352 ZOOLOGY 



growth ; these have been much studied of late, because 

 it appears that they may be used to interpret the past 

 history of the fish. In the case of salmon,, especially, 

 the study of scales has thrown light on the life history, 

 and has come to be of practical importance in relation 

 to the regulation of the fishing industry. The salmon is 

 a migratory fish, and the different surroundings affect 

 the growth of the scales., and are recorded in the annuli. 

 It has even been possible to infer that an extinct fish 

 used to migrate, from a study of its scales. In addi- 

 tion to the marks just described, there are often radi- 

 ating (occasionally transverse) lines (radii} representing 

 grooves. These may extend in every direction from 

 the nucleus of the scale, or may be all basal, or all apical ; 

 or apical and basal, but not lateral. In typical ctenoid 

 scales, there are nearly always strong basal radii, and 

 where they reach the margin the latter is often crinkled, 

 producing a scalloped effect. Taking all these different 

 characters together, it is often possible to classify a fish 

 if we have no more than a single scale from the middle 

 of the side, where the characters are best shown. 



6. Some of the principal groups of bony fishes are the 

 following : 

 Salmon, ( a ) Isospondyli. Marine and fresh-water fishes with 



trout, and so f t ns anc j the pectoral and pelvic fins far 



herrings ' r . 



apart. The group is a large one, with very 



diverse families, the most important being the 

 herrings and their relatives, and the group in- 

 cluding the salmon and trout. The latter are 

 especially distinguished by the second dorsal 

 fin, a little fin above the root of the tail. Some 

 other fishes have such a fin, but they have 

 either very different scales, or none at all. The 

 salmon or trout scale is cycloid, and without 



