36 
Proceedings of Royal Society of Edinburgh. [sess„ 
The typical gill cover I would represent thus : — 
The general appearance of the head will be seen in the photographs 
of the male and female clean run fish (figs. 2 and 3). Relatively 
to the total length of the fish, I find that the head is contained 
from 4J to 5f times. The males examined in August varied from 
4J to 4f times. The females in each case had the head measure- 
ment 5f times in the length of the fish (measured on the flat). 
The caudal fin is also a well-marked feature. At a comparatively 
early age this tail fin becomes truncate or rounded at its outer 
margin. In salar and in trutta proper this never happens, so far 
as I am aware, except in distinctly large fish. In the Tweed trout,, 
however, fish between 6 and 7 pounds, or about 25 inches long,, 
show this rounded tail — hence the name ‘ round tail.’ 
The female specimen photographed is 7J lbs. and 26 inches in 
length. The rounded tail is well seen. An example weighing 
2J lbs., and which was 18 \ inches long, was found to have the 
caudal fin slightly forked when fully extended. From this 
slightly forked condition in young fish, the tail fin becomes first 
‘straight,’ then, with increased size and age, the rounded outer 
border appears. Finally, in fish of 10 lbs. and upwards, a stunted 
aspect is frequently noticeable, the tail being not only rounded, but 
apparently so much thickened and grown-over by the caudal 
peduncle as to have the free portions of the caudal fin rays notice- 
ably short. All large specimens have not this appearance, but it is 
