GENERAL CHARACTERISTICS 2683 



has lost many of the characteristics common to the two classes, it will be evident 

 that Fishes and Amphibians are very closely -allied groups; the latter of which has 

 been directly derived from the former. Geologically, fishes are older than any of 

 the classes hitherto described, their fossil remains occurring in strata belonging to 

 the upper part of the Silurian division of the Palaeozoic epoch. 



The form of a typical fish is so well known that it will be quite unnecessary to 

 describe it; and it may be mentioned that this typical form, which is the one best 

 adapted for progress through water, is very general among fresh-water fishes, al- 

 though the eels constitute an exception in this respect. Much greater diversity 

 exists, however, among the marine representatives of the class; and we may cite as 

 extreme types a shark, a flat fish, a ribbon fish, and a globe fish. 



The structure of the skeleton, both external and internal, being of 

 the utmost importance in the classification of fishes, it is essential that 

 the attention of the reader should be more fully directed to this point than has 

 been done in the case of the higher Vertebrates. It should first be mentioned 

 that fishes are divided into four subclasses, namely, the Lungfishes or Dipnoi; the 

 Chimseroids or Holocephali; the Bony Fishes and Ganoids or Teleostomi; and the 

 Sharks and Rays or Elasmobranchii. These may be further subdivided into orders 

 ^s follows: 



1. L/ungfishes Subclass DIPNOI. 



(1) True L,ungfishes Order SIRENOIDEI. 



(2) Berry-Boned Fishes Order ARTHRODIRA (extinct). 



2. Chimseroids Subclass HOLOCEPHALI. 



3. Bony Fishes and Ganoids Subclass TELEOSTOMI. 



(1) Fan-Finned Fishes Order ACTINOPTERYGII. 



(2) Fringe-Finned Ganoids Order CROSSOPTERYGII. 



4. Sharks and Rays Subclass ELASMOBRANCHII. 



(1) Acanthodians Order ACANTHODII (extinct). 



(2) Fringe-Finned Sharks Order ICHTHYOTOMI (extinct). 



(3) True Sharks and Rays Order SELACHOIDEI. 



In regard to the external skeleton, the most characteristic type takes 



the form of scales. When these overlap and their posterior border is 

 Skeleton 



entire, such scales are termed cycloid, but when serrated, ctenoid. The 



external skeleton may, however, take the form of plates or granules, which in the 

 chimseroids and sharks and rays are generally isolated, and have a structure 

 precisely similar to that of teeth, consisting of a base of ivory or dentine capped 

 with enamel. The so-called ganoid scales, like those of the bony pike, are, on the 

 other hand, quadrangular, and often connected by a peg and socket arrangement; 

 they are formed of true bone capped with an enamel-like substance termed ganoin, 

 and true bone likewise occurs in the plates of the sturgeons. A series of specially 

 modified scales, running along the sides of many fishes, constitute the so-called 

 lateral line, which is partly connected with the supply of mucus; and certain large 

 "V-shaped scales on the borders of the fins of many extinct bony fishes are known as 

 fulcra. The fin rays, which also come under the designation of dermal structures, 



