EXTINCT UNGULATE MAMMAL EROM PATAGONIA. 
181 
ceroses, as all the latter have these teeth either quite rudimentary and deciduous, or, 
when functionally developed, greatly reduced in number and separated from the molars 
by a wide diastema. There is, however, an American genus from the Lower Miocene of 
Dakota, to which Leidy has given the name of Hyracodon , which, as proved by the 
sockets in the alveolar border, possessed the full complement of incisors and canines as 
in Homalodontotherium *. Unfortunately the characters of these teeth are at present 
imperfectly known ; but they appear to have been more differentiated than those of 
Homalodontotherium, and in fact to occupy an intermediate position between that genus 
and trae Rhinoceros ; so that, judging by the teeth alone, we may place Homalodontothe- 
rium, Hyracodon, and Rhinoceros as three terms of one series of modifications ; and it is 
quite possible that as Hyracodon is of greater geological antiquity than Rhinoceros, so 
Homalodontotherium may be a still more primaeval typef. 
The discovery of this new form throws some light upon the affinities of the very 
enigmatical Nesodon and Toxodon. If, as observed by the first describer of those genera, 
“ the interval between Toxodon and Macrauchenia is evidently partly filled by Nesodon'%, 
Homalodontotherium is another link in the same chain connecting Nesodon with the true 
Perissodactyles. The modifications required to convert the molar teeth of Homalodon- 
totherium into those of Nesodon, especially the lengthening of the crowns, if carried to 
a further degree would result in the rootless persistent-pulped teeth of Toxodon ; and in 
the characters of the incisors and the canines Nesodon is obviously intermediate between 
the other two genera. 
We have thus a new form of mammal, which, as far as the evidence of dental characters 
(by no means sufficient for deciding affinity, as the case of Hyrax shows) allows us to 
judge, is an extremely generalized type, related on the one hand to Rhinoceros through 
Hyracodon, also, though more remotely, to Macrauchenia , and apparently connecting 
these true perissodactyle forms with the more aberrant Nesodon and Toxodon. 
To the fragments of bone sent with the teeth comparatively little importance can be 
attached, as it is quite uncertain whether they belonged to Homalodontotherium or to 
some other animal. The most characteristic among them are :■ — 
1. The greater part of the right innominate bone of an animal about the size of the 
common American Tapir, and more resembling that species than any other with which 
it is comparable, though differing notably in many details, especially the form of the 
acetabulum. 
2. A mutilated dorsal vertebra, which from its size and age (the epiphyses of the 
centrum being detached, as is that of the crest of the ilium in the last-mentioned spe- 
cimen) probably belonged to the same individual. It is, however, far more equine or 
* Extinct Mammalia of Dakota and Nebraska (1869), p. 232. Ancient Fauna of Nebraska (1853), p. 81, 
pis. xiv., xv. 
t A more detailed comparison with the teeth of Macrauchenia would be desirable, but unfortunately the 
materials are not as yet forthcoming; judging from an unpublished drawing kindly lent me by Professor 
Gervais, the incisors and premolars of that genus have quite a different shape. 
i Owen, British-Association Reports (1846), vol. xvi. p. 66. 
