2 3 TEETH AND SPINES 



fossil shark (?), Sandalodus, which probably represents a 

 condition of complete fusion ; it would accordingly cor- 

 respond to the sum of the dental elements of half of the 

 jaw of Fig. 27. 



In more highly modified fishes the tooth-producing 

 region has become greatly extended ; teeth are present not 

 only on the jaw rims, but deep in the mouth cavity, 

 studding its floor and roof, and occurring even on the 

 tongue, gill bars, and pharynx. 



Fin Spines 



Primitive dermal defences appear to have played a 

 prominent part in the formation of fin spines. The clus- 

 tering of dermal cusps on the exposed margin of a fin 

 may have been an important initial step toward the for- 

 mation of a rigid cutwater. The anterior margin of the 

 fin of Fig. 49 is whitened with a fusion of dermal tuber- 

 cles which must have formed a firm encrusting support ; 

 the extension of the calcification of the bases of the tu- 

 bercles would accordingly be the mode of origin of a fin 

 spine. In Fig. 32 is shown a spine that appears largely 

 of this origin. A similar spine (Fig. 33) shows its dermal 

 tubercles not only at its sides, but in a most marked 

 way at its hinder margins. In Fig. 34, representing the 

 " sting " of the sting ray, a series of dermal spines, bear- 

 ing rows of minute denticles are seen to arise in a meta- 

 meral succession. A condition somewhat similar is known 

 in the Carboniferous shark, Edestus (Fig. 35), whose spine, 

 often of gigantic size, is of special interest, since it shows 

 how important a part in spine-formation may be taken by 

 the dermal defences of many successive metameres. The 

 spine is clearly segmented, and as its separate elements 

 (Fig. 37) are bilaterally symmetrical (Figs. 36 and 38), its 



