of the Fishery Board for Scotland. 



363 



passes backwards to the body, and both the pectoral and ventral fins 

 begin to degenerate. Those of the eyeless side grow smaller than those 

 of the eyed side, and in the end do not develop at all. As might be 

 imagined from their relative positions, the ventral fins display the 

 various stages in specialisation much better than the pectorals do, 

 although we can trace the specialisation along both lines in separate 

 groups. Following firstly the fate of the ventrals, we find that the 

 ventral fin of the eyed side pushes forward in front of its fellow until 

 it reaches and is joined on to the base of the clavicles. This forms a 

 well-marked stage limiting off the Citharus from the Turbot-group, and 

 is accompanied by other characters which display its differentiation from 

 the Halibut-group. Not only is the asymmetry of the Hippoglossus- 

 type increased, but the type itself seems to be wavering in the balance. 

 In the Halibut group the eyes are almost invariably on the right side,* 

 in the species of the Citharus-group the eyes may be either on the left or 

 right side, but most frequently on the left, f This peculiar phenomenon 

 seems to be connected with some changing element in the environment. 

 In the Flounder ( P.flesus ), for example, whose young migrate at an early 

 stage from salt to fresh water, and in which the eyes are normally on 

 the right side as in the other species of the Plaice-group, we find that 

 from six to ten per cent, of the individuals on certain grounds may have 

 their eyes on the left side. Why this change should occur in some 

 individuals and species and not in others is still a mystery, but 

 meanwhile we may note that it is a characteristic of the Hippoglosso- 

 bothinse. 



Another great change from the Halibut type, whose meaning and 

 importance are not yet clear, occurs in the olfactory organ. It is some- 

 what difficult to imagine any transitional stage between the lamellar 

 form of olfactory epithelium and the rosette-shape form, but we may 

 possibly yet find that both forms occur within the bounds of one of the 

 transitional genera, e.g. Paralichthys, which is the most nearly allied to 

 the Halibut -group. On the other hand, one might think that the 

 rosette-shaped form was of greater physiological importance than the 

 other, and we can thus understand its universal prevalence amongst the 

 forms of the sub-temperate and tropical seas, where the eyes have 

 degenerated and the sense of smell is the chief means of detecting food. 

 These are problems for the future, however ; at present it is only 

 possible to point out their significance. 



The Citharus- group appears to have sprung directly from the 

 flippoglossinse. In the European fauna we have only one representa- 

 tive of the group, namely Citharus (Hippoglossus of Cuvier, 11), but in 

 the American fauna the transitional stages are more in evidence, from 

 Paralichthys with the ventral fins as in the Halibut -group, through 

 PseudMrhombus, in which the ventrals are close to the base of the 

 clavicles, to Hemirhombus and Citharichthys, in which the ventral fins 

 have become slightly asymmetrical, but are still behind the clavicles as 

 in Citharus. With regard to distribution, also, the American forms 

 range further to the north than Citharus, overlapping the hippoglossoid- 

 forms. As a sub-family this group has the widest range of any. Its 

 altered hippoglossoid characters make it well adapted to the warm seas, 

 and it ranges all round the globe within the tropical and sub-temperate 

 zones of the southern apparently as well as of the northern hemisphere. 



* Jordan (33, p. 2615) cites Hippoglossoides elassodon as being sometimes sinistral. 



f The only writer, to my knowledge, who has examined any specimens of Citharus 

 itself with the eyes on the right side, is Linneus (35, p. 1233). There he describes his 

 genus Linguatula, which apparently applies to C. linguatula, as now known, as 

 Pleitronectes oculis a desctra. 

 Y 



