216 LEIDY'S OSTEOLOGY OF THE HEAD OF HIPPOPOTAMUS. 



oblong quadrilateral, and presents a plane or very slightly convex surface, presenting 

 outwards and forwards, and continuous with the oblique plane of the malar surface. 

 Superiorly it articulates with the outer margin of the angular process of the os frontis, 

 anteriorly with the os maxillare, inferiorly with the os raalse, and posteriorly constitutes 

 part of the margin of the orbit. It is completely separated from the ossa nasi by the 

 angular process of the os frontis. In Hippopotamus the corresponding portion of the 

 lachrymal bones forms a large, square, transversely concave plate, presenting 

 upwards, and sloping downwards and outwards, from which proceeds a very narrow 

 portion outwardly to the margin of the orbit, continuous with the orbital portion of 

 the same bones. Its inner or superior margin articulates with the corresponding os 

 nasi ; its posterior margin with the orbital process of the os frontis and lateral process 

 of the OS nasi, anteriorly with the os maxillare, and inferiorly with the os malse, and 

 a very small portion contributes to the margin of the orbit. It separates entirely the 

 OS frontis from the os maxillare. 



In Choeropsis the anterior suture of the lachrymal and malar bones forms one 

 continuous oblique line: but in Hippopotamus the suture of the latter bone is 

 considerably in advance of that of the former. The ossa nasi are relatively 

 much shorter in Choeropsis than in Hippopotamus. In the former they gradu- 

 ally widen from the points of the ossa intermaxillaria to their posterior fourth, 

 when they converge in a slightly curved line to their pointed commencement. Their 

 posterior fourth fills up the notch between the angular processes of the os frontis. In 

 Hippopotamus the ossa nasi are slightly constricted between the ossa lachrymalia, and 

 posterior to these expand laterally into a triangular curved process projecting be- 

 tween the ossa lachrymalia and the os frontis. Their posterior margin articulating 

 with the latter is a broad convexity occupying the middle half of the width of the 

 forehead. 



In Choeropsis the external malar surface presents a uniform oblique plane inclining 

 from the lachrymal surface downwards and backwards below the orbit, and does not 

 advance upon the face beyond the anterior line of the lachrymal bone. In Hippopo- 

 tamus the anterior extremity of the malar bone is more prominent outwardly than 

 in Choeropsis, and that portion of its supero-external face anterior to the orbit, is 

 bent inwards from its oblique plane below the latter on a line with the anterior 

 orbital margin. This anterior portion of the malar surface forms part of the same 

 concavity of the face, to which the lachrymal bone contributes. In Choeropsis the 

 masseteric surface of the malar bone is relatively narrow and directed downwards; 

 in Hippopotamus it is broad and presents outwards and downwards. The ossa 

 maxillaria in Choeropsis are relatively shorter, but less narrow at their middle 

 than in Hippopotamus. The ridge proceeding forward from the malar articulation 

 is very much shorter and less prominent in the latter than the former. Postero- 

 superiorly the ossa maxillaria in Choeropsis present a continuation of the same 



