EXTINCT UNGULATE MAMMAL EEOM PATAGONIA. 
175 
very fragmentary condition, owing to the friable nature both of the bone itself and the 
matrix, that they afford no satisfactory evidence of the structure or affinities of the 
animal ; on the other hand, the teeth are in a remarkably good state of preservation, 
and consist of the almost entire permanent series of an individual just arrived at maturity, 
and therefore in the best condition for affording such information as to the general 
characters of the animal as may be obtained from the dental structures of a single 
specimen. The teeth that are missing are certain of the inferior incisors and the poste- 
rior upper true molars. As nearly all the bone of the cranium had perished, and it has 
been necessary to adjust and cement the teeth artificially in place *, the exact number that 
were present cannot be ascertained with absolute certainty ; but I have scarcely any 
doubt but that the animal possessed the complete typical number, forty-four, and that 
therefore four incisors are wanting from the lower jaw. In the upper series there were 
certainly eleven on each side. As in the specimens of Nesodon from the same locality, 
they have acquired a very dark, almost black colour. 
All the teeth have crowns distinctly separated from the long and tapering roots, and 
with a well-marked cingulum around their base ; but as compared with most Ungulates 
having distinct crowns to the teeth, they are decidedly “ hypsodont,” or long-crowned, 
contrasting especially in this respect with the early “ brachyodont ” forms, as Palceothe- 
rium, Anoplotherium , Dichodon , Ilyopotamus, &c. 
They are arranged in both jaws in a perfectly unbroken series, being, in fact, in some 
places (especially at the antero-lateral region of the mouth, where gaps are so frequent 
in recent Ungulata) so crowded as to be pressed out of the straight line and to overlap 
one another, as in the lower jaw of Nesodon imbricatus. The adaptation of the form of 
the teeth on both sides to this position, and the accurate adjustment of their contiguous 
surfaces, shows that it is a natural conformation. They are, moreover, of very nearly 
even height throughout the series, and in their configuration present a remarkable and 
gradual transition from the first incisor to the last molar, easily traced in both jaws, 
and more even and regular than in any other known heterodont mammal. Indeed it is 
only by the analogy of other forms that they can be separated into the groups, con- 
venient for descriptive purposes, designated as incisors, canines, premolars, and molars. 
They are therefore most instructive in throwing light upon the homology of the com- 
ponent parts of the different teeth throughout the series. 
The enamelled surface, especially of the molars, is not smooth and polished, but 
covered with fine intersecting reticulations, surrounding shallow pits or depressions. 
There is no distinct layer of coronal cement. 
As the teeth are drawn in the accompanying figures of the exact natural size, I have 
not thought it necessary to give any detailed measurements of them. 
Description of the upper teeth (Plate XVI. figs. 1 & 4). — The incisors increase slightly in 
size from the first to the third. Each has a single, long, tapering and slightly curved root, 
and at the base of the crown a well-marked cingulum, developing both on the outer and 
* This was done by Mr. E. T. Newton, Assistant to Professor Huxley, at the Royal School of Mines. 
