226 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



fully formed, the odontoblasts cease to act, but exceptionally, even in 

 mammals (tusks of elephants, incisors of rodents) they function 

 through life and the tooth continues to grow. In the mammals an ad- 

 ditional layer of modified bone, the cement, is formed around the 

 root of the tooth and may extend on to the crown. 



Just as the scales are arranged in quincunx on the surface of the 

 body, so are the teeth in the mouths of skates and some other elasmo- 

 branchs, where they form a tessellated pavement above and below, 

 the teeth being flattened and used for crushing the molluscs on which 

 these animals feed. More commonly the teeth are flattened in the 

 anteroposterior direction and have sharp cutting edges. In such 







^mt:i^t: 



Fig. 236. Fig. 237. 



Fig. 236. — Diagram of germs of milk and permanent dentitions in a mammal, 

 based on Rose, b, basal layer of e, ectoderm; dr, dental ridge; ds, dental shelf; eo, 

 enamel organ of milk tooth; m, mesenchyme; p, pulp of milk tooth; pg, germ of per- 

 manent tooth. 



Fig. 237. — Diagrammatic section of incisor tooth, c, cement; d, dentine; e, enamel; 

 p, pulp cavity. 



cases, as a rule, only the anterior row of teeth is functional, the others 

 lying folded down behind, ready to come into use when one of the 

 first row is lost. 



Most vertebrates have a succession of teeth (pol5rphyodont denti- 

 tion) and the elasmobranchs (fig. 238) show how this has come about. 

 The second arises on the (morphologically) posterior side of the first 

 and so on. In the non-mammalian classes the number of such den- 

 titions is indefinite (polyphyodont), but in the great majority of mam- 

 mals there are two, the first or milk dentition and the second or per- 

 manent dentition (diphyodont condition). 



In a few mammals only one dentition has been retained (monophyodont) ; 

 among these may be mentioned the monotremes, sirenians and cetacea. In the 



