PROFESSOR OWEN ON SOME SPECIES OF THE EXTINCT GENUS NESODON. 309 
Concluding Remarhs. 
The osteological characters defining the orders of hoofed quadrupeds, called Pro- 
hoscidia, Perissodactyla and Artiodactyla, are associated with modifications of the 
soft parts of such importance, as not only to establish the accuracy of the principles 
of that ternary division of the great Natural group of IJngulata*, but to indicate that 
the known modifications of the skeleton of the extinct Toxodons and Nesodons of 
South America, in the degree in which they differ from the osteology of the already 
defined orders of Ungulata, must have been associated with concomitant modifica- 
tions of other parts of their structure, which would justify, and indeed compel, the 
consistent classifier to place them in a distinct division of the Ungulata, of equal 
value, if not with the Perissodactyla and Artiodactyla, at least with the Proboscidia. 
Like the Proboscidia, this group, which I propose to call Toxodontia, is more nearly 
allied to the Perisso- than to the Artio-dactyle orders. 
This is shown by the large and complex third and fourth premolars {p 3 and p 4), 
by their close similarity with the true molars, by the unsymmetrical oblique foldings 
and islands of the enamel, and by the great length of the crowns of the molars, to 
which the Horse alone offers any near approach amongst existing Ungulates. By 
the form and proportions of the eminentia articularis, of the glenoid cavity, and of 
the post-glenoid process, — and by those, also, of the lacrymal bone, of the zygomatic 
arch, and of the orbit, — the Toxodontia are most closely matched by the Tapir and 
Rhinoceros in the Perissodactyle order. 
The dental and osteological characters detailed in the text, whilst they illustrate 
the closer mutual affinities between the Nesodons and Toxodons, establish their claim 
to be regarded as types of a distinct order of Ungulata-, and they also tend to dis- 
sipate much of the obscurity supposed to involve the true nature of the genus Toxodon, 
and to reconcile the conflicting opinions as to its proper place in the Mammalian 
Class. 
The fossils above described were discovered on the coast of Patagonia to the south 
of Port St. Julian, and my friend Mr. Charles Darwin, F.R.S., has kindly com- 
municated to me the following opinion as to the formation in which they were 
imbedded: ‘‘These beds resemble mineralogically the upper ancient tertiary forma- 
tion of Patagonia, but Ehrenberg found the included microscopical organisms wholly 
different from those of the ancient tertiary formation, being of freshwater and 
brackish origin (p. II7 of my Geological Observations on South America). Hence 
these beds are of unknown age, probably younger than the old tertiary and older 
than the superficial beds in which Macrauchenia was found.” 
* Memoir on the Anthracotherioid Animals, in ‘ Quarterly Journal of the Geological Society,’ Nov. 1847, 
and < Osteological Catalogue of the Museum of the Royal College of Surgeons,’ 4to, p. 629. 
