91 COMPARATIVE ANATOMY. 



between the two, except in the matter of size being trifling), 

 belongs to the latter. 



And here (as from want of specimens I shall be obliged to 

 pass over the Proboscidia and Toxodonlia,) I will endeavour to 

 point out with some minuteness the distinctions that mark the 

 Artiodactyla and Perissodactyla. 



In the Sheep there are no incisors in the upper jaw, its 

 formula being — 



o— 3—3 3—3 

 I PM M 30 



3—3 3—3 3 3 



In the Pig as I have already stated the typical number of 

 44 is attained — 



3—3 I— I 4—4 3—3 

 I C PM M 44 



3—3 I— I 4—4 3—3 



If, however, you will attentively examine these three skulls, 

 the Sheep, the Pig, the Fawn, you will notice the following 

 characteristics common to them all. 



The pre-molars are only half as complex as the molars, 

 that is have one fang while the molars have two. 



The last molar does not project beyond the palatal bones. 



They have their nasal bones of equal width throughout 

 their length. 



They have no foramen penetrating lengthways the ectop- 

 ter}'goid bone. 



We will now examine this skull of an Ass or Horse, and 

 we shall see that the pre-molars are equally complex with 

 the molars. 



The last molar projects beyond the palatal bone. Its nasal 

 bones expand posteriorly. 



The ectopter)'goid bone is penetrated lengthways, as you 

 will see by the direction that this bristle takes, when inserted 

 in this small foramen. 



Any one of these characteristics in the skull of an ungulate 

 animal would be sufficient to enable you to decide whether it 

 belonged to the class Artiodactyla ox Perissodactyla ; you would 

 alsobe able to infer other peculiarities in its skeleton. 



