LEIDY'S OSTEOLOGY OF THE HEAD OF HIPPOPOTAMUS. 215 



riorly between the canine teeth it is relatively less in Choeropsis than in Hippo- 

 potamus. 



The temporal fossae are alike in both genera. 



In a comparison of the side view of the skull, we are struck with the relative 

 shortness of the face and the advanced position of the orbit in Choeropsis, being placed 

 immediately posterior to the middle ; while in Hippopotamus it is situated at the 

 middle of the posterior two-thirds of the head. In Hippopotamus the orbits also are 

 elevated to so remarkable a degree that a considerable portion of the vertical extent 

 of their entrance is above the level of the forehead : but in Choeropsis, so far as eleva- 

 tion is concerned, they occupy the more ordinary position of ungulata, as in the Hog. 

 They are rendered very prominent laterally in Hippopotamus, from the elevation and 

 projection outwards of the anterior extremity of the ossa malarum, and the prolongation 

 outwardly, upwards and forwards, of the orbitar portions of the os frontis. In Choe- 

 ropsis, they do not project laterally, but have very much the same relative position in 

 this respect as in the Hog, but are removed rather farther from one another by the 

 greater breadth of the forehead. 



The face, anterior to the position of the orbits, is relatively shorter in Choeropsis 

 than in Hippopotamus. 



In the anatomical details of the upper part of the skull, the differences are so great 

 between Choeropsis and Hippopotamus, that any one finding a fragment constituted 

 by this part of the former, from his previous knowledge of the latter alone, would 

 not suspect it even of being closely allied to the genus Hippopotamus.* 



A very conspicuous difference is observed in the transversely slightly convex 

 forehead of Choeropsis, and the deep concavity of that of Hippopotamus produced by 

 the extraordinary elevation of the orbits. 



In the adult of Choeropsis, the forehead is more plane transversely than in the 

 younger animal, or is even a little depressed in the middle line. 



The form and proportions of the os frontis are also quite different in the two 

 genera. In Choeropsis it is absolutely as long, but relatively less broad than in Hip- 

 popotamus. In the former it extends into an angular process on each side of the ossa 

 nasi in advance of the ossa lachrymalia, to join the ossa maxillaria ; in the latter it 

 terminates anteriorly in two divergent supra-orbitar processes, enclosing in front a 

 broad concave notch receiving the sutural connexions of the ossa nasi and lachry- 

 malia. In Choeropsis the facial portion of the latter bones is relatively very small, is 



• De Blainville says in his Osteographie, p. 69 of the Mem. sur les Hippopotames, "je viens d'apprendre toUt 

 a I'heure (8 Septembre dernier,) de M. R. Owen, a son dernier voyage a Paris, que le docteur Morton avait I'annee 

 derniere decrit et figure, comme espece nouvelle, sous le nom de H. minor, un Hippopotame de petite taille, pro- 

 bablement du Gabon en Afrique. II paratt cependant, d'apres ce que m'en a apris M. R. Owen, qui en a vu le 

 crane, que les differences specifiques porteraient essentiellement sur la difference de taille, et du reste seraient 

 peu importantes." 



