III. 



The Evolution of Fiat-Fish. 



IN the May number of Natural Science, Mr. J. T. Cunningham 

 criticised a prehminary note of mine in the Societe de Biologic 

 (i6 Jan., 1892), deahng with a curious monstrosity in a turbot found 

 at Wimereux. In this note, which my critic appears to have 

 imperfectly read, I have carefully distinguished those monstrous 

 Pleuronectids which show arrested development (without stoppage 

 in growth) from the double flat-fishes whose existence Cunningham 

 says is recorded in almost every ichthyological work of any 

 importance. 



The former are sufficiently rare to have been recorded carefully by 

 the authors. Pleuvonectes cyclops of Donovan is probably the first example 

 cited. Couch speaks of a similar case among the turbots (Rhombus 

 niaximus). Macintosh and, more recently, Filhol have also met with 

 examples, while the case noted by Yarrell relates to a Brill (Rhombtis 

 IcBvis). The same monstrosity, according to Couch, is not very rare 

 among Zeugopterus ptmctatus, and we know that Zeugopterus differs but 

 little from Rhombus. It has also been observed among the flounders, 

 whose development is as slow as that of the turbot. Higgins records, 

 under the name of Pleuvonectes melanogaster, a flounder notched above 

 the eyes, and of a very dark brown colour, almost black, on both 

 sides. Newman, Thompson, and Ritzema Bos have noticed the 

 same malformation in individuals of this species. These monstrous 

 specimens differ from the normal not only by the colouring of the 

 blind side, but also by the notch in the dorsal fin, by the much 

 greater thickness of the muscles on this side, which changes the 

 form of the vertical section of the fish, by the existence in the turbot 

 of tubercles like those on the upper side, and by other anatomical 

 characters to which I shall allude later, in a special paper. 



I have stated that these monstrosities, by arrest of development, 

 are more frequently to be found among Rhombus than among the other 

 Pleuronectids. I ought rather to have said among Rhombus and the 

 flounders, or in a more general way, as I have indeed indicated, among 

 the Pleuronectidae of gradual metamorphosis (palingenetic develop- 

 ment). The accuracy of this assertion is not only verified by the authors 

 quoted, but also by the fact that since I drew the attention of the 



