SOFT-RAYED BONY FISHES 87 



some marked changes. The original gristly upper jaw of the 

 Shark no longer functions as such, but has become modified 

 almost out of recognition, and, in association with some new 

 bones, forms part of the bony palace or roof of the mouth in 

 the Bony Fish. To replace it there has been formed an 

 entirely new upper jaw made up mainly of 2 dermal bones — 

 a praemaxillary in front and a maxillary behind on each 

 side — bones that originated in the tissues of the upper lip. 

 The lower jaw of the Shark still retains its original function, 

 but has become entirely invested or even replaced by bones. 



The body of a Bony Fish is nearly always covered with 

 scales, and the dermal denticles have disappeared. In the 

 earliest Bony Fishes these scales took the form of 4-sided, 

 shining, enamel-covered, bony plates, all in close contact one 

 with the other, and together forming a complete investing 

 armour. Such ganoid scales, as they aie called, are still 

 found in certain primitive Bony Fishes existing to-day, but 

 in the vast majority of living forms they have been replaced 

 by thin, rounded scales, which overlap one another like the 

 tiles on a roof. 



As in the Selachians, there are a series of internal gill-clefts 

 on each side in the walls of the gullet, and these are supported 

 by similar hoop-like gill-arches, although the latter are here 

 composed of bone. The partitions between the clefts have 

 been reduced to such small proportions that the delicate red 

 gill-plates or lamellae project outwards as filaments. Instead 

 of each opening separately to the exterior, the internal clefts 

 open into a common branchial chamber on each side, which 

 has a single external gill-opening at the back of the head. 

 The outer wall of this chamber is provided by the movable 

 gill-cover or operculum, which forms so characteristic a feature 

 of most Bony Fishes (see p. xix). 



The organs associated with the sense of smell are paired, 

 and are placed on the sides of the snout. In most Bony 

 Fishes each nasal organ is provided with 2 external nostrils. 

 Finally, in the majority of Bony Fishes an air-bladder is 

 present, which generally functions as a hydrostatic organ : 

 this is never developed in any Selachian. 



The class Pisces is split up into a number of orders, which 

 most modern authorities group into three great sub-classes ; 



