1907.] AN ABNORMAL TURBOT. 175 



at the bases of the caudal and ventral rays. These ai-e probably 

 the beginnings of marginal tubercles. 



The anterior end of the dorsal fin and the basal tissue which 

 carries it forma projecting hook-like process over the dorsal eye — 

 that is, the originally left eye, which has moved to the right side 

 of the head. This projection, due to the absence of attachment 

 between the base of the fin at the anterior end and the head, 

 occurs commonly in ambicolorate specimens of the turbot and less 

 frequently in ambicolorate specimens of other species of Pleuro- 

 nectidce (see Cunningham and MacMunn, " Coloratioii of Skins of 

 Fishes, &c.," Phil. Trans. 1894). 



A letter from Miss Fox to Dr. Allen, dated Jan. 7, 1907, 

 explains that the fish was caught on Sept. 28 last year, and 

 lived in captivity till Nov. 28, when it died from some unknown 

 cause. When caught it was, unlike all the others seen at the same 

 time, quite stationary on the sand, which Miss Fox thought might 

 imply a certain blindness. In captivity, however, it was very 

 active, and certainly saw food very quickly, so that there is no 

 reason to think the function of the eyes was afifected. 



In Plate XI. I have figured the two sides of the abnormal 

 specimen. The normal specimen was 4-2 cm. long. Its meta- 

 morphosis is complete, but there are still a few scattered black 

 chromatophores on the right or lower side. Similar black 

 chromatophores are present on the right side of the abnormal 

 specimen, and they appear to be lai-ger and slightly moi-e 

 numerous ; but the difference is slight, so that the exposure of 

 the right side to light during the two months it was in captivity 

 had not produced much efiect. 



It seems to me that the only way to attempt an explanation of 

 the condition of this specimen is to base the explanation on the 

 view of the constitution of the ovum which was developed by 

 Weismann, and which is adopted in the Mendelian doctrine of 

 heredity. If the right side of the anterior or cephalic region were 

 more completely pigmented, we might regard the fish as consisting 

 of an anterior smaller part which was reversed, and a posterior 

 part which was normal. The condition would then be explained 

 by supposing the specimen developed from an ovum consisting of 

 parts usually occurring in separate ova. We know that reversed 

 specimens occur in various species of flat-fishes, e. g. the Flounder 

 {Pleuronectes fles'us). In this species in some localities reversed 

 specimens are not only common, but abundant. At Plymouth I 

 found about 30 per cent, of the specimens captured had the eyes 

 and pigment on the left side instead of on the right. It is neces- 

 sary here to consider the precise terms to be used to indicate the 

 structural peculiarities which present themselves. It has been 

 usual to speak of a Flounder with eyes on the left side as reversed • 

 but if we use the substantive corresponding to this adjective, 

 namely reversion, we are using a term which has been employed 

 in an entirely different sense, namely as meaning atavism, or the 

 recurrence in a species of some more or less remote ancestral form. 



