56 MINOR ABNORMALITIES AMBICOLORATION 



(c) THE SO-CALLED "TAILLESS" TROUT. 



Eeference is made to this condition by Couch (4.5), Day (54 II. p. 102), Stoddart 1 (239), 

 Peach (185), and Traquair (351-2). The latter gives a full account of the general appearance and 

 the structure of the fins affected, as well as a discussion of the probable causes, and his papers may 

 with advantage be consulted for fuller details. In these trout, neither tail nor tail-fin is awanting, 

 but the rays of the latter are abnormally shortened and thickened at their extremities, which tend 

 to be bent upwards or downwards towards a median posterior angular projection. In the specimens 

 described by Traquair from Islay and Kirkcudbrightshire, the extremities of the rays, besides showing 

 a tendency to coalesce, were deficient as regards dichotomisation and also as regards the number of 

 their transverse joints. Specimens from the Eiver Carron showed the same general features, except 

 that the amount of dichotomisation at the ends of the fin rays was not reduced. 



It is important to note that the caudals are not the only fins affected. The anals usually 

 share in the malformation, the pectorals often, and more rarely the pelvics. The dorsal fins seem 

 never to suffer. 



The causation is unknown. Traquair (254) gives an excellent analysis of the available 

 evidence on the subject. 



ABNORMALITIES OF COLORATION. 



(a) In Fishes generally. 



Ordinary variations and abnormalities of coloration hardly fall within the scope of this work. 

 The larger systematic books on fishes contain mention of such variations in numerous species, and 

 reference may be made to the literature index (pp. xi-xvii) for the following papers or notices 

 dealing with the subject: Bateson (19 p. 466); Bellotti (21) ; Berg (22); Bird (24); Bolan (26); 

 Dean (55, albinism) ; Giinther (87) ; Kershaw (.?##); Ninni(.770); Panceri (17 9, albinism); Pettis 

 (191, albinism); Schneider (1880); Suomalainen (43); Toruier (849); Traquair (255); Trois 

 (257-60). Attention should also be called to the important work of Cunningham and MacMunn 

 (50a) on the coloration of the skins of fishes, and to that of Schondorff (221) and Wagner (Intern. 

 Rev. ffydrobiol. Leipzig 4 1911 pp. 1-33) on the normal pigmentation of the Salmonidae. 



(b) In flat Fishes. 



This group requires separate mention. To begin with, its members are subject to the same 

 minor colour variations as occur in other fishes, for example to albinism, usually partial in character 

 and patchy or local in distribution. 



Of much greater importance is the so-called ambicoloured condition, in which the whole or part 

 of the under side is more or less deeply pigmented. If not very pronounced or complete, the 

 condition may be unassociated with any other abnormality. Instances of the kind have been 

 recorded in Pleuronectes flesus, PI. oblongus, PL microcephalus, PI. j^tessa, PL limanda, Solea mdgaris, 

 Rhombus maxim-us, and R. laevis, while PL cynoglossus normally has many chromatophores (50 per 

 sq. cm. (Elmhirst 64)) on the under side. Bateson calls attention to the fact that in the better 

 marked examples of this and the succeeding sets of cases there is often correspondence in character 

 and position between the abnormal colour markings on the under side and the normal ones on 

 the upper side. This correspondence seems to depend primarily on the persistence and further 

 development of chromatophore groupings normally present in the larva. It does not accordingly 

 furnish a pure illustration of Homoeosis, i.e. the acquirement of secondary or " imitative " bilateral 

 symmetry (see Bateson, 17, 18), although this principle must be in operation in regard to minor and 

 specific details. 



Pronounced and widespread pigmentation of the under side, particularly when the head is 

 wholly or partly involved, is found to be associated : 



1 See footnote on p. 46. 



