CLASSIFICATION 373 



The code of rules lays it down that a name is nothing niore than 

 a name, and cannot be ignored because of its unclassical form 

 or its unsuitability, or for any other reason except the existence 

 of a prior name. Generally speaking, it is customary in books on 

 systematic zoology to give, not only the full scientific name, but 

 to add the name of the author who first described the species 

 and gave it the name. Thus, ''Clupea harengus Linnaeus" means 

 that the common Atlantic Herring was first named and described 

 by Linnaeus; '' Alosa fitita (Cuvier) " means that the trivial 

 name finta was first given by Cuvier to the Twaite Shad, but 

 he placed it in the Linnean genus Clupea, from which it was 

 removed by a later authority and placed in the genus Alosa. 



Of the four main classes constituting the " fishes," the Marsipo- 

 branchs and Placoderms have been dealt with in some detail 

 in the previous chapter and need not be discussed further here. 

 The latter are entirely extinct, and the hving Marsipobranchs, 

 the Cyclostomes, are divided into two families, the Petromyzonidae 

 or Lampreys and the Myxinidae or Hag-fishes. The arrangement 

 of the sub-classes, orders, sub-orders, etc., of the other two 

 classes may be very briefly outlined, but this will consist of 

 little more than a list of their scientific names : however, taken 

 in conjunction with the accompanying "trees" (Figs. 134, 135), 

 this will serve to give some idea of the manner in which modern 

 authorities classify the Selachians and Bony Fishes. 



Of the five main sub-classes into which the Selachians are 

 divided, the first three {Pleuropterygii, Acanthodii, Ichthyotomi) 

 contain only extinct forms and have been considered in the 

 previous chapter [cf. pp. 351-352)- The fourth, the Euselachii, is 

 spht up into two orders: in the first, the Pleurotremata or Sharks, 

 the front margin of each pectoral fin is free, the external gill- 

 clefts are on the sides of the head, and the eyes have free 

 margins (Figs, i; 14A; 23c; 32A, etc.); in the second the 

 Hypotremata or Rays, the front margin of the pectoral fin is 

 joined to the side of the body or head, the gill-openings are on 

 the lower surface of the head, and the upper edges of the eyes 

 are not free (Figs. 8a; 14B; 107, etc.). The Pleurotremata are 

 again sub-divided into three sub-orders, represented on the 

 tree by the branches d, e, and f. The Notidanoidea are the 

 most primitive of existing Sharks, and in these the gill-clefts 

 number six or seven, the vertebral column is of simple form, 

 and there is only one dorsal fin (Figs. 320; 66a). The group 

 includes the rare Frilled Shark {Chlamydoselachus) and the 

 Comb-toothed Sharks (Hexanchidae) . The Galeoidea, with five 



