TROUT— VARIATIONS IN COLOUR. 



147 



present, but it is by no means rare to see only one irregularly placed row. While 

 in very large specimens tbese teeth (unless they have entirely disappeared) are 

 alvrays in a single row, and the vomer may be found even toothless or with one or two 

 teeth at the bind edge of the head of that bone. Equally incorrect is the statement 

 that the teeth disappear differently in different forms, for in all they first assume 

 a single row and then fall out, first commencing from behind. But in the 

 rapidly growing sea trout the vomerine teeth are shed sooner than in the brook 

 trout. 



^:^^Ti A 



Fig. 18. Head of young hum 

 trout, natural size. 



Fig. 19. Head of old male brook 

 trout, 1-21 the natural size. 



If the external conformation of the head is examined it is found very similar 

 in the young trout, fig. 18, to what is seen in the salmon par {see p. 52, fig. 13) 

 while in the old trout, fig, 19, there are many points of resemblance with the old 

 salmon (fig. 14, p. 52) ; but should the skulls be referred to (see plate i) the bones 

 of the trout will be seen to be much stronger and denser than in the salmon, 

 and this is especially remarkable in those of the jaws and snout. At first the 

 posterior extremity of the upper jaw in the young trout extends to beneath the 

 eye, but with increasing age it reaches to behind it, and for two reasons, first the 

 eye with age does not augment in size so rapidly as the rest of the head, so 

 becomes compaiutively smaller in old fish, and secondly the jaw on the contrary 

 grows proportionately with the rest of the skull : as a rule the jaws are stronger 

 in fresh-water than anadromous trout. As i-egards the size of the eye, should two 

 trout of the same length but different ages be examined, the one which is the 

 younger will have the larger eyes. 



Respecting the form of the preopercle in adult salmonidEe, much has been 

 written, but, as already remarked (p. 17), what has been asserted to be specific 

 differences are often simply such as have been induced by age, sex, or local 



Fig. 20. 



Fig. 21. 



Fig. 22. 



circumstances. Fig. 22, is that of an adult salmon; fig. 21, of a female Lochleven 

 trout, 19 in. long; and fig. 20, of a salmon-trout, S. triUfa, also 19 in. long, and 

 having 52 crecal appendages. Trout from other localities, of the same size, show 

 but very little individual differences to what is seen in the above figures.* The 

 forms of the opercles will be subsequently alluded to and figured. 



Respecting the size of the fins of trout, those in fresh-water forms are generally 

 more developed than in marine ones, in young fish than in old ones, and in males 

 than in females. The pectorals, which are shorter in trout than in salmon, 



* For figures showing the gradual change of the form of the preopercle with age, see 

 Lochleven trout. 



10 * 



