5 
chromatophores and iridocytes occurs outside the scales, 
and there is another layer of both on the inner surface of 
the skin and between it and the muscles. In the skin of 
the eyeless side no chromatophores whatever are normally 
present, and also only the external layer of iridocytes is 
found. Internally, however, there is a ‘ thin perfectly 
opaque layer of material giving a dead-white reflection. 
Examined with the microscope, this layer presents only 
a minutely granular structure, and is everywhere uniform 
and continuous.” On account of its capacity of reflecting 
light in such a way as to produce a silvery appearance it 
is called the argenteum. It is doubtful whether the 
chromatophores, iridocytes or argenteum are cellular 
structures. It may be mentioned that Cunningham and 
MacMunn were able by experiment to induce a coloura- 
tion of the under side of the flounder. 
The dorsal fin commences vertically above the left 
eye, a short distance behind the left posterior nostril. It 
extends back to the root of the tail, and is highest about 
two-thirds of its length from the snout. The number of 
fin rays varies considerably,t and in six specimens selected 
at random ranged from 66 to 74. The anal fin commences 
very far forward, immediately behind the so-called “ anal 
spine,’ and stretches as far back as the dorsal fin. It is 
highest at about its anterior third. In the same six 
examples above the fin rays varied from 52 to 57. The 
caudal fin belongs to the masked heterocercal or homocercal 
type, and has usually about 20 fin rays. The pectoral fin 
is situated immediately below the posterior angle of the 
operculum. It has usually the same number of fin rays 
on both sides (about 10), but on the eyeless side one is 
small and may be overlooked. The pelvic fin is jugular 
+ See especially Cunningham, Jour. M. Biol, Assoc. N. S., vol. iv., 1897. 
