FLATFISHES. XORMAX. 279 



are united posteriorly as in Boivenia, but are not con- 

 nected with the anal fin. On 'account of the abnormal 

 development of the upper part of the head, the rostral 

 process characteristic of R. tapirina is wanting in this 

 example. 



The phenomenon of ambicoloration in Flatfishes is 

 particularly interesting on account of the other varia- 

 tions towards symmetry which accompany complete (or 

 almost complete) pigmentation of the blind side. 10 



The assumption by the scales of the blind side of the 

 structure of those of the ocular side is a characteristic 

 variation of this nature. In the European Dab (Pleu- 

 ronectes limanda), which normally has ctenoid scales on 

 the ocular side and cycloid scales on the blind side, the 

 scales are more or less ctenoid on both sides of the body 

 in ambicolorate examples. In the Turbot (Rhombus 

 maximus] bony tubercles are normally developed only on 

 the ocular side, but in ambicolorate examples they are 

 also strongly developed on the blind side. Another cor- 

 related variation is the delayed or arrested migration of 

 the eye, which interferes with the growth forward of the 

 dorsal fin and leads to the development of the charac- 

 teristic fleshy hook. Examination of a series of examples 

 shows that this feature is only present where the ambi- 

 coloration is almost complete, and pigmentation extends 

 over the greater part of the body on the blind side, and 

 frequently also over a portion of the head. Where the 

 pigmentation of the blind side is confined to the hinder 

 half of the body, the structure of the scales may be 

 affected to some extent in the pigmeuted area, but the 

 form of the head remains quite normal. The tendency 

 for ambicoloration in Flatfishes to affect the head last of 

 all may be noted; this is the region where asymmetry 

 is most marked. 



The development of the pelvic fin of the blind side 

 in the above mentioned examples of Rhombosolea is of 

 considerable interest, and there is little doubt that this 

 is another variation towards symmetry associated with 

 ambicoloration. 



10 Schnakenbeck (Wiss. Meeresunters., N.F., XV, Abt. Helgo- 

 land, Heft 1, No. 10, 1923, 22 pp., 1 pi.) has recently investigated the 

 general problem of abnormal coloration in the Pleuronectidae, but 

 has not dealt with these correlated variations. 



