cliromatopliores and iridocytes occurs outside the scales, 

 and tliere is another layer of both on the inner surface of 

 the skin and between it and the muscles. In ihe skin of 

 the eyeless side no chroniatophores 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 leflecting 

 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, f and in six specimens selected 

 at random ranged from 6G 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 



t See especially Cunningham, Jour. M. Biol. Assoc. N. S., vol. iv., 1897. 



