588 UNGULATES. 



subcubical, and the grinding surface, when moderately worn, sub- 

 quadrate, and penetrated by two folds or valleys of enamel : the 

 principal valley (PL 138, fig. 5, b) commences at the middle of the 

 inner side, and extends obliquely outwards and forwards towards 

 the antero-external angle of the crown about two-thirds across, 

 where it terminates, according to the species, in a more or 

 less expanded, sometimes bilobed, cul de sac, (e) : the second 

 and shorter valley {ib. c) is usually of a triangular form, and 

 indents more or less deeply the posterior border of the crown : 

 in most Rhinoceroses this is wanting in the last molar, which has 

 a trihedral conical crown : both valleys are usually deepest at their 

 blind terminations : the outer surface of the crown is gently un- 

 dulated by one of the convexities (ib. o') being sometimes produced 

 into a longitudinal ridge. In the lower jaw the molars (ib. fig. 9) 

 have an oblong, laterally compressed crown, divided into two cres- 

 centic lobes, placed obliquely, wdth their convexities (o o) outwards 

 and a little backwards ; the anterior horn of the hinder crescent, 

 before it is worn down, abuts against the middle of the convexity 

 and below the upper margin of the crescent in front. 



The normal formula of the molar series is : — p. J-Ef, wi. |e|: = 28. 

 There are no canines. As to the incisors, the species vary not only 

 in regard to their form and proportions but also their existence ; and 

 in the varieties of these teeth we may discern the same inverse 

 relation to the development of the horns which is manifested by the 

 canines of the Ruminants. Thus, the two-horned Rhinoceroses of 

 Africa, which are remarkable for the great length of one(l) or both(2) 

 of the nasal weapons, have no incisors in their adult dentition (PL 138, 

 fig. 2); neither had that great extinct two-horned species {Rh. tichorhinus), 

 the prodigious development of whose horns is indicated by the sin- 

 gular modifications of the vomerine, nasal, and intermaxillary bones 

 in relation to the firm support of those weapons. (3) 



The Sumatran bicorn Rhinoceros, combines with comparatively 



(1) Rhinoceros bicornis, Rh. simus, 



(2) Rb. Keitloa, Smith. 



(3) These bones in the fossil skull of the species cited are confluent with each other, 

 forming a solid obtuse termination to the upper jaw, and are anchylosed to a strong bony 

 partition wall extending from the vomer to the anterior outlet of the nasal passages, and thus 



