258 PATAGONIAN EXPEDITIONS! PALAEONTOLOGY. 
horn curving inward and backward and taking the place of the internal 
pillar of the premolars, with which it is perhaps homologous, though 
probably it is not. The valley of the anterior crescent is narrow, but 
deeply impressed and thus differs strongly from the same structure in 
the premolars ; that of the hinder crescent is much broader, but relatively 
shallower. In the valley of the posterior crescent is a vertical pillar, or 
spur, such as is found in almost all Santa Cruz ungulates and is highly 
characteristic of the South American types generally. In the rare cases, 
such as the horse-like Thoatherium , in which this pillar is lacking, its 
absence is obviously due to a secondary reduction. In Homalodontotherium 
the pillar is very conspicuous, and, after a short period of abrasion, becomes 
connected with the hinder horn of the anterior crescent, enclosing an enamel 
lake, which is worn away in old teeth. The cingulum is quite strongly 
developed on the internal and external sides of the crown, but not on the 
anterior and posterior ends. The three teeth of the series are very closely 
appressed, the anterior face of each being flattened or concave and the 
posterior face convex. On these approximate surfaces the enamel is in- 
complete, being reflected over from the inner and outer faces, but becomes 
very thin and dies away in the middle. Needless to say, the hinder end 
of m3, which is freely exposed, is completely covered with enamel. 
Nothing has yet been learned of the milk-teeth in this genus. 
The dentition of Homalodontotherium is obviously of the same plan as 
that of the Toxodonta, but is in a somewhat simpler and more primitive 
stage of development. That there is a certain resemblance, especially in 
the grinding teeth, to the dentition of the rhinoceroses, is not to be 
denied and Flower goes so far as to say: “On comparing a lower molar 
of Homalodontotherium with an equally worn tooth of Rhinoceros , it will 
be seen that they are formed on precisely the same type.” “The molar 
and premolar teeth of both upper and lower jaws thus without question 
show strongly marked Rhinocerotic characters” (/. c., p. 180). A study 
of perfectly fresh and unworn teeth, however, leads to the conclusion that 
the rhinocerotic resemblances are superficial and without special signifi- 
cance, the toxodont affinities being far stronger, and, as has been shown 
in a preceding section (p. 112), the toxodont dentition has certain distinct 
likenesses to that of the rhinoceroses. 
In making the comparison, the peculiar and simplified pattern of the 
grinding teeth in the Pampean toxodonts should be omitted from con- 
