Fig. 4.— The Evolution of Teeth and Scales. A. Portion of skin 

 of a sliark showing symnietrically disposed shagreen denti- 

 cles. B.C. Shagreen denticles of varying form and size ; 

 showing how clusters of scales are formed : these may later 

 conjoin to form either (a) large bony plates investing the 

 sknll— skull bones; or (5) more or less complete teeth. D. 

 Two rows of shagreen denticles (teeth) from the jaws of a 

 sliark (compare with A.). E. is a tooth of a shark (Noti- 

 danus), and shows the result of fusion of a row of separate 

 denticles such as in D. to form a single comb-like tooth. F. 

 The jaws of the Port Jackson shark {Cestracion), to show 

 the remarkably modified teeth for crushing purposes. G. 

 The jaws of an eagle-ray (Aft/liobatis), also sliowing teeth 

 modified for crushing purposes. In fig. £. note the spine 

 s. resting in the bony base of the isolated "odontoid." 



