702 



CYCLOTD AND CTENOID SCALES. 



with a third but incomplete layer interposed between them. The 

 outer layer is composed of several concentric laminae of a structure- 

 less 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 

 laminae of a fibrous structure, the fibres of each lamina being in- 

 clined 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 moi*e 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, &c. The structure of the Ctenoid scales (Fig. 363), 



which we find in the Sole, Perch, 

 Fig. 363. Pike, &c, does not differ essen- 



tially from that of the Cycloid, 

 gave as to the projection of the 

 comb-like teeth from the posterior 

 margin ; and it does not appear 

 that the strongly-marked division 

 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 either 



kind may become consolidated to 

 a considerable extent by the cal- 

 cification of their soft substance ; 

 but still they uever present any 

 approach to the true Bony struc- 

 ture, such as is shown in the two 

 Orders to be next adverted-to. 



549. In the Ganoid Scales, on 

 the other hand, the whole sub- 

 stance of the scale is composed of 

 a substance which is essentially 

 Bony in its nature; its inti- 

 mate structure being always comparable to that of one or other 

 of the varieties which present themselves in the bones of the 

 Vertebrate skeleton ; and being very frequently identical with that 

 of the Bones of the same fish, as is the case with the Lepidos- 

 teus (Fig. 357), one of the few existing representatives of this 



Scale of Sole, viewed as a trans- 

 parent object. 



