VII. 3 



FISH SCALES 



193 



and hardly at all in the winter. The age of the fish can therefore be 

 determined from these rings (Fig. 117), or from the similar markings 

 on the ear stones (p. 216). While an adult salmon is in fresh water no 

 growth occurs, leaving a spawning mark on the scale. 



The head of the trout shows some of the most specialized and typical 

 teleostean features (Fig. 119). There are two nostrils on each side, but 

 no external sign of ears. The mouth is very large and its edges are 

 supported by movable bones, to be described below. The maxillary 

 and mandibular valves are folds 

 of the buccal mucosa, serving to 

 prevent the exit of water during 

 respiration. The tongue, as in 

 Selachians, has no muscles, but 

 may carry teeth and taste-buds. 

 Behind the edge of the jaw is the 

 operculum, a flap covering the 

 gills and also supported by bony 

 plates. In connexion with these 

 special developments of jaws and 

 gills the skull has become much 

 modified and has developed com- 

 plex and characteristic features 

 (Fig. 118). 



Fig. 117. Spawning mark (sp. ?nk.), the 

 result of erosion or absorption of the scale 

 margin due to a calcium deficiency fasting 

 period. (After J. W. Jones, The Salmon.) 



1st river winter. 2. 2nd river winter. 

 3. 1 st sea winter. 



3. The skull of bony fishes 



The main basis for the skull 

 is a chondrocranium and set of 



branchial arches, exactly comparable to those of the elasmobranchs. 

 In the early stages of development there is a set of cartilaginous boxes 

 around the nasal and auditory capsules, brain and eyes, and a series 

 of cartilaginous rods in the gill arches. Bones are then added in two 

 ways: either (i) as cartilage bones (endochondral bones) by the re- 

 placement of some parts of the original chondrocranium, or (2) as 

 membrane or dermal bones, laid down as more superficial coverings 

 and considered to be derived from a layer of scales in the skin. This 

 outer position of the bones can be clearly seen in many cases by the 

 readiness with which the membrane bones can be pulled away from 

 the rest of the skull. 



The skull bones are arranged in a regular pattern, whose broad 

 outlines can be seen in all fishes and in their tetrapod descendants. 

 However, there are many confusing variations and the naming of 



