1880.] R. Lydekker — Notes on the Dentition of Ehinoceros. 139 



there occur two large and well-worn permanent incisors not differing to 

 such an extent in size as do those of the figured specimen. No trace of a 

 second incisor is to be found in the left premaxilla, and I cannot, indeed, 

 find any instance of the development of the two upper incisors of both 

 sides in the same individual of R. indicus. The occasional development 

 on one side only of the second permanent incisor in the last-named species, 

 would seem to be a pretty clear indication that it is descended from an 

 ancestor in which two pairs of upper incisors were normally present. It 

 seems, indeed, that, when teeth normally absent do present themselves, 

 they usually appear only on one side, as in the instance of the lower jaw of 

 a tiger with an extra premolar, described by myself in a former volume of 

 the Society's Journal.* 



In all species of the genus, the normal number of permanent upper 

 incisors (if any are present) appears to be one only on either side, and I 

 have not come across any instance of the abnormal development of the 

 second upper incisor in any species but R. indicus. It may not improba- 

 bly be, however, that such abnormal development may occur in other 

 species. 



It has, indeed, been stated on the authority of the late Dr. Falconerf 

 that the extinct Indian iB. sivalensis was furnished with three pairs of 

 upper (and lower) permanent incisors ; none of the numerous specimens of 

 the skull of this species figured in tlie ' Fauna Antiqua Sivalensis,' how- 

 ever, exhibit any incisors at all, and we have, therefore, no tangible evidence 

 whatever to support the new genus Zalalis lately proposed by Professor 

 Cope J for the reception of this species on the ground of the unusual 

 number of incisors with which it was provided. 



Turning now to the lower jaw, we shall find that there is some consi- 

 derable dilficulty in arriving at a satisfactory conclusion as to the homo- 

 logies of the teeth in advance of the molar series. 



In jB. indicus, there normally exist in the young animal an inner pair 

 of very small conical teeth, and an outer pair of larger teeth. The outer 

 pair are succeeded from below by a pair of much larger triangular and pointed 

 teeth, which, therefore, evidently belong to the permanent series. Normally, 

 I believe, the inner pair are not succeeded by permanent teeth, as I can 

 find no trace of such in most lower jaws ; in the lower jaw of the skull 

 drawn in fig. 1, however, there occurs, a little above and internal to the 

 middle pair of teeth, a second pair of small teeth, which are less protruded 

 from the jaw, and which, I think, certainly belong to the second dentition. 



* Vol. xlvii, pt. ii, pi. 2. 



t Owen, loc. cit. p. 689. 



+ Bui. U. ti. Gfcol. Geog. Siu'v. Vol. v, p. 229. 



