590 UNGULATES. 



I have shown in many parts of this Treatise how strikingly the 

 duly defined law of Unity of Organization is exemplified by the dental 

 system; and, anticipating that this system would adhere to the typical 

 formula by transitory representatives of the defective teeth in the adult, 

 I made search for the germs of incisors in the dried jaws of a fcetal 

 Rhinoceros bicornis of South Africa. The alveolar border of the 

 short symphysis of the lower jaw was apparently edentulous, and 

 thickly coated with the dried gum ; after soaking this for some hours 

 in warm water, and cutting down into its substance, I detected the 

 germs of the four lower incisors, which are figured, of half the natural 

 size, in PL 138, fig. 14, i 1, i 2. Although these teeth are destined 

 to be absorbed and never to make their appearance above the gum 

 in the living animal, they manifest the typical relative proportions 

 to one another, the outer pair {i 2) being more than double the size 

 of the inner pair (i I). The outer incisor is six lines in length, 

 and in great part lodged in the socket ; the crown is one line and a half 

 in breadth, convex anteriorly, flattened behind ; it protrudes from the 

 jaw about five lines in advance of the alveolus of the first molar, 

 and close to the anterior border of the jaw. At the same border, 

 about one line nearer the symphysis, the first or inner incisor is 

 situated ; it is about two lines in length, and half a line across the 

 crown, which just peeps above the bone. There were no sockets 

 of reserve beneath or behind these deciduous germs of incisors. 

 The anterior end of the thin and small intermaxillary lamella of 

 the same skull is expanded, and was covered by a thick, dried 

 gum, but I could find no calcified rudiments of upper incisors ; it 

 is highly probable, however, that germs of these teeth or their 

 matrices, may be manifested at an earlier period. 



The permanent median incisors of the upper jaw have a peculiar 

 and easily recognizable generic form in all the species possessing 

 them ; they are short, broad, much compressed, rhomboidal, or 

 sub-triangular, the crown forming the base of the triangle and the 



contain any reference to the teeth : Mr. Macleay, in the Entomological number of the same 

 work, exaggerates in affirming of the genus Rhinocei-os, " that the dentition varies extensively 

 in almost every species." No. iii, p. 6. 



