scott: toxodonta OF THE SANTA CRUZ beds. 
203 
tion. The most obvious and constant differences are (1) the palatine 
constriction at p 1 and the forward expansion of the muzzle from that 
point, which is much more marked than in Nesodon and suggests the 
character of Toxodon ; (2) the presence of small rugosities on the frontals, 
which Ameghino regards as the bases of incipient horns ; (3) the greater 
length of the sagittal crest ; (4) the different character of the fronto-nasal 
suture. 
The upper profile of the skull is somewhat sinuous, but does not 
depart widely from a straight line and lies nearly in the same horizontal 
plane throughout. The sagittal crest is slightly convex in the antero- 
posterior direction, and the nasals are also somewhat convex in the same 
direction ; between the two, there is a moderate descent at the forehead. 
The occiput is even lower and wider proportionately than in Nesodon , 
though the difference is not a marked one, and the zygomatic arches 
rise rather higher upon the lambdoidal crest, giving the skull a some- 
what different appearance in side-view. (See PI. XII, figs. 1 and 2.) 
The remarkable peculiarities of the auditory region are identical in 
the two genera except that the mastoid and its process are proportionately 
less developed in Adinotherium. The parietals are relatively somewhat 
longer than in Nesodon and are contracted by a more definite postorbital 
constriction, and the brain-case is slender and of small capacity. 
The frontals differ from those of Nesodon in several respects ; the supra- 
ciliary ridges are rather better defined and extend nearer to the postorbital 
processes. Between the supraciliary ridges, there is, in a certain proportion 
of the skulls, a raised and more or less roughened area, which may 
be single, but is much more commonly divided by a median depres- 
sion. Ameghino (’07) regards this rugosity as the attachment for 
an incipient horn and this is very probably the correct explanation, but 
there is no such definitely rounded form as the dermal horns have in the 
Perissodactyla, even in the earliest stages, and the median depression 
would seem to indicate the presence of a pair of very small horns rather 
than of a single one. Further, there is some indication of a sexual 
difference in the development of the horns, as in some skulls the 
rugosities are much less prominent, or may be absent altogether, a 
difference which is not correlated with the age of the various individuals. 
Whatever its significance, this roughened area is an almost constant differ- 
ence from the smooth, plane or even concave surface, which occupies the 
