560 UNGULATES. 



of the crown has two lateral ridges ; the internal surface presents a 

 strong basal ridge. The margins of the crowns of the lateral incisors 

 are notched. In the lower jaw the incisors are long, narrow, and 

 procumbent ; similar in form, as well as position, to those in 

 the Hog. 



The upper canines have moderately long, narrow, compressed, 

 two-edged, straight, descending crowns, with an entire covering 

 of enamel ; the fang has a longitudinal groove on both the outer 

 and inner surface ; the socket is strengthened by an anterior semi- 

 lunar ridge. The canine is separated from both incisors and molars 

 by an interspace of nearly an inch, both before and behind it. 

 The three premolars are contiguous, and gradually increase in size ; 

 their crowns are more or less triangular : each has two outer cusps 

 and one inner cusp, with an accessory tubercle behind the inner 

 one, which progressively increases in the second and third premolar. 

 The true molars are quadricuspid, with an accessory tubercle at 

 the fore part of the interspace of each transverse pair of cusps, 

 and with an anterior and posterior crenate ridge. There is a slight 

 increase in the size, and in the development of the accessory ridges 

 of the grinding surface in the second and the last molars. 



The lower canines have a long, slightly curved, and trihedral crown; 

 the narrow posterior side having no enamel, and playing upon the 

 anterior margin of the upper canine, which thus becomes blunted 

 and deprived of enamel. The first premolar has one blunt tubercle, 

 the second has two : both have also anterior and posterior ridges, 

 the latter being developed in the third premolar into a second pair 

 of tubercles. The true molars of the lower jaw are quadri-cuspid, 

 with an accessory tubercle at the posterior interspace of each 

 transverse pair of cusps, the second of which is developed into a 

 small third lobe in the last molar tooth. Each true molar has also 

 an anterior basal ridge. 



This type of dentition was closely adhered to, or rather fore- 

 showed, by some very interesting forms of ancient extinct Pachy- 

 derms, amongst which, remains of the following genera have been 

 found in the oldest tertiary deposits in England and on the 

 Continent. 



