PIGMENTS ON THE LOWER SIDES OF FLAT FISHES. 57 



held slightly elevated above the slate bottom, which accounts for the 

 pigmentation of the lower side of that fin. 



It is now necessary to give the history of these fish. (See the previous 

 number of this Journal, vol. iii. p. 273.) They were the survivors of a 

 number put into the tank in the summer of the year 1893, and judged 

 to be hatched in the early part of that year. In April and May were 

 put in five soles, about 1.5 cm. long, and a few flounders from 1.0 to 

 1.5 cm. long. In June a few young turbot and brill, in the pelagic 

 transformation stages, were put in. In July there were added seven 

 plaice, 6,5 to 8.5 cm. long, and judged to have been hatched the 

 preceding January. On October 19th there were found to be in the 

 tank seven plaice, one sole, three brill, three turbot, and several 

 flounders, but no note was made of the presence or absence of pigment 

 on the lower side. The fish were simply reared in order to see their 

 growth, and it was not supposed that any conditions affecting pigmen- 

 tation were present. On April 4th, 1894, some of the fish died, and 

 the notes of their condition taken at the time are as follows : 



One turbot 10.8 cm. long, a little pigment on the lower side, on the 

 marginal fins, and within their bases. 



One brill 11*3 cm. long, pigment on lower side near bases of 

 marginal fins. 



One plaice 16'7 cm. long, no pigment on lower side. 



One flounder 12 cm. long, some pigment on lower side near bases 

 of fins. 



One flounder 11 cm. long, no pigment on lower side. 



Thus, in three out of five of these specimens there was some pigment 

 on the lower side in the same region as in the fishes above described 

 from the same tank. But at this time I did not pay much attention to 

 it, and thought it was only an instance of casual variation. 



The five fish first described had thus been living in the tank about a 

 year and a half. The tank having a glass front, and being at a height 

 of about 5 feet from the ground, opposite a north window, many of the 

 rays of light entering it must have been nearly horizontal. The glass 

 front, however, did not extend quite to the bottom of the tank, the 

 lower 4 inches of the front being formed of slate. Although it is 

 difficult to make an exact comparison, it certainly seems that the 

 pigmentation was produced more constantly and more rapidly in the 

 unintentional experiment here described, than in those recorded in 

 the previous memoir, which I took so much trouble to arrange. Thus 

 the survivors of the specimens of the brood of 1892, reared in the 

 glass-bottom tank, died on October 20th, 1893, and had therefore been 

 exposed to light very nearly as long as the five described in this paper. 

 They were ten in number, and four of them had no pigment at all on 



