28 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 
