NATURAL HISTORY OF BRITISH SALMONIDA£. 155 



when in season : upper part of head and body bluish black, lighter on sides, 

 which are marked (principally above the lateral line) with numerous spots 

 somewhat resembling in form the letter X. Lower part of sides and belly, 

 cheeks, and gill covers silvery white ; back tins and tail nearly same colour as 

 back ; pectoral fins small, and bluish white ; anal fin and ventral fins while. 

 Vertebras 58. 



Fin rays: D. 12: P. 13: V. 9: A. 10: C. 19. 



DISTINCTIVE MARKS OF SALMON, BULL TROUT, 

 AND SEA TROUT. 



The points upon which ichthyologists principally rely in 

 distinguishing between the three species composing the migra- 

 tory Salmonidi^ of the British Islands are the form of the gill 

 covers, the arrangement of the teeth, the shape and relative 

 position of the tail and of the fins, and the colouring. 



A represents the pre-opercitbim, or fore gill cover ; B, the operculum, or gill cover 

 proper ; c, the sub-operculuttt, or under gill cover ; D, the inter-operculuni, 

 or intermediate gill cover ; e, the branchwstegous rays, or gill rays. 



To begin with the form of the gill covers or opercula, which 

 consist of four pieces, three movable, and one— the pre-oper- 

 culum—f[y.G^± These afford the readiest, and probably the surest 

 mode of distinguishing between the true salmon, sah/io salar, 

 and the sea or salmon trout, Salmo triitta. In the former the 

 hinder margin of the whole gill cover forms almost a semicircle, 

 whilst in the latter it approaches more nearly a right angle, or 

 a semicircle with a slice taken off the circumference above and 



