288 
PATAGONIAN EXPEDITIONS I PALAEONTOLOGY. 
be noted between the more ancient genera, like Tliomashuxleya , and the 
Santa Cruz forms, the latter stage marking the end of the series. 
In the Toxodonta the canines are greatly reduced and have lost their 
functional importance, but the second upper and third lower incisors 
develop into tusks, which grow from permanent pulps. The Typotheria 
show great variety in the character of the incisors ; in some, e. g. Pvo- 
typotherimn , all of the anterior teeth are of moderately large size and very 
much alike in form ; in others, such as Interatherium and the Hegeto- 
theriidae, there is a marked tendency for the first upper incisor to enlarge 
greatly and become scalpriform and, in the latter family, the tooth is root- 
less and grows from a permanent pulp. Coincident with this enlargement 
of the median incisor there is great reduction and ultimate loss of the 
other incisors, the canine and one or more of the anterior premolars. In 
the mandible the median and, in less degree, the second incisor become 
scalpriform, while the succeeding teeth in a varying number atrophy and 
disappear. In the larger and later genera, such as Typotherium itself, 
which has the formula, ii, c§, pf, mf, the scalpriform incisors are very 
large and the rodent-like appearance of the dentition is so strongly 
marked that these animals have actually been referred to the Rodentia. 
Protypotherium displays the exceptional peculiarity of having the lower 
incisors deeply cleft and fork-like, somewhat resembling those of the 
Recent Procavia (Hyrax). In the Entelonychia the incisors and canines 
have undergone less modification from the primitive type than in the other 
suborders. Aside from the early family of the Notostylopidae, in which 
the median incisors are scalpriform, the incisors are all retained and have 
simple, conical or hastate crowns, and the canines are quite large, though 
not very prominent tusks. The canines are relatively reduced in size in 
the Santa Cruz genera, Homalodontotherium and Diorotherium , especially 
in certain individuals which are presumably females. 
The premolars are almost always smaller and simpler than the molars, 
though the hinder ones approximate the molar pattern so closely, espe- 
cially in the Santa Cruz Typotheria, that only in unworn teeth is the dif- 
ference appreciable. The first premolar, particularly the upper one, is 
generally very small and not infrequently wanting and in Typotherium 
the premolars are reduced to f. The pattern of the upper molars is some- 
what rhinocerotic in character, a feature which is least marked in the 
Typotheria and is lost in the Pampean Toxodonta. The outer cusps 
