22 EVENINGS AT THE MICEOSCOPE. 



trie lines, following the irregular sinuosities of the out 

 line. These lines are the edges of the successive layers 

 of which the scale is believed to be composed, each 

 layer being added in the process of growth to the under 

 surface, and each being a little larger every way than 

 its pi'edecessor ; thus the scale is a very depressed cone, 

 of which the centre is the apex. There is a marked dif- 

 ference (indicated in the figures) between that part of 

 the surface which is exposed, and that which is covered 

 by the other scales ; the concentric marks in the former 

 are much coarser and less regular, often being inter- 

 rupted, and seeming to run into each other, and fre- 

 quently swelling into oval scars. Tliis may, perhaps, 

 be owing to the surface having been partially worn 

 down by rubbing against the gravel of the bottom, or 

 against other objects in the water. Besides the concen- 

 tric lines, there are seen on many of the scales, espe- 

 cially those of the lateral line, radiating lines varying 

 in number from one to twentv, or more, divcrffino; from 

 the centre towards the circumference, and frequently 

 connected by cross lines forming a sort of net-work 

 around the centre (see c). Under the microscope, these 

 lines appear to be elevated ridges, dividing the concen- 

 tric lines ; but of their use I am ignorant. 



AVhat I have just stated is the ordinary explanation 

 of these fine concentric lines ; but a careful examination 

 of the structure with much higher powers than we have 

 been using, induces me to doubt its correctness. Ee- 

 verting to the scales of the Perch, let us notice the clear 

 diverging bands, which look as if the whole scale had 

 been split in several places, and the openings thus made 

 filled with uniform clear substance. The same struc- 

 Uirc is seen in many other scales, as in this cycloid one 



