SKIN, SCALES, AND SPINES 



89 



along the body from the head to the tail, which have the same 

 structure as the rhomboid scales. In young fishes the scutes 

 are all touching one another, and each is armed with a strong 

 knife-like spine, but as growth proceeds they become separated 

 and the spine disappears (Fig. 41 a). Between the scutes are 

 obhque series of small denticles. The related Paddle-fishes, of 

 which the curious Spoon-bill {Polyodon) of North America is 

 best known (Fig. 470), are even more degenerate in their 

 bodily armour, the skin being naked and the rudimentary 

 scales of the tail still further reduced. 



The Gar Pikes (Lepidosteus) of the rivers and lakes of North 

 America (Fig. 48), not to be confused with the marine Gar- 





d N\o 



Fig. 39. SCALES. 



a Portion of skin and isolated scales of the Gar Pike (Lepidosteus sp.), X about i ; 



b The same of the Bichir (Polypterus bichir), X i ; c. Isolated scute of large 



Sturgeon (Acipenser sp.), X i ; d. Cycloid scale of Tarpon (Megahps atlanticus), 



X i ; e. Ctenoid scale of Soldier-fish {Holocentrum ascensioms), X 8. 



fishes (Belonidae), have an armour of shining diamond-shaped 

 plates similar in appearance to those of the Bichirs, but they 

 differ in lacking the middle layer of cosmine (Fig. 39A). They 

 are very much the same in other respects, and articulate with 

 one another by peg-and-socket joints. 



Body armour composed of articulated rhomboid plates, 

 although more efficient than soUd mail, has certain disad- 

 vantages, and must restrict to some extent the flexures of the 

 body m swimming. As fishes came to rely more and more upon 

 their speed and agility rather than upon the strength of their 

 armour for protection, the ganoid plates were gradually super- 

 seded by thinner and more flexible structures. Mechanical 

 factors may also have played some part in the change, for an 



