Zoological Society. 405 



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 suliicient 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 naturaUst 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 incliiied 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. 



