248 FISHES CHAP. 
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 
Fic. 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.p, dermal papillae ; ep, epidermis lining buccal 
funnel ; ep!, epidermis which has formed the horny functional tooth jf; ep?, 
epidermis forming the horny cone of the successional tooth Af!, (From 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which calcification superseded cornification as a method of 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, Quart. Journ. Micr. Sci. xlv. 1902, p. 631. 
2 See Ridewood, Wat. Sci. viii. 1896, p. 391, for references. 
