GENERAL CHARACTERS. 315 



Although the bony fishes of the present day form a specialised side-branch, which 

 has lost many of the characters 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 amongst fresh- water fishes, 

 although 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 



OUUWlflOAuOXL 



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 Lung-Fishes 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 

 as follows : 



1. Lung-Fishes Subclass DIPNOI. 



(1) True Lung-Fishes 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. 



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



Skeleton. takes the form of scales. When these overlap and their posterior 

 border is 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 

 chimaeroids 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, 



