NATURAL HISTORY QF BRITISH SALMONIDZ. 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 fins and tail nearly same colour as 
back; pectoral fins small, and bluish white ; anal fin and ventral fins white. 
Vertebrze 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 Salmonide 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 Ave-oferculum, or fore gill cover ; B, the oferculus, or gill cover 
proper ; c, the svb-opercudum, or under gill cover; D, the zuter-operculumt, 
or intermediate gill cover ; E, the dranchiostegous rays, or gill rays. 
To begin with the form of the gill covers or ofercula, which 
consist of four pieces, three movable, and one—the fre-oper- 
culum—fixed. These afford the readiest, and probably the surest 
mode of distinguishing between the true salmon, sa/mo salar, 
and the sea or salmon trout, Salmo trutta. 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 
