220 LEIDY'S OSTEOLOGY OF THE HEAD OF HIPPOPOTAMUS. 



by a ridge ; one antero-internal near the end of the tooth, produced by the contact of 

 the upper mid incisors, the other postero-external to the former, produced by the 

 upper lateral incisors. 



The exserted portion of the upper canines projects much less obliquely outwards 

 in Choeropsis than in Hippopotamus, and the posterior groove is not only much nearer 

 the middle of the tooth, but it is very much deeper. Its outer side, near the bottom, 

 for one line wide, is uncovered by enamel, approaching, in this respect, more the 

 Hippopotamus Capensis than the Hippopotamus amphibius. The outer and inner 

 sides of the upper canines of Choeropsis are also a little depressed longitudinally, 

 more on the former than the latter, so that the anterior vertically abraded surface 

 presents a somewhat trilobate character, instead of the reniform appearance exhibited 

 by Hippopotamus. 



The inferior canines are situated at their alveoli, much more internally than 

 in the latter genus, but their exserted portion diverges relatively more obliquely 

 outward. They are uniformly convex externally and antero-internally. Their 

 enamel is relatively less ridged than in Hippopotamus. 



The true molars above and below in Choeropsis are very like those of Hippo- 

 potamus, but their lobes are less constricted laterally, so that the trefoil appearance 

 of their summits when abraded is not of so striking a character as in the latter, 

 presenting rather the form of triangles with concave sides and rounded angles. The 

 posterior lobes of the middle true molar, and all those of the last, are exceedingly 

 slightly constricted laterally. The lobes of the molars are also less obtuse on the 

 outer and inner sides of the teeth, so that the entering angles or interspaces are also 

 relatively wider. The basal ridge is also more horizontal in Choeropsis, or, in other 

 words, does not rise upon the lobes anteriorly and posteriorly in a crescentic manner, 

 as is the usual disposition in Hippopotamus. 



The superior fourth premolar presents no variation of importance in the two genera. 

 In the third and second premolars of Choeropsis the basal ridge is nearly obsolete, 

 while in Hippopotamus it is well developed, especially in the former tooth. In 

 Choeropsis these two teeth also possess a posterior conical tubercle, or short 

 accessory lobe, rising out of the base, and becoming confluent with the principal lobe. 



The first premolar is retained even to old age in Choeropsis, is small, curved in its 

 course, and has a simple conical crown. 



In the lower jaw the fourth premolar has a slight basal ridge externally, and a 

 posterior tubercle, more or less confluent with the principal lobe of the crown. 



The third and second premolars are quite simple, compressed conoidal, without a 

 basal ridge, and are relatively longer than in Hippopotamus. 



The first premolar is smaller than the corresponding one above, is curved, and has 

 a simple conoidal crown. 



