596 UNGULATES. 



a more marked difference by its position in relation to the sy-p^isial 

 termination of the jaw; in the Rhinoceroses with ^" ^^^^^^^^^ mcisive 

 teeth the symphysis is prolonged beyor^^ ^^'^ ^'^^ premolar ; but 

 in the African species this tooth - ^^^^^.^^^ ^^^^^ to the anterior 



J ^ ,, . T„ +u« .Acmct tichorhine Rhinoceros in which 



end of the jaw. In thf^ 



, r -1 ^^, ^1 incisors endured longer and attained a greater 



, ^. . .aan they do in the livinar Rh. bicornis, the depressed 

 relative st'' ■^ ° ' r 



^,^id,te symphysis is prolonged, as in the Indian and Sumatran 

 Rhinoceroses, beyond the molar series, which begins opposite the 

 posterior border of the symphysis, as shown in PL 138, fig. 10. 

 In the leptorhine Rhinoceros (ib. fig. 11) the molar series extends 

 closer to the anterior end of the symphysis, which both in form 

 and relation to the molar teeth, more resembles that of the two- 

 horned African Rhinoceroses. The first premolar {p 1) is soon 

 shed, and all traces of its socket soon obliterated ; this has led to 

 the supposition that some of the fossil Rhinoceroses had only six 

 molars on each side of the lower jaw ; but specimens of young 

 individuals have demonstrated the normal number in species where 

 it has been most formally denied. (1) The second lower molar {p 2) 

 has its lobes distinctly defined by the vertical furrow on the outer 

 surface, but the variations of the form of its inner surface in different 

 species are such as to make it, perhaps, the most characteristic tooth 

 of the lower jaw ; the anterior lobe is always the smallest and 

 thinnest ; and its internal indentation is shallow. The two lobes 

 assume a more equal size and crescentic figure in the third {p 3) 

 and fourth {p 4) premolars, which progressively increase in size : 

 the three true molars resemble the fourth premolar, the last tooth 

 not being distinguised by an accessory lobe. 



216. Microscopic Structure. — Retzius(2) describes the dentinal 

 tubes as he saw them in longitudinal sections of the root of a molar 

 tooth of a Rhinoceros, to proceed transversely from the pulp-cavity 



(1) The specimen in Dr. Buckland's Museum, from Lawford near Rugby, PI. 138, fig. 10, 

 shows the first of the four premolars in situ, in the lower jaw of the Rhinoceros tichorhinus. 

 M. Christol, who has figured a very perfect lower jaw of an older individual of the same 

 species, in which the first premolar has been shed, describes it as ' raunie de toutes ses 

 molaires.' Annales des Sciences, 1835, p. 46, PI. 2, fig. 1. 



(2) Loc. cit., p. 32 



