[Illustration: 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.p_, dermal papillae; _ep_, epidermis lining buccal funnel; _ep^1_, epidermis which has formed the horny functional tooth _ht_; _ep^2_, epidermis forming the horny cone of the successional tooth _ht^1_. (From Warren.)] True calcified teeth first make their appearance in Fishes, where they assume the form of modifications of exoskeletal structures.[224] 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 {249}dermal spines form a continuous series with those which invest the jaws and eventually become teeth (Fig. 151). It is only later, when lips become apparent, that the continuity of the teeth and dermal spines is interrupted, and the two structures assume their distinctive characters. The tissues of which the teeth of Fishes are composed are (1) _dentine_, which is a non-vascular, calcified tissue, traversed by numerous radiating, branched, dentinal tubuli, into which extend protoplasmic prolongations from the cells (scleroblasts) by which the dentine is secreted. Dentine forms the greater part of the body of a tooth. (2) _vasodentine_ and (3) _osteodentine_ are modifications of ordinary dentine, the former containing blood-vessels ramifying in its substance but no dentinal tubules, and the latter more closely resembling bone. (4) _enamel_, an exceptionally dense, non-vascular, non-tubular tissue, which may or may not exhibit traces of the prismatic structure so characteristic of this tissue in the higher Vertebrates, forms the outer investment of the teeth. [Illustration: FIG. 151.—Transverse section through the lower jaw of an embryo _Scyllium_, to show the gradual transition from dermal spines (_d_, _d_, _d_) on the outer surface of the jaw to teeth (_t_, _t_, _t_) on the oral surface. _c_, Cartilage of the lower jaw. (From Gegenbaur.)]