HIPPOPOTAMUS. fj67 



proportions, and relative position of the canines and incisors of the 

 Merycopotamus closely accord with the Hippopotamic type of these teeth. 



207. Anthracotherium. — In this genus although the molars, espe- 

 cially those of the lower jaw, depart from the Hippopotamic type less 

 than the molars of the Merycopotamus, yet the canines and incisors 

 retain more of the ordinary forms and proportions of those teeth in 

 other Pachyderms : the canines, for example, though large are simply 

 conical, the premolars have strong conical crowns with a basal 

 ridge ; the upper true molars have four principal cones, anci a basal 

 cingulum, the two outer cones encroach upon the two inner ones, 

 and manifest, like those of Merycopotamus, an approximation to the 

 Ruminant type. (I) In the lower jaw, the cones are more equal 

 and symmetrical : the last molar, of which a reduced view is given 

 in PI. 135, fig. 10, has four distinct cones, and a large and slightly 

 bifid posterior lobe : the surfaces of the four chief cones which are 

 turned towards each other are somewhat angular and support 

 projecting ridges, the opposite surfaces are simply convex. The 

 entire dentition of the Anthracotherium is not yet known, but 

 sufficiently so to prove that this extinct genus formed, with the 

 closely allied Merycopotamus, links connecting the Hippopotamidcs 

 with the Ruminantia^ 



208. Microscopic Structure. — ^The dentine is hard and unvascularin 

 all the teeth of the Hippopotamus, and the sections of this compact 

 substance take a fine polish : the dentinal cells are very conspicuous 

 in the deciduous teeth, in an incisor of the first dentition they are 

 subhexagonal, about ^th inch in diameter, arranged in layers 

 parallel with the outer surface of the dentine and are pierced by the 

 dentinal tubes which proceed at right angles to that surface ; about 

 twelve tubes are included in the area of a single cell. There is no 

 peculiarity in the size, course, division, branching or termination of 

 the dentinal tubes of the molars of the Hippopotamus. The 

 calcigerous tubes of the dentine of the tusks are smaller, their 

 diameter does not exceed i^„th of an inch, and their intervals not 

 more than g^Jj^th of an inch. They radiate from the pulp-cavity to 

 the periphery of the tusk, inclined obliquely towards its extremity, 

 and describe numerous subspiral curves, or waves succeeding each 



(1) See Cuvier, Ossemens Fossiles, torn, iii, pi. Ixxx. fig. 1. 



