HOW FISH ARE CLOTHED. 33 



Polypierus or "bichir" of the Nile. In form, it 

 may be described as rhomboid. As in the 

 representative of the scale in the shark, so here 

 the main part of the scale is made up of dense 

 bony tissue. This is covered externally by a 

 hard glistening substance known as "ganoine," 

 a substance which bears some resemblance to, 

 but differs from, the enamel coating which we 

 found in the shark. 



These ganoine - covered plates are closely 

 packed, investing the body in a kind of mosaic, 

 and forming a most perfect armour. In many 

 of the old fossil fishes these scales were still more 

 perfectly united one to another by an arrange- 

 ment which constituted a peg and socket joint. 



The gradual rise, perfection and decline of the 

 heavy armour plating, so conspicuous a feature 

 amongst the earlier fishes, is a matter of very 

 considerable interest. Of the condition which 

 fostered the development of such cumbrous 

 clothing we know nothing. In some cases, as in 

 fishes of the genus Mesodon, for instance, only the 

 head and forepart of the body were thus protected, 

 but in the majority, as in the surviving forms, 

 Polypterus (fig. 15, B.} and Lepidosteus the whole 

 body was completely invested. Amongst some 

 recent fishes we find armour-plating has once 

 more been adopted, as in Amphisik (tortoise-fish), 

 the coffer-fish and its allies, and the sea-horses, but 

 in all these cases the armour is of a quite different 

 type. 



The ancient scale-mail, if we may so call it, re- 

 calls forcibly the ancient chain-mail and kindred 

 forms of armour adopted by our ancestors of the 

 C 



