180 ON AN ABNORMAL TURBOT. [Feb. 19, 



the dorsal fin and the head, but in the course of the dorsal fin 

 itself. The anterior end of the fin then belonging to the same part 

 of the composite fish as the head, the relations of the two struc- 

 tures in development are not disturbed. Holt*, however, records 

 a specimen of Turbot in which there was some pigment on the lower 

 side of the jaws and on other parts in front of the preoperculum, 

 and yet the head and fin were structurally normal. This excep- 

 tion is not enough to disprove my hypothesis, for it may happen 

 exceptionally that the two parts in the composite gamete are so 

 exactly fitted together in their new relations, that the fin attaches 

 itself to the skull in development as in a normal specimen. 



The fact which gives most suppoi't to my hypothesis, is that in 

 the great majority of ambicolorate Tvirbot the lower side of the 

 bead is white and destitute of pigment. The same condition has 

 been seen by me in two specimens of Pleuronectes niacrocephalt(s 

 and one specimen of Plaice, P. platessa. In the Flounder [P.flesus), 

 however, I have not found the absence of pigment from the lower 

 side of the head in ambicolorate specimens. In the Phil. Trans, 

 memoir I have recorded four specimens in which the lower side 

 was completely coloured and in which the usual abnoi-mality of 

 the dorsal fin was pi'esent. 



There is, however, another fact concerning ambicol oration 

 which is diflacult to reconcile with the view that it is due to the 

 metastrophe of the jjosterioi' region, namely, that in ambicolorate 

 specimens not merely both sides are pigmented, but both sides are 

 equally armed — that is to say, the scales, spines, or tubercles are 

 equally developed on both sides. In normal flat-fishes the arma- 

 ture is much reduced on the lower side. In the Turbot the 

 scattered tubercles present on the upper side are almost entirely 

 absent fi'om the lower side ; in the Flounder the rough spiny scales 

 alone- the lateral line and alon^ the bases of the dorsal and ventral 

 fins are absent from the lower side. In ambicolorate specimens 

 the armature is not only present on the lo\A'er side, but also on 

 the upper ; the abnormal specimens are not merely ambicolorate, 

 but ambiarmate. If, according to the hypothesis I liave sviggested, 

 the postcephalic region were metastrophic, the upper side should 

 be originally not merely without pigment, but without armature, 

 like the lower side in normal specimens. If the pigment on the 

 upper side in ambicolorate specimens were due to light, the arma- 

 ture should remain absent, unless the action of light prodvices 

 armature as well as pigment, for which there seems no reason and 

 for which we have no evidence. Something might be attributed 

 to the absence of friction from the upper side, but this is excluded 

 by the strong development of the ai'mature on the lower side in 

 these abnormal specimens. These considerations are in favour 

 of the alternative hypothesis to explain ambicoloration and ambi- 

 armature, namely, that in the gamete two right or two left sides 

 are united instead of a right and left. In this case of course all 



* E. W. L. Holt, " Studies in Teleostean Morphology from the Marine Labora- 

 tory at Cleethorpes," P. Z. S. 1894, p. 413. 



