364 COMPARATIVE ANATOMY AND PHYSIOLOGY. 



better mode of protection in its well-developed electric 

 organ. In various Elasmobranchs the more prominent 

 fins are provided with strong ^spines. 



The Ganoidei received their name from the posses- 

 sion by some of them of bright shining scales, which 

 owe their appearance to the investing layer of enamel. 

 Such "ganoid scales" are, however, found in 

 perfection only in Lepidosteus and Polypterus among 

 recent members of the group; in the sturgeon, for 

 example, there are bony plates, and Spatularia is 

 naked. In the two Ganoids first mentioned the 

 scales overlap, and the whole body is protected by 

 a closely and firmly set coat of mail. Among fossil 

 forms we find the typical ganoid arrangement some- 

 times carried to a remarkable extreme, as in Pterich- 

 thys, where large bucklers are found not only on the 

 dorsal but also on the ventral surface. The allied 

 Dinichthys is thought to have reached a length of 

 more than fifteen feet ; and we see in it, as in other 

 gigantic forms, such as the Irish elk, that individual 

 protection has been only attained at the cost of the 

 disappearance of the species. 



The simpler smaller scales that are found in some 

 Ganoids, and very commonly among the Teleostei, 

 may be, when we look at extremes, classed under 

 the head of cycloid scales, in which the free pro- 

 jecting margin is rounded, or as ctenoid, in which 

 the margin, or part of the surface, is denticulated or 

 comb-like ; between these, however, there are a 

 number of intermediate stages ; the ctenoid scales may 

 be supposed to have given rise to those in which part 

 of the surface is continued into fine non-denticulated 

 spines (sparoid scales of sea-breams). While some 

 fishes, such as Stomias, have the scales deciduate, 

 and others, not to speak of electric forms, are, 

 like the eel, scaleless, the Teleostei, almost as much 

 as the Ganoids, present us with examples of forms in 



