THE TEETH OF VEETEBEATES 51 



lrypsodont teeth. In most of these teeth we find 

 a layer of cementum over the enamel. 



The tooth of continuous growth is one in which 

 there is a persistent tooth germ and has been 

 called a tooth with a "persistent pulp." There 

 is a persistent pnlp but there is also a persistent 

 enamel organ. In all of the teeth which are at- 

 tached by gomphosis, there is a persistent ccmen- 



Fig. 14. — Radiograph of the mandible of a Wolf (Canis lupus), show- 

 ing brachydont tooth attached by gomphosis. 



turn organ. With the tooth of continnons growth 

 the dentin, enamel and cementum continue to de- 

 velop and the tooth continues to emerge from the 

 socket. A great many of the animals which have 

 teeth of continuous growth have but one set of 

 teeth. All of the rodents have some teeth which 

 are of continuous growth. Fig. 16 shows the 

 incisor of a squirrel. 



Tooth Forms. The forms of the teeth in the 

 Vertebrates present great variety. In the lowest 

 classes, the Fishes and Eeptiles, the simple conical 



