1880.] E. Lydekker — Notes on tlie Dentition o/Rliiiioceros. 



137 



however, in fig. 2 are still in germ, and as being totally unworn must be 

 o£ a later development than the third and fourth milk-molars : consequent- 

 ly, the former must be the first and second premolars, which have replaced 

 the first and second milk-molars. In this instance, therefore, the first 

 milk-molar, which, as we have seen, is normally persistant, has been re- 

 placed by a vertically succeeding premolar, from which replacement there can 

 be no question as to the correctness of the serial position assigned to the 

 former tooth. The replacing premolar (fig. 2, p.m.^,) is of considerably 

 la,rger size and more complex structure tliau the replaced milk-molar 

 (fig. 1, m.vi.^). 



In the lower jaws of all the skulls of It. inclicus which have come 

 under my notice, I cannot find any instance of the vertical replacement of 

 the first milk-molar, which generally persists until the permanent dentition 

 is well in wear, and subsequently falls out at a comparatively early period. 

 Neither can I find any instance of the replacement of the first milk-molar 

 of either jaw in R. smnatrensis {suinatranus) or i2. javanicus (sotidaicm). 



The formula of the molar dentition of S. indicus, taking into account 

 the abnormal form, may be written as follows: — m.m. — b.^;^. ^lilni^ 



4 — 4 ^ 3 — 3 



m.^^; the adult molar dentition of the normal ioxm, m.m.^^p.m.^^^ 



m- ^— ^ ; 'in'l of abnormal form, m.m. 5^ p.m. ^ m. 



The succession and homology of the anterior tooth of the molar series 

 appears to have given rise to a certain amount of confusion among natura- 

 lists. Thus Professor Huxley when treating of the dentition of the genus 

 Mhinoceros, observes:* "Of the four milk-molars, the first, as in the 

 Horse, is smaller than the others, and is not replaced ;" two pages back in 

 the same work, however, the Professor gives the formula of the premolars 

 as which would imply either that the first tooth of the molar series 



is replaced, or else that it is reckoned as a premolar, in which case there 

 would be only three milk-molars. f Professor Owen appears to have come 

 to a conclusion totally opposite to that of Professor Huxley, aud seems to 

 consider that tlie first milk-molar is always replaced. Thus on page 592 

 of his ' Odontography' the Professor observes that " the first of the 



* ' Anatomy of Vertebrated Animals,' p. 362. 



t In a work explanatory of the homology of the teeth, as is Professor Huxley's, 

 there can bo no doubt that this homology should be given with the most strict accu- 

 racy. In descriptive zoology and palosontology, however, it will still be convenient, 

 in referring to the dentition of the genus Ithiiioccros, to coimt the first milk-molar, when 

 persistent, as a premolar, in order to avoid introducing another term into the dental 

 series. The same conventional arrangement may be adopted in regard to the perma- 

 nent and milk-incisors, referred to below. 



