CHAPTER III. 



The Boxy Fishes and Gaxoids, — Subclass Teleostomi. 



Formerly the typical bony fishes of the present clay were regarded as indicating a 

 primary group (Teleostei) of equal rank with a second one known as the Ganoidei ; 

 the latter containing the American bony pike, and the African bichir, together 

 with a host of extinct genera possessing a similar armour of hard ganoid scales. 

 A fuller study of these and other allied fossil forms has, however, shown the 

 existence of such a complete transition from these so-called ganoids to the typical 

 bony fishes that it has become necessary to include the whole of them in a single 

 subclass, under the title heading this chapter. Although there is still some degree 

 of uncertainty as to the best mode of arranging certain groups of the bony fishes, 

 the following scheme may be temporarily adopted : — 



1. Order Actixopterygii — Fan-Finned Group. 



(1) Suborder., f Acaxthopterygii — Spine-Finned Fishes. 



(2) „ >• Lophobraxchii — Tuft-Gilled Fishes. 



(3) ., 1 1 Plectognathi — Comb-Gilled Fishes. 



(4) ,, (5 [Axacaxthixi — Soft-Finned Fishes. 



(5) „ Physostomi — Tube-Bladdered Fishes. 



(6) „ Isospoxdyli — Leptolepis. 



(7) ., /Etheospoxdyli — Bony Pike 



(8) „ Protospoxdyli — Amioids. 



(9) ., Choxdrostei — Sturgeons. 



2, Order Crossopterygii — Fringe-Finned Group. 



In this wide sense the subclass differs broadly from the two preceding ones in 

 the structure of the skull, which is formed on what may be termed the hinged type 

 (hyostylic); that is to say, the palato-pterygoid bar remains separated from the 

 cranium proper, to the hinder-part of which it is movably articulated by the 

 intervention of the hyomandibular. The internal skeleton is more or less ossified, 

 with the development of membrane-bones on the jaws; the gill-clefts are but 

 slightly separated from one another, and are fully protected by an operculum : the 

 membrane-bones of the pectoral girdle (that is to say. the scapula, claviculars, etc.) 

 are connected with the hinder-part of the skull: and the external skeleton takes 

 the form either of plates of bone or of calcified overlapping seal.-. In existing 

 forms the eggs are small, numerous, and generally masse,! together; the two optic 

 nerves may either simply cross one another, or may give off mutually interlacing 

 fibres; an air-bladder— with or without a duet— is very generally present : and the 

 intestine may sometimes be furnished with a spiral valve. 



