medial to the replacement tooth. The usual sequence in 

 reptiles is for new teeth to be formed medial to the old teeth. 

 The new teeth move outward as they form, and the base 

 of the old tooth is resorbed until the crown drops free. A 

 new tooth may move outward below the tooth it is going 



to replace or it may grow next to the old tooth and then 

 crowd it out. Replacement also involves a sequence along 

 the jaw margin — every other tooth or every third tooth 

 being replaced at any one time. 



In the lizard Tupinambis, one sees the beginnings of 



lateral (or medial) ridge 



labyrinthine"plicidentine 



fine structure as in Bent/iosuchus (Fig. 8-61) 



pulp cavity 



osteodentine of pulp covity 



B 



Figure 8-60. External appearance of a tooth of Holoptychius, A, with a section near the middle, 

 B, and at the base, C, to show details of structure. The fine structure of the wall is like that of Benfho- 

 suchus shown in Figure 8-61. (After Pander and Bystrow, 1936) 



?M section shown in detail 



-compact dentine 



inner globular layer 

 outer globular layer 



B 



Figure 8-61. Tooth structure of a lobyrinthodont amphibian, Benfhosuchus. A, cross section near 

 base; B, histological detail of wall. (After Bystrow, 1938) 



250 • THE SKIN AND ITS DERIVATIVES 



