SCOTT: ENTELONYCHIA OF THE SANTA CRUZ BEDS. 
241 
The premolars are always less complex than the molars, though the 
transition from one to the other is gradual, except in some of the earlier 
genera, in which the molars are extremely simple. Probably even in 
those cases, however, perfectly unworn teeth would show a difference 
between the two classes. Ameghino states (’02, 34-5) that in Prochalico- 
therium , of the Colpodon Beds, the upper premolars have isolated and 
conical inner cusps, which are not connected with the external wall till a 
very advanced age. I have not seen these specimens. The molar-pattern 
is essentially that of the Toxodonta. In the upper molars the external 
wall is completely formed, even in the most ancient genera, and is nearly 
smooth, the lobes being very obscurely, or not at all, indicated ; there are 
two oblique transverse crests, of which the posterior one is much the shorter, 
and a varying number of spurs or “combing plates” from the outer wall, 
which are much less prominently developed than in the Toxodonta. 
When well worn, these teeth have as a conspicuous feature an elongate 
and narrow enamel-lake, the remnant of the main valley, which has a very 
oblique direction, more antero-posterior than transverse. The lower 
molars are made up of two crescents, the hinder one of which is much 
more elongate antero-posteriorly and has in its internal valley a spur or 
pillar, such as is found in all of the groups of indigenous South American 
ungulates. That the grinding teeth of the Entelonychia have a certaiu 
resemblance to those of the rhinoceroses is not to be denied, but perfectly 
fresh and unworn examples show this likeness to be but superficial, the 
approximation to the Toxodonta being much closer and more fundamental. 
As Roth has pointed out (’03), all of the Toxodontia, or Notoungulata, 
agree in the unusual structure of the auditory region, but each of the three 
suborders has its particular modification of the general plan. In the 
Entelonychia the inflated post-tympanic region of the squamosal makes up 
less of the occipital surface than in the Toxodonta; the mastoid portion 
of the periotic is excluded from the surface of the skull, by the junction of 
the post-tympanic and postglenoid processes, and the external auditory 
meatus is tubular in the Notostylopidae, and a mere hole in the Homalo- 
dontotheriidae. Only in these two families is the skull known, if, as I 
think should be done, the Leontiniidae are excluded from this suborder. 
In the Notostylopidae the skull is depressed and flattened and has a very 
short, pointed muzzle, which, with the enlarged and scalpriform incisors, 
gives it quite a rodent-like appearance. Little is known of the skull in 
