AUl« OF TMt PAST _ A FOSSIL TARPOM 



CHAPTER VIII : KEY TO THE PRINCIPAL 

 FISHES DESCRIBED IN THIS BOOK. 



The following key deals with nearly all the different kinds 

 of fishes described in detail in these pages, and is intended to 

 be used in conjunction with the descriptions and illustrations 

 for the identification of any large fish that the reader is able 

 to examine. The arrangement is, naturally, somewhat 

 artificial, and the species brought together in a certain section 

 of the key are not necessarily in any way closely related. 



Some difficulty may be experienced at first in using a key 

 of this nature, but with a little practice the reader should be 

 able to " run down " most of the fishes he is likely to encounter. 

 The key is so arranged that alternative characters are con- 

 trasted, one character or set of characters being placed under 

 a letter (a), the other under a corresponding double letter 

 (aa). If, on consulting the key, the characters given under a 

 particular letter do not fit the specimen at hand, try those 

 given under the corresponding double letter. If these fit, 

 then continue to read on in this section as far as they continue 

 to fit ; when they do not fit, skip to the alternative double 

 letter where they do. Thus, proceed as far as possible and 

 the name of the 'fish under consideration will be found. 



Suppose, for example, that we have a specimen of the 

 Greenland or Sleeper Shark before us, and, being ignorant of 

 its identity, wish to find its name by using the key. Under 

 the first letter in the section devoted to Selachians (a) we have 

 to consider whether the external gill-openings are on the sides 



