6 



SALMONID^. 



poma) exceedingly short. The adjoined woodcuts will readily show 

 the difference : — 



PrjBoperculum. 



S. salar. 



S. brackypoma. 



In young specimens of all Salmonoids the prseoperculum has a very 

 short lower limb ; but whilst in some species it lengthens with age, 

 its development in a horizontal direction is arrested in others. 



2. The tvidth and strength of the masclllary of the adult fish. To 

 show this character in two distinct species, we have given woodcuts 

 of the maxillaries of females (12 inches long) oiS. fario and S. leve- 

 nensis of the same size. 



S. fario. 



S. levenensis. 



In young specimens of all Salmonoids the maxillary is compara- 

 tively shorter and broader, somewhat resembling that of a Coregonus ; 

 yet this bone offers a valuable character for the determination of the 

 young of some species ; for instance, in a yoxing S. cambricus it ex- 

 tends scarcely to below the centre of the eye, whilst in a S. fario of 

 the same size it reaches to, or even beyond this point. 



3. The size of the teeth, those of the intermaxillaries excepted. 



4. The arrangement and the permanence or deciduousness of the 

 vomerine teeth. In some species the vomer is normally armed with 

 a double or single series throughout life, although of course some of 

 the teeth are frequently accidentally lost ; in others these teeth dis- 

 appear gradually with age, the hinder ones first, so that finally the 

 anterior only remain. In order to ascertain the arrangement of the 

 teeth, it is necessary to remove the ^giva. Frequently the teeth 

 stand in a distinctly double or single series, or they are placed alter- 

 nately ; but frequent irregularities occur which render this character 

 vague, or even unsafe, so that some zoologists have rejected it entirely 

 as unreliable. However, when a greater number of individuals, really 

 belonging to the same species, are examined, a pretty safe conclusion 

 may be arrived at as regards the arrangement of these teeth. 



