Whence the Shadows? 225 



easy to assume that the family connection is tenuous. This is not true. 

 The Selachians are all intimately related to one another. 



How does the hypertrophy, the increased size and changed shape 

 of certain denticles, show this kinship? Shagreen denticles, which are 

 seized by a hypertrophic urge for nonconformity, modify in various 

 ways. One such modification is the fin spine, a thorn-like quill, which 

 emerges in such species of shark as the Spiny or Piked dogfish {Squalus 

 acanthias) and the Port Jackson shark {Heterodontus portus-jacksoni). 

 The fin spine projects in front of the dorsal fin (and is similar to the 

 ichthyodorulites, the prehistoric fin spines mentioned before). Another 

 modification is the saw tooth which is found in the Sawfishes {Fris- 

 toidea). The Sawfish has a long, flat, narrow rostrum, or snout, which 

 resembles a saw because along both edges are large, sharp teeth. A third 

 denticle modification is the stinger of the Sting rays (Dasyatidae and 

 other families), the defensive weapon which has earned the Sting ray 

 well-deserved respect among both men and other fish. The spine of the 

 Dogfish, the saw tooth of the Sawfish, and the sting of the ray— all are 

 versions of the same "tooth," the denticle, the persistent sign of the Se- 

 lachian. 



In the embryonic Dogfish, for example, there is virtually no dis- 

 tinction between the denticles near the mouth and those elsewhere on 

 the body. As the embryo develops, however, the denticles around the 

 jaws become bigger and complete their growth as distinctive teeth. The 

 teeth of all fishes, the higher vertebrates— and man himself— have as their 

 origin modifications of the dermal layer of the skin. Nowhere is this 

 fact better demonstrated than in a shark's transmutation of the denticle 

 into the tooth. 



The teeth of sharks, skates, and rays are lined up in several orderly 

 rows, as many as a thousand or more. The variety which characterizes 

 so many other aspects of different Selachians is present in the teeth, too. 

 They vary from the stiletto-shaped teeth of the Sand shark (Carcharias 

 taurus) to the blunt teeth arranged like pavement stones in the mouths 

 of most skates and rays. Other sharks have the more familiar triangular- 

 shaped teeth, and these in turn vary, some having finely serrated edges 

 and others flanking the triangle with cusps. In some rays, there is even 

 a variation by sex in the teeth, the female having flat teeth and the 

 male sharp ones. 



Some sharks may call into action as many as five rows of teeth which, 

 in fearsome phalanxes, obey muscular orders that erect or depress what- 

 ever teeth are needed on any predaceous occasion. And behind these 

 teeth on active duty are row upon row of reserves lying in deep grooves 

 inside the jaw. 



When a tooth is worn or lost, another moves up to replace it. The 



