262 THE MICROSCOPE AND ITS REVELATIONS. 



supposed to be), but concretions of Carbonate of Lime. "When the scale 

 of the Eel is examined by Polarized light, its surface exhibits a beautiful 

 St. Andrew's cross; and if a plate of Selenite be placed behind it, and 

 the analyzing prism be made to revolve, a remarkable play of colors is 

 presented. 



658. In studying the structure of the more highly developed scales, 

 we may take as an illustration that of the Carp; in which two very dis- 

 tinct layers can be made-out by a vertical section, with a third but incom- 

 plete layer interposed between them. The outer layer is composed of 

 several concentric lamina of a structureless transparent substance, like 

 that of cartilage; the outermost of these laminae is the smallest, and the 

 size of the plates increases progressively from without inwards, so that 

 their margins appear on the surface as a series of concentric lines; and 

 their surfaces are thrown into ridges and furrows, which commonly have 

 a radiating direction. The inner layer is composed of numerous laminge 



FIG. 447. FIG. 448. 



Portion of Skin of Sole, viewed as an opaque object. Scale of Sole, viewed as a trans- 



parent object. 



of a fibrous structure, the fibres of each lamina being inclined at various- 

 angles to those of the lamina above and below it. Between these two 

 layers is interposed a stratum of calcareous concretions, resembling those 

 of the scale of the Eel: these are sometimes globular or spheroidal, but 

 more commonly ' lenticular,' that is, having the form of a double-convex: 

 lens. The scales which resemble those of the Carp in having a form 

 more or less circular, and in being destitute of comb-like prolongations, 

 are called cycloid; and such are the characters of those of the Salmon, 

 Herring, Roach, etc. The structure of the ctenoid scales (Fig. 448), 

 which we find in the Sole, Perch, Pike, etc., does not differ essentially 

 from that of the cycloid, save as to the projection of the comb-like teeth 

 from the posterior margin; and it does not appear that the strongly- 

 marked divisions which Prof. Agassiz has attempted to establish between 

 the 'cycloid' and the ' ctenoid' Orders of Fishes, on the basis of this 

 difference, is in harmony with their general organization. Scales of every 

 kind may become consolidated to a considerable extent by the calcifica- 

 tion of their soft substance; but still they never present any approach to 



