124 REMINISCENCES OF 



Lepidasteus, or bony pike of the Mississippi and 

 other of the larger rivers of America, and by the 

 Polypterus, a smaller type found in the River Nile. 

 But these Ganoid forms are most abundant in a 

 fossil state, more especially in the strata older than 

 the chalk. The second class, or Placoids, are much 

 more widely represented at the present day, and are 

 also comparatively abundant in all the strata from 

 the clay down to the Devonian rocks. This group 

 contains all the tribes of sharks and skates and 

 rays. These are specially characterised by having 

 their skins studded thickly with those enormously 

 sized teeth commonly known as shagreen and used 

 as a covering for cases of compasses and other 

 similar instruments. These fishes have no proper 

 scales beyond these dermal or skin teeth. 



The Cycloids and Ctenoids comprehend most of 

 the ordinary fishes, with which every one is familiar. 

 The scales of the Cycloids have the upper surface of 

 each scale marked with a series of concentric ridges, 

 which are parallel with the margin of each scale. 



The Ctenoids have scales of which the anterior 

 part is identical with those of the Cycloids, but at 

 the posterior part each concentric ridge terminates 

 in one or more minute spines pointed backwards. 



The scales of the Ganoids furnished some remark- 

 able results ; each scale presented three layers, a 

 lower one of bone, usually with magnificent lacunae ; 

 a middle one composed of true tooth structure, and 

 .an upper one of shining enamel. 



