76 Vcrtebrata. 



skull, beneath the ear. The shape of the articular 

 surfaces which form this joint is variable, and de- 

 pends on the nature of the food and the character of 

 the motions which are necessary for mastication. 

 Thus, in flesh-eating animals the lower jaw has a 

 transversely elongated, cylindrical condyle, which can 

 allow only of a vertical motion, while in gnawing 

 animals the lower jaw slides forwards and backwards. 

 51. Teeth. The jaw arches, and they alone, bear 

 teeth, which are arranged in one row ; no accessory 

 teeth are developed on the palate as in reptiles and 

 fishes. The teeth are always in sockets, and are 



FIG. 34. 



Skull of anteater, a perfectly toothless mammal. 



rarely absent, as in the anteaters (fig. 34), though 

 sometimes they are rudimental and disappear early, 

 as in whales. There are usually two sets of teeth ; 

 one an early developed or milk set, which soon drop 

 out and are succeeded by a second or permanent set ; 

 thus reminding us of what we found in crocodiles, 

 where successively growing teeth follow each other in 

 the one row almost indefinitely as long as growth 

 continues. Those teeth in the upper jaw which are 

 rooted in the foremost bone, or premaxilla, are called 

 incisor teeth, and have usually a cutting edge and a 

 single root. When the first tooth in the maxilla or 

 jaw proper, is placed near the suture or line of con- 

 tact between that bone and the premaxilla, it is 



