20 



MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



tinue for a long time, even through life, new teeth thus arising 

 to make good the loss of others. In the mammals, however, 

 there are at most but two of these sets of teeth, —a temporary 

 or milk dentition, and a second or permanent dentition, the details 

 of which are given in connection with that group. 



In the lower vertebrates teeth may appear 

 in any part of the mouth where there are 

 soHd parts (bones or cartilages) to support 

 them. Thus in fishes and amphibia we may 

 find them not only along the edges of the 

 jaws, but upon any of the bones which lie 

 in the walls of the oral cavity. In the higher 

 vertebrates they are confined solely to the 

 edges of the jaws. 



Teeth are very variable in shape, a fact 

 largely correlated with differences in food. 

 In the lower vertebrates all of the teeth of 

 an individual are closely similar to each other. 

 This is the homodont condition ; the hetero- 

 dont dentition appears in the mammals, and 

 occasionally in the lower groups, where the 

 teeth in the different regions of the mouth 

 are of different shapes. Usually in the lower 

 vertebrates each tooth possesses but a single 

 root and a single cusp ; while in the mam- 

 mals, besides these simple teeth, there are 

 others, with two, three, or several roots, the 

 crowns also showing a corresponding duplica- 

 tion of parts. 



In the elasmobranchs the teeth rest upon 

 but are not firmly united to the skeletal parts. In the other 

 ichthyopsida they are usually firmly united to the bones of the 

 mouth by continuous growth, and the same is true of many 

 reptiles. In others they may be implanted in sockets (alveoli) 

 in the jaws, a condition which is universal in the mammals. 



Besides these true or calcified teeth, horny teeth occur here 

 and there, as in the cyclostomes, where the oral hood and the 

 tongue are armed with such structures resting upon epithelial 



Fig. 22. Dia- 

 grammatic section of 

 mammalian incisor 

 tooth, f, cement; i/, 

 dentine ; e^ enamel j 

 /, pulp cavity. 



