

468 



MERISTIC VARIATION. 



[part I. 



(Such a development of tubercles 1 on the lower side may however occur 

 without any correlated change of colour.) It is also stated that in 

 the "double" tui'bots the muscles of the lower side are thicker than 

 they normally are, thus approximating to the upper side, a feature 

 that may be taken as an indication that the manner of swimming is 

 different from that of normals. 



A flat-fish having pigmentation on the lower side does not 

 necessarily present any other abnormality 2 . The Plaice, for in- 

 stance, just mentioned, was, colour apart, quite normal. But 

 some specimens of fiat-fishes darkly coloured below present in 

 addition a very singular structural variation. This consists essen- 

 tially in the presence of a notch of greater or less depth occurring 

 below the anterior end of the dorsal fin above the eye (Fig. 152). By 

 this cleft the anterior end of the dorsal fin is separated from the 

 back of the head and is borne on a process or horn project- 

 ing anteriorly so as to continue the contour of the body above the 



Fig. 152. Head of a Brill (Rhombus lavis) having the dorsal fin separated from 

 the head as described in the text. (From Yarrell.) 



1 The literature relating to discontinuous variations consisting in the presence 

 of bony tubercles upon the blind side of Rhombi is extensive. See especially 

 Demidoff, Voy. dans la Russie Merid., 1840, in. p. 534, Pis. 28, 29 and 30. 

 Steindachner, Sitzb. Ak. Wiss. Wien, 1868, lvii. (1), p. 714. Rathke, Mem. Ac. 

 Sci. Pet., 1837, in. p. 349. Gunther, Cat. Brit. Mus. Fishes, iv. p. 409. These 

 cases will not be confounded with those of supposed hybrids between R. maximus 

 and R. Icevis, which bear upon both sides scales of various sizes. 



- I know no detailed description of a flat-fish wholly pigmented on the underside, 

 having the dorsal fin normal, but numerous authors (Gottsche, Duhamel, &c.) make 

 mention of such cases. Since this chapter was written I have seen two recent papers 

 on the subject by Giard (Comptes rend. Sue. Hiol, 1892, S. 9, iv. p. 31 and Nat. Sci., 

 1893, p. 356) contributing further evidence on the subject and giving new cases in 

 the Turbot. According to Giard, of flounders (P.flesus) at Wimereux 3 °/ are fully 

 coloured on the blind side, in addition to many that are piebald. This must be a 

 very much higher proportion of abnormal specimens than is found in English 

 fisheries. 





