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Part III. — Eighteenth Annual Report 



Geographical Distribution of the Sub-Families. 



It has at times been asked whether the flat-fishes could not be 

 considered as having sprung from more than one stock, that is, as having 

 two or more distinct sources of origin. The doubt concerning this point 

 is centred round the great differences between the Turbot-group and 

 the Plaice-or Halibut-groups. The Sole-group — in the European fauna 

 at any rate — is readily recognised to have close affinities with the Plaice- 

 group, whereas no other groups are so widely separated from one 

 another as the latter and the Turbot-group. Nor are these differences — 

 which have been displayed in the earlier portions of this paper — confined 

 to the adult structures, they are also found in the earliest stages. The 

 eggs of all the species of the Plaice- and Halibut-groups yet known are 

 transparent and without oil-globules like those of the Gadidae, whereas 

 all the species of the Turbot-group have, without exception up to the 

 present time, an oil-globule which may be coloured or colourless. The 

 larvse and post-larvae of the latter group, again, are readily distinguished 

 from those of the former by the very early anterior prolongation of the 

 dorsal fin over the migrating eye, and by the presence, in several species, 

 of spines on some part of the head, in other species of tentacular 

 prolongations of the anterior rays of the dorsal fin. 



These great differences throughout might lend countenance, therefore, 

 to the possibility of there being two separate origins for the flat-fishes. 

 But such an hypothesis could only obtain a firm ground in fact if it 

 were shown that no transitional stages existed between the Turbot and 

 Halibut-or Plaice -groups, and is at once disposed of if such transitional 

 stages are found. In the preceding pages it has been shown that the 

 Cithamis-gi'oup intervenes between the Halibut-and Turbot-groups, and 

 that the transitional forms are all present in the existing fauna. It 

 becomes necessary, then, to take a broad survey of the various characters 

 in order to display their gradual transition from the one group to the 

 other, and this can be most readily accomplished by following the 

 geographical distribution of the various sub-families. 



Beginning in the northern parts of the northern hemisphere, and 

 restricting attention mostly to the European forms, we find that the 

 Hippoglossinae are the sole representatives of the flat-fishes in the extreme 

 north. It is there that we find the nearest approximation to the round- 

 fishes in species which have the eye of the blind side on the ridge of the 

 head. Closely allied to these we find other species, e.g. the Halibut, 

 which have a broader or deeper body, and both eyes on one side of the 

 head. The chief characters of these, according to the foregoing classifica- 

 tion, are the large and nearly symmetrical mouth, the symmetrically- 

 placed ventral fins, the eyes on the right side of the head, and the 

 lamellar type of olfactory organ. As we approach nearer to the coasts 

 of northern Europe we find another form making its appearance, which 

 only differs essentially from the above in the shape of its mouth. This 

 second form — the Plaice-group — differs from the former in having a 

 smaller mouth, which is twisted towards the under side. A further 

 difference, slight in itself but of some significance, is the more anterior 

 position of the anterior extremity of the dorsal fin. 



The Halibut and Plaice-groups are found together in the northern 

 portions of the temperate region, the latter extending further south 

 than the former, but as we proceed towards the warmer regions of the 

 temperate zone both groups decrease in number to the vanishing point, 

 and we find their places taken by forms which have undergone some 

 remarkable changes. The asymmetry of the flat-fish, which in the above 

 groups was restricted to the bones round the eyes and mouth, now 



