346 



ANATOMY OF VERTEBRATES. 



sometimes subdivided into smaller islands. These islands or 

 lobes are the abraded ends of long and slender columns of dentine, 

 encased by thick enamel, and the whole blended into a coherent 

 crown by abundant cement, which fills up all the interspaces, and 

 forms a thick exterior investment of the entire complex tooth. 



The milk-molars are J :f- in number ; but only the two last are 

 succeeded by premolars. These are small, and, after the wearing 

 out of the first true molar, are shed, leaving the remnant of the 

 second true molar, fig. 275, in 2, with the last large one, m 3, to 

 which the work of mastication is confined in old Wart-hogs. 

 This interesting modification, as to order and number, in the 



276 



Deutition, lower jaw, old Wart-hog 

 (Phacochoerus). 



Dentition of Hippopotamus. 



change of the dentition, has thrown 

 important light on the more ano- 

 malous dentition of the Elephant. 1 

 The tendency to excessive development which characterises the 

 canine teeth in the Suidce, affects both these and the incisors in 

 the genus Hippopotamus. The two median inferior incisive tusks, 

 fig. 276, z, are cylindrical, of great size and length ; the two outer 

 incisors are likewise cylindrical and straight, but much smaller. 

 The upper canines curve downward and outward ; their exposed 

 partis very short, and is worn obliquely at the forepart; they are 

 three-sided, with a wide and deep longitudinal groove behind. 

 The lower canines, ib. c, are massive, curved in the arc of a 

 circle, subtriedral, the angle rounded off between the two an- 

 terior sides, which are convex and thickly enamelled, the posterior 

 side of the crown being almost wholly occupied by the oblique 

 abraded surface opposed to that on the upper canine. The im- 

 planted base of each of these incisive and canine teeth is simple, 

 and excavated for a large persistent matrix, contributing to their 

 perennial growth by constantly reproducing the dental matter to 



1 CLXXiii". p. 495. 



