Slade.] ^^ IMarch 15, 



Ungulata. In the Perissodactyla, the sagittal crest, ridges and extensive 

 parietal surface are correlated with increased insertions of the temporal, 

 while the large, strong and complicated arch has equal reference to a 

 powerful masseter. So in the Artiodactyla, especially in the Ruminantia, 

 the diminished surface for the temporal, and the smaller, weaker arch, 

 both denote lessened energy in the above muscles, while the enlarged 

 pterygoid muscular insertions show that the required action has been pro- 

 vided in another direction. As Prof. Cope has shown, " Forms which 

 move the lower jaw transversely have the temporal muscles inversely as 

 the extent of the lateral excursions of the jaw. Hence these have a 

 diminished size in sucli forms as the ruminants, and are widely separated." 



The singular fact that the Tylopoda alone of the selenodont Artiodactyla 

 possess the sagittal crest, is explained by Prof. Cope, by the presence of 

 canine teeth, which are used as weapons of offense and defense, and 

 which demand large development of the temporal muscles. 



The energy of the action of these muscles has reference to the position 

 of the dental series. In the primitive Mammalia, as Cope shows, a con- 

 siderable portion of the molar series is below and posterior to the vertical 

 line of the orbit, and this condition has been preserved in the Rodentia 

 and Proboscidia, forms which have the proal mastication. But in those 

 which have lateral movements of the jaw, the molar series has gradually 

 moved forwards. The camel alone retains the primitive condition. 



The bunodont Artiodactyla, as the Dicotylidae, have the molar series 

 posterior to the orbit ; those with lateral movement of the jaw, the Suidae, 

 have them more anterior. 



In the relation of the arch to the orbit, it is obvious that the position of 

 this last must exert its influence upon the strength of the arch. When the 

 orbit is above or below the articulation a longer and consequently a 

 weaker arch is demanded, than when it is on a level with it. The same 

 may be said when the orbit occupies an anterior rather than a posterior 

 position upon the cranium. A comparison of the crania of the Tapiridae 

 with those ol the Suidse will corroborate this fact. 



Then again the union of the bones by suture imparts a degree of 

 elasticity to the arcli which must serve to disperse over a given space the 

 effects of shocks and blows, which might under other circumstances 

 prove injurious. 



We have already noted the peculiar vertical curvature of the arch down- 

 wards in the Rodentia. This is a decided manifestation of weakness, and 

 is compensated in some of the families by the unusual arrangement made 

 in the distribution of the muscular insertions of the masseter through the 

 infraorbital opening, by which increased energy is imparted to the powers 

 of mastication, and whereby the action of the mandible is rendered equal 

 to the demand upon its efforts. 



Where this does not exist, it is evident that the strength of the arch is 

 still sufficient for the antero-posterior movement of tlie articulation so 

 peculiar to the Rodentia and so characteristic of the act of gnawing. The 



