THE HEAD AND NECK. 



121 



were simple, and chisel- or awl-sliaped. When their 

 crowns became worn in consequence of long-contin- 

 ued use, they presented an external ring of enamel, 

 surrounding a core of the dentine, or ivory, of which 

 the bulk of the tooth is composed. This is the con- 

 dition of the incisor teeth in the great majority of 

 mammals. The first modification from this simple 

 form consisted in the development of a ridge along 

 the hinder border of the base of the crown, as seen 

 in Fig. 17. There was then a groove between this 

 ridge and the rest of the tooth. By the continuous 



Fig. 17. — Incisor tooth 

 of Anehitherium au- 

 relianense. 



Fig. 18. — Unworn crown 

 of temporary incisor 

 tooth of young horse. 



increase of the ridge, and its union with the edges of 

 the main part of the crown on each side, the groove 

 became converted into a deep pit (infundibulum), 

 the orifice of which is transversely elongated, and 

 placed behind and rather below the cutting-edge of 

 the tooth. This is the condition seen in a colt's in- 

 cisor which has just cut the gum (Fig. 18). As wear 



