248 



FISHES 



epidermic cells into horny layers.-' The old teeth are vertically 

 replaced by new teeth developed beneath the functional teeth. 

 With the exception of a median tooth above the oral aperture, 

 Myxine and its allies have only lingual teeth. These are comb- 

 like, and they are formed by the basal fusion of primitively 

 distinct tooth -germs. The structure and development of the 

 teeth of the Cyclostomes lend no support to the view that the 





^'-h(' 



Fig. 150. — Vertical section of developing tooth in Petromyzon marinus, showing a 

 successional tooth, which is just beginning to cornify at its apex beneath the 

 functional tooth, d, Dermis ; d.}], dermal papillae ; ej], epidermis lining buccal 

 funnel ; e^ji, epidermis which has formed the horny functional tooth lit ; ff, 

 epidermis forming the horny cone of the successional tooth hO-. (Prom Warren.) 



teeth are degenerate calcified structures. With greater prob- 

 ability they represent a stage in the evolution of teeth and 

 dermal spines, which has been succeeded by a later stage in 

 which calcification superseded cornification as a method of 

 hardening. 



True calcified teeth first make their appearance in Fishes, 

 where they assume the form of modifications of exoskeletal 

 structures.^ The teeth of Elasmobranchs are identical in essential 

 structure, as well as in the manner of their development, with 

 the ordinary dermal spines of the skin, and in the embryo the 



1 Warren, Quarl. Journ. Micr. Set. xlv. 1902, p. 631. 



" See Ridewood, Nat. Sci. vlii. 1896, p. 391, for references. 



