298 PROFESSOR OWEN ON SOME SPECIES OF THE EXTINCT GENUS NESODON. 
the mouth backwards as far as, and a little beyond, the last molar m place ; but this 
part of the palate is formed for some extent externally by the maxillary bones (Plate 
XV. fig. 3, 21 ), whilst in the Artiodactyles it is formed by the palatines, except where a 
very thin plate of the maxillary completes the socket of the last molar on its inner side. 
Neither family nor subordinal correspondence resulting from the comparisons with 
existing Ungulata, they were next extended to the extinct forms, and especially to 
those that had previously been discovered in South America. But not to prolong 
the description with needless details, I may confine myself to the remark, that, in 
this inquiry, the nearest resemblance to the Nesodon was found to be offered by that 
hitherto anomalous genus for which I have proposed the name of Toi'odoti*. 
In the Toxodon platensis the bony palate is continued, as in existing Artiodactyles, 
beyond the molar series ; but the proportion of this back part which is contributed by 
the maxillaries resembles that in the Nesodon, and is greater than, and diffm'ent m form 
from, that in the Hog-tribe, the Hippopotamus and the Ruminants. The palate is 
arched in the Nesodon, and also, but with a deeper concavity, in the Toxodon : the 
post-palatal canals in both have the same advanced position of their oblique outlets 
upon the maxillary; in both Perisso- and Artio-dactyles those outlets are close to the 
maxillo-palatal suture. In the form of the ‘ cavitas et eminentia articularis,’ and of 
the post-glenoid process, the Toxodon more resembles the Nesodon than does the 
Tapir or the Horse ; the convex and protuberant front and lower part of the process 
and the excavation of its back part are repeated in the Toxodon. The strong, deep, 
sigmoidally bent zygoma, and the vertically extended orbit communicating widely 
be^hind with the temporal fossa, are characters common to the Nesodon and Toxodon ; 
but the orbit is relatively wider, the malar bone is narrower, and its post-orbital 
process less prominent in the smaller extinct South American Herbivore. In the 
concavity of the outer surface of the deep facial part of the maxillary, and the 
anterior expansion of the premaxillaries, the Toxodon resembles the Nesodon ; but 
the most important evidence of their mutual affinity is afforded by the dental 
system. 
The molar series has a similar arrangement in the Toxodon, describing a line 
gently convex externally, the teeth gradually decreasing as they advance, and having 
the same close-set, imbricated disposition. The length, curvature and deep implanta- 
tion of the undivided crown of the upper molars of the A esodon, as exemplified in 
the fifth molar of the specimen described, fig. 5, m 1 , are important marks of agree- 
ment with the upper molars of the Toxodon, in which those characters are present in 
a greater degree. The same molars of the Toxodon also present an obvious similarity 
to those of the Nesodon in structure. The enamel is very thin : a deep oblique fold, 
penetrating from the inner side, divides the crown into two lobes ; and a short fold 
indents the inner side of the hinder lobe. The anterior external angle of the tooth is 
produced like a ridge to form the overlapping part in the imbricated molar series : 
* Zoology of the Voyage of the Beagle, Fossil Mammalia, p. 16, Plates I. to V. 4to, 1840. 
