196 NATURAL SCIENCE. „^^_ 



Proceedings of the Societe de Biologic. Giard has considered the 

 evolution of the Pleuronectidae before, although he has not made a 

 special study of their anatomy or development. Years ago he 

 suggested a new theory in explanation of their origin, to which he 

 refers in this note. His familiarity with the problem, as well as 

 his studies in the doctrine of evolution generally, caused me to 

 expect to find his remarks both sound and suggestive ; but I must 

 confess to disappointment. The immediate occasion of this note 

 was the capture of what is commonly known as a double turbot. 

 The right eye in this specimen was on the edge of the head, 

 so that the dorsal fin, instead of extending as it usually does to the 

 anterior part of the head, was separated therefrom by a concavity. 

 The right side was pigmented all over like the left, but the colour was 

 a little less intense. The specimen was ig cm. long. 



In almost every ichthyological work of any importance the occur- 

 rence of double flat-fishes having the characters just mentioned is des- 

 cribed. Yet Giard proceeds to remark that the relative frequency in 

 Rhombus maximus of this monstrosity, which has not been stated to occur 

 in other Pleuronectids, is due without doubt to the fact that, as Day 

 and Mcintosh have pointed out, the young turbot swims in a vertical 

 position for a longer time than the other flat-fishes. It is perfectly 

 well known that double specimens occur in many species of flat-fish. 

 They are as common in the flounder as in any species, and a specimen 

 of this species is described and figured by Traquair in his memoir 

 published by the Linnean Society in 1865. I have in my possession 

 adult specimens both of the flounder and the plaice which exhibit 

 this monstrosity. Day himself, to whom Giard refers, states in his 

 " British Fishes " that double specimens have been observed in the 

 flounder, turbot, plaice, soles, &c. Ritzema Bos records a case in 

 the flounder in the Biologisches Centvalhlatt of July, 1886. There is, 

 therefore, no evidence whatever that the monstrosity in question is 

 more frequent in the turbot than in other species. 



After describing his abnormal specimen, Giard remarks : 

 " L'animal devait nager en se tenant verticalement et se poser 

 rarement sur le cote droit." // devait nager, the fish must have swum 

 in a vertical position, and must have rested rarely on its right side. 

 The question is, did it swim vertically and lie on its side rarely ? 

 Day states that these abnormal double specimens are seen to swim 

 vertically, and to be more frequently found near the surface of the 

 water than those which progress in the usual manner. He gives no 

 actual observations in support of this statement. I have for some 

 time past been endeavouring to obtain evidence on this particular 

 point. Apart from observations on living adult double specimens, 

 I have good ground for believing that they do not swim vertically, 

 for I have observed living young flat-fishes many times during the 

 period of their metamorphosis, and I know that they never swim 

 vertically after the moment when the eyes have ceased to be perfectly 



