Slade.] 



G2 



[March 15, 



rounded surface, are but slightly raised above the level of the alveolar 

 surface The pterygoid surface is extensive and the fossa deep. 



In the Hippopotamida3, the arch is broad and strong. Its superior bor- 

 der presents a marked sigmoid curvature, and the convexity which is 

 always posterior, is in this case much shorter in proportion. The tem- 

 poral fossfe, as also the surfaces for the muscular insertions, are extensive. 

 The pterygoid surface is not so large as in the SuidiB. The glenoid fossa 

 is slightly concave, but not bounded externally by a continuation of the 

 jugal. The condyles of the mandible are nearly on a level with the 

 molars, and the coronoid process is small and recurved. The angle is 

 greatly modified for muscular attachment. 



In the Hyracoidea, the arch is composed of three bones, of which the 

 jugal is the most important. Resting anteriorly upon the maxilla, the 

 jugal sends backwards a process to form the external boundary of the 

 glenoid fossa. It also sends upwards a postorbital process to meet a cor- 

 responding one from the parietal alone or from the parietal and frontal 

 combined, thus completing the bony orbit. Both horizontal and vertical 

 curvatures are slight. The surface for the temporal muscle is largely 

 developed while the pterygoid fossae are well marked. The ascending 

 ramus of the mandible is high, and the angle ia rounded and projects 

 very much behind the condyle, which last is wide transversely, and 

 rounded on its external border. The coronoid process is small, slightly 

 recurved, and not on a level with the condylar surface. 



In the Proboscidea, the arch is straight, slender, and composed of three 

 bones. The maxilla forms the interior portion, while the jugal sup- 

 ported upon the process of the maxilla, meets that of the squamosal in 

 the middle of the arch, and is continued under this as far as the posterior 

 root. This modification is unlike that of any other ungulate. There is a 

 small postorbital process from the frontal. The temporal surface is exten- 



