HIPPOPOTAMUS. 15 



I. 2 (Text-fig. 1, o) is a very much smaller tooth with only about 3 cm. pro- 

 jecting from the alveolus in a full-grown skull. Friction tends to reduce it to a 

 somewhat conical form, the postero-internal and external surfaces being bevelled 

 away. 



C (Text-fig. 1 , d) is a very large tooth, far larger than c. In cross-section it is 

 triangular; the base of the triangle, which is the shortest of the three sides, is 

 posterior. This face consists of dentine only, while the other sides are somewhat 

 thickly coated with enamel marked by shallow longitudinal grooves. 



The 'premolars have a general similarity to those of the upper jaw. 



Pra. 1 is a small one-rooted tooth which falls out early. 



I'm. 2 is a conical tooth with two strong divergent roots. There is a slight 

 cingulum developed on the anterior and inner faces. This tooth is sometimes 

 missino- and its alveolus closed in a very old skull. 



I'm. 3 and 4 are of the same type, but somewhat larger and with the cingulum 

 better developed, especially in pm. 4. 



The molars are very like those of the upper jaw, especially as regards the 

 thickness of the enamel, but they do not show much tendency to the development 

 of small cusps at the ends of the transverse valleys bisecting the tooth. 



M. 1 and 2 have four roots. The cingulum is less marked than in the corre- 

 sponding teeth of the upper jaw. 



M. 3 has five roots. It is the largest of the molar teeth and the only one 

 possessing a well-marked posterior cone. 



Milk Dentition (Plate VI). 

 The formula of the milk dentition is : — di. f dc. j dm. f . 



Milk Dentition of the TJfpev Jaw. 



The milk-incisors are minute, subequal, conical teeth, with a complete enamel 

 cap. 



The canine is a little conical tooth, in character similar to the incisors, and only 

 very slightly larger. 



Of the milk-molars dm. 1 is unrepresented unless the tooth here regarded as 

 pm. 1 is really dm. 1. Dm. 2 is a simple conical tooth with two widely divergent 

 roots. There is a minute cusp placed postero-internally to the principal cone. 

 Dm. 3 is a considerably larger tooth and more triquetral — wider behind. It consists 

 of an anterior cone preceded by a small cusp and separated by a deep valley from a 

 bilobed posterior cone. " After prolonged wear it shows four distinct enamel lobes, 

 two longitudinal in front and two transverse behind" (de Blainville). Dm. 4 is a 



