
The skull, though of very much larger size and quite different appear- 
ance and proportions, is yet constructed upon essentially the same plan 
as that of the Typotheria, the toxodont skull having much less resemblance 
to that of the Rodentia. The cranium is relatively narrow and the facial 
region broad, without such a long and slender rostrum as characterizes 
the Typotheria. On the other hand, the skull and mandible are deeper 
dorso-ventrally, which gives a peculiar appearance to the head. The 
occiput, which differs considerably in the various genera and species of the 
group, is proportionately narrower and higher than in the Typotheria. 
The auditory apparatus is of the same exceptional structure as is found 
throughout the order and in the Toxodonta has few special peculiarities. 
The post-tympanic portion of the squamosal (or perhaps, as Roth main- 
tains, the mastoid) contains a large, oval cavity, which communicates 
with the cavity of the tympanic bulla, but it is not so inflated as to form 
an external protuberance on the surface of the skull, as it is in such 
Typotheria as Pachyrukhos and Hegetotherium. On the other hand, 
this part of the squamosal forms a larger proportion of the occipital 
surface than in the Typotheria or Entelonychia and the external audi- 
tory meatus has a more elevated position than in the other suborders, 
owing to the extensive union of the post-tympanic and postglenoid pro- 
cesses with the intervening mastoid portion of the periotic (the protuber- 
