FISHES. 307 



possible friction in swimming. To this end also, as well as 

 for purposes of defence, the body is usually enveloped with a 

 coating of scales developed in the inferior or dermal layer of 

 the skin. The more important modifications in the form of 

 these dermal scales are as follows: I. Cycloid scales (fig. 272), 

 consisting of thin, flexible, horny scales, circular or elliptical 

 in shape, and having a more or less completely smooth outline. 

 These are the scales which are characteristic of most of the 

 ordinary bony fishes. II. Ctenoid scales (fig. 273), also con- 

 sisting of thin horny plates, but having their posterior margins 



Fig. 272. Cycloid scale. Fig. 273. Ctenoid scale. Fig. 274. Ganoid scale. 



fringed with spines, or cut into comb-like projections. III. 

 Ganoid scales, composed of an inferior layer consisting of bone, 

 covered by a superficial layer of hard polished enamel (the so- 

 called "ganoine"). These scales (fig. 274) are usually much 

 larger and thicker than the ordinary scales, and though they 

 are often articulated to one another by special processes, they 

 only rarely overlap. IV. Placoid scales, consisting of detached 

 bony grains, tubercles, or plates, of which the latter are not 

 uncommonly armed with spines. 



It is very important for the geologist to recognise the charac- 

 ters of these different scales, as he may have to decide upon 

 the characters of a fossil fish merely from detached scales. 

 Such decisions, however, are always more or less hazardous, 

 since the scales of the different orders of the living fishes are 

 not invariably of the same kind in all the forms of the order. 

 Thus, ganoid scales are not peculiar to the order of the Ganoid 

 fishes, but occur also in some of the Bony Fishes (Teleostei). 

 The scales, also, form at best but one character, and they can 

 hardly be said to constitute the most important character of 

 any fish. A classification, therefore, which is based primarily 

 upon the nature of the scales, necessarily is more or less "artifi- 

 cial," and is liable to bring into juxtaposition forms which have 

 no real affinity to one another. For these reasons, most 

 zoologists do not accept the classification of the Fishes into the 

 four orders of the Cycloidei t Ctenoidei, Ganoidei, and Placoidei, 

 since this classification, though sanctioned by such an eminent 



