i8g6. 



THE TEETH OF FISHES. 



385 



each jaw. The teeth which are situated upon the edge of the jaw are 

 usually erect, while the rows which lie behind them, farther within 

 the mouth, point backwards, and are more or less recumbent, not 

 having yet come into full use. New teeth are continually being 

 formed on the buccal surface of the jaw, beneath a protecting fold of 

 the mucous membrane (Fig. 9, m'), which acts as a composite enamel 

 organ, locally differentiated into the enamel organs of the individual 

 teeth. The developing teeth increase in size and thickness as, by a 

 continual, but gradual, sliding of the gum over the inner surface of 

 the cartilage, and outwards over its border, they are brought to the 

 edge of the jaw, where they become functional (Fig. 9). They are 

 finally shed, and their place is taken by the rising teeth behind. 

 Owen (4, p. 383), with that happy choice of metaphor which always 



Fig. 9. — Transverse section through the jaw of Carcharias : c, cartilage of jaw ; 

 cc, its calcined crust; m, mucous membrane covering the jaw; m', the fold of 

 mucous membrane beneath which the young teeth develop ; t, the tooth in use ; 

 t', its successor ; t", the youngest tooth of the series. Fig. 10. — Upper jaw of 

 Cestracion philippi. Fig. 11. — Lower jaw of Cochliodus contortus, Lr. Carb. : a, anterior 

 dental plate ; b, posterior do. Fig. 12. — Section of posterior dental plate of 

 Cochliodus : e, external surface ; i, internal or buccal surface. Fig. 13. — Vertical 

 section through a tooth (t) of Lepidotus maximus, Kim. Clay, showing its successor 

 (f) in process of reversal (after Etheridge, Q.J.G.S., xlv.). Fig. 14. — Section 

 through the lower jaw of Diodon : a, marginal dental mass ; b, second do. ; c, alveolar 

 cavity. Fig. 15. — Section through the lower jaw of Scants : a, amalgamated teeth ; 

 b, younger teeth, still free ; c, alveolar cavity. 



characterised his teaching, describes the whole phalanx of teeth as 

 ever marching slowly forwards over the border of the jaw, the front 

 ranks, after having engaged in active service, being successively lost, 

 while the rear ranks are continually being recruited by newly 



