71 
show at one glance so striking a similarity of plan, that if we can 
but divest ourselves of prejudged notions, which the great contrast 
in external form may have imparted, we cannot doubt their close 
alliance ; and the little Hyrax, the only other living genus of this 
family, when we make allowance for those differences of proportion 
invariably existing between species organized upon one plan, and 
differing much in size, will also be found closely to resemble its 
more gigantic relatives. In this group the bony palate is curtailed 
in length, its level not much below that of the base of the cranium, 
and the size of the posterior nasal orifice made up chiefly by its great 
increase of length in the antero-posterior direction. Here also there 
is no fissure between the wide-spreading walls of the nasal canal 
and the nidus of the last molar tooth. The form of the auditory 
bulla presents a nearer approach to the Ruminant than the Hog, as 
also does the occipital bone, especially in the form and relative 
position of its condyles and paroccipital processes; but the under 
surface of its basal portion is flat, and very convex in the transverse 
section; the lower jaw articulates on a fulcrum similar to that de- 
scribed in the true Hogs, but posteriorly is a smooth concave surface, 
which terminates behind in a characteristic salient process. Among 
the characters of the foramina this division is well-distinguished 
from either of the others by the presence of the ali-sphenoid canal, 
which exists in all the living genera, and, as already pointed out, is 
wanting both in the Ruminants and in the Hog-tribe*. 
This canal is also present in the Elephant and Mastodon, an 
aberrant division of the order, also possessing toes in uneven number, 
and still further characterized by being the only members of the 
Ungulate division that have a distinct canalis caroticus, and by their 
wanting the foramen condyloideum. 
A sufficient number of examples have now I think been adduced, 
to show, that although the instances may be few in which a group 
could be isolated by characters drawn from the foramina alone, yet 
in most cases they will be found to be of some assistance in marking 
the limits of closely-allied divisions; and even in those cases which 
I have brought forward, it is far from my intention to advocate that 
* Since this paper was communicated to the Society, it has struck me that the 
similarity of structure (which must have been observed by every naturalist who 
has given attention to the subject of dentition) existing between the premolars. and 
true molars in the members of the Perissodactyle division, may prove an important 
addition to the zoological characters of that group. This must of course depend 
upon the confirmation, by further researches, of the idea, that in most Mam- 
malia a premolar represents, in the homologies of its component parts, only 
the half of a true molar; but there can be no doubt that in the group alluded to 
the premolars each represent the whole of a true molar, for the resemblance in 
most cases is very striking. Among all the genera, the extinct Lophiodon is that 
which looks most like an exception: unfortunately, I have never had access to any 
specimens of this genus; but so far as I can judge of it by the figures published, 
I should be inclined to the belief that further researches will show, that notwith- 
standing the apparent dissimilarity, the premolars of the Lophiodon, as well as 
those of the other members of the group, each represent the whole of a true molar; 
and that in the other divisions of the Ungulata, as well as in most members of the 
class, the half only of a true molar is typified. 
