SKIN, SCALES, AND SPINES 



99 



points which enables an expert to tell them apart, by far the 

 most reliable method consists in counting the number of scales on 

 the tail, a sure means of identifying almost any specimen. In the 

 Salmon the number in an oblique series from the hinder edge of 

 the adipose fin downwards and forwards to the lateral line varies 

 from 10 to 13, but in the Trout this number ranges from 13 to 16. 

 Mention may be made here of the so-called hybrid between 

 the Pilchard (Sardina) and the Herring (Clupea), specimens of 

 which turn up from time to time. At first sight this fish appears 

 to have about 30 rows of scales along one side of the body, and 

 more than 50 on the other. This is not, of course, a genuine 

 hybrid, and the explanation of the abnormality is that the 

 scales of the Pilchard are unequal in size, the oblique rows being 



Fig- 43- 

 Carp (Cyprinus carpio), X^. 

 To show arrangement of scales. 



alternately of larger and smaller scales, the latter being quite con- 

 cealed by the former in normal fish. In the so-called hybrid all 

 the scales of one side of the body are equal in size and regularly 

 arranged, while those of the other are large and small as usual. 

 In many Bony Fishes there is an enlarged and somewhat 

 modified scale in the angle where the upper edge of the pectoral 

 fin joins the body, and often a similar axillary or accessory scale 

 in the outer angle of the pelvic fin. It usually takes the form 

 of a pointed dagger-like process, sometimes stifi:^ and hard, 

 sometimes quite soft and flexible. As a rule, it is present in the 

 more generalised forms, but is lacking in the more specialised 

 fishes. Again, it is often well developed in actively swimming 

 forms, and absent or much reduced in those living at or near 

 the bottom, a fact which suggests that it is in some way con- 



