LEIDY'S OSTEOLOGY OF THE HEAD OF HIPPOPOTAMUS. 217 



inclined plane of the ossa lachrymalia and malarum. In Hippopotamus the corres- 

 ponding portion of surface presents a continuation of th econcave conditiono f the lach- 

 rymal and malar bones. Below the latter the maxillary bones are relatively less deep 

 in Choeropsis than in Hippopotamus. In both genera the infra-orbitar foramina 

 occupy the same relative position. The canine alveoli in Choeropsis project relatively 

 less outwardly than in Hippopotamus, and the boss or protuberance so conspi- 

 cuous upon them in the latter, is less distinct, smaller and more external, so that the 

 deep transverse concavity existing above the alveoli in Hippopotamus, hardly exists 

 at all in Choeropsis. 



The antero-internal portion of the ossa intermaxillaria is less everted in Choerop- 

 sis than in Hippopotamus, and these bones, in the former, possess but a relatively 

 small protuberance antero-externally. 



The inion, the relative position of the auditory meatus, and the existence of a 

 partial or complete post-orbital arch, are the same in both genera of Hippopotamidae. 



Upon an examination of the basal aspect of the skull, we observe an important 

 difference in the direction of the glenoid articular surface. In Hippopotamus it 

 slopes from the post-glenoid tubercle forwards and outwards, and for the external 

 half of its extent presents downwards and backwards; while in Choeropsis it is 

 nearly horizontal, presenting downwards and a very little forwards, except the 

 portion on the post-glenoid tubercle, which presents forwards and outwards. 



In this view, also, of the skull, may be particularly noticed the less degree 

 of divergence of the canine alveoli from the face in Choeropsis than in Hippopotamus. 

 In the former the inner margin of the orifices of these alveoli is immediately to the 

 outside of the line of the molars, whereas in the latter it is removed several inches 

 to the outside. 



In consequence of the reduction of the relative length of the face of Choeropsis, the 

 molar teeth, with the exception of a hiatus of about from 1 to 3 lines between the 

 first and second premolars, form on each side an unbroken row. The canine, at its 

 inner margin, is only 4 lines from the first premolar, and the lateral incisor is in a 

 nearly straight line forward from the molars, and about three-fourths of an inch 

 antero-internal to the canine. 



The rami of the lower jaw are relatively not so much divergent in Choeropsis as in 

 Hippopotamus, nor are they so concave externally. The angles in the former are 

 relativelv nearly equally distant as in the latter, and project outward and downward 

 but not forward as they do in Hippopotamus. 



The condyles are nearly horizontal convexities in Choeropsis, and present slightly 

 backward, except their inner end, which turns down and looks inwards. In Hippo- 

 potamus, on the contrary, the condyloid surfaces incline inwards and forwards. 

 The coronoid process in Choeropsis rises more rapidly and vertically from behind 

 than in Hippopotamus. The inferior canine alveoli relatively project little forward 



55 



