22 EVENINGS AT THE MICROSCOPE. 



radiating bands, but underlies the whole of the lined parts. 

 The concentric lines of the upper layer do not now appear 

 to be edges of successive plates, but irregular canals run- 

 ning through the solid substance. This, however, is decep- 

 tive : for, by delicate focussing, we perceive that each 

 portion marked by these lines is really in a different plane 

 from the others, that the highest is at the centre of radia- 

 tion of the scale, and that each is successively lower till we 

 reach the margin. But now, if with very sharp scissors 

 we cut one of these scales longitudinally through the 

 centre, and examine the cut edge, we find that each of 

 these lines forms a distinct ridge. On the other hand, 

 the under layer of clear substance is quite smooth, and 

 always a little exceeds the margin of the concentrically 

 lined portion. The clear substance that fills the radiating 

 slits agrees both in texture and level with this lower layer, 

 and is manifestly continuous with it. 



Hence, I think that, in these slit scales, the upper layer 

 is formed, as commonly believed, by successive deposits 

 from beneath ; but that, after a few have been deposited, 

 they begin to slit, probably by contraction in becoming 

 solid ; that the lower layer is formed after each upper one 

 is hardened, exceeding its length by a little, and filling up 

 the slit ; that this lower layer becomes the upper layer of 

 the next course, slitting, and turning up its terminal edge 

 as it hardens ; that then the lower layer is deposited on 

 this, filling up the slit as before ; and that this process 

 goes on as long as the fish lives. 



It is curious that, in the scales of the Pike, the portions 

 thus separated by slitting, instead of expanding and leaving 

 spaces to be filled up, actually close over each other, the 

 divided parts overlapping considerably, as you may see in 

 these specimens. The left hand scale (a) is from the 

 back ; the central one (b), which has only a deep narrow 

 incision instead of a tube, is from the lateral line ; and the 

 third (c) is from the belly of the fish. 



