136 R. Lydekker — Notes on tlie Dentition 0/ Rhinoceros. [No. 3, 



The true molars (in?-, m?', mP) in all species of ItJiinoceros, whether 

 living or extinct, are invariably three in number, corresponding with the 

 typical maiTimalian series, and, therefore, require no further notice on this 

 occasion. In advance of the first of the three true molars, there occur, in 

 all young skulls of Rhinoceros, four teeth in serial apposition, but in older 

 skulls there may he only three. It is to these anterior teeth of the milk- 

 molar and premolar series (the one or the other present, according to the 

 age of the animal) to which I now desire to draw attention. 



An examination of the skull of which the left dentition is drawn in 

 fig. 1, shows that, of the four teeth (m.m?, m.m?, m.m.^, m.in.^) in advance 

 of the first true molar (in?), tlie three last {m.in?, mm?, m.m^) have 

 their fangs and bases absorbed away by the germs of other teeth, which are 

 succeeding them from above : there can, therefore, be no doubt that these 

 three teeth are the three last milk-molars of the typical series. This is 

 also shown by the last tooth of the anterior series {m.m.'^) being more 

 worn than the first of the true molar series {m}) : if the tooth preceding 

 the latter were a premolar, it would be the less worn of the two. The first 

 tooth of the whole series (m.m?) shows, however, no signs of being about 

 to be replaced by a vertically succeeding premolar. I have carefully 

 examined another skull of the same age, in which the alveoli of the teeth 

 hav6 been opened, and I can find there no trace of a replacing premolar 

 above the first of the seven teeth of the molar series. Were this tooth to 

 be replaced by a premolar, such replacement would take place before that 

 of the tooth next in the series. Several other adolescent skulls of i2. wdi- 

 cus which I have examined show no trace of the replacement of the anterior 

 tooth, and it may, therefore, be considered to be proved that in many 

 instances no such replacement ever takes place. 



From the development of the tooth in question with the milk-molar 

 series (though it sometimes appears rather later than the next tooth), 

 there would seem to be no doubt that it is the first of that series, and I 

 shall show below that such is undoubtedly the case. From the fact of 

 this tooth having in most instances no vertical successor and persisting 

 for a considerable time during the period of use of the permanent denti- 

 tion, it is not unfrequently referred to as the first premolar, and though, 

 as I shall show, such a nomenclature is altogether inaccurate, yet it has p. 

 certain amount of convenience wliich may justify its conditional use. 



The dentition drawn in fig. 2 also exhibits four teeth in front of the 

 first true molar {m?), but they are not all homologous with those in the 

 preceding specimen. The two teeth (m.m.^, m.mA) in advance of the first 

 true molar {m? ) in fig. 2 are more worn than the former, and will conse- 

 quently be the third and fourth milk-molars, or the homologues of the 

 corresponding teeth in fig. 1. The first and second teeth {p.m}, p.m.^), 



