364 



Part III. — Eighteenth Annual Report 



Thus far we have traced one of the main lines of descent from the 

 Halibut-group from the Arctic Seas to the tropics. There is still 

 another main line, but before proceeding to its discussion it is necessary 

 to refer to a sub-branch or offshoot which has sprung from the 

 Hippoglosso-bothinse. This, the Turbot-group, has the essential 

 characteristics of the former, and has varied from it through the eyes 

 becoming definitely fixed on the left side, and through the ventral fin of 

 the eyed side, or both fins, developing forwards anterior to the base of 

 the clavicles. This group belongs almost exclusively to the northern 

 hemisphere, the various genera having migrated northwards from the 

 sub-temperate and tropical regions — the original home of the group. 

 The northern forms, the Turbot, Whiff, and Topknots, show that they 

 have become specialised, so that if we wish to retrace their aiiinities we 

 must return through the Brill to Arnoglossus, and thence to Citharus. 

 It is of interest to note also that whilst the Hippoglosso-bothinse have 

 remained predominant in the American fauna, and the Turbot-group is 

 barely represented there, the reverse is the case in the European fauna. 

 The Hippoglosso-bothinse are represented by only one form, and the 

 Turbot group is predominant. 



The second main line of descent from the Halibut-group passes down- 

 ward through the Plaice-group to the Soles. This at least is one of the 

 sources from which the Soles have sprung, and it is represented mostly 

 by the European Soleinse. In these the ventral fins are symmetrical, 

 and the eyes are on the right side as in the Pleuronectinse. The 

 specialisation within this sub-family displays itself in the gradual 

 decrease in size of the pectoral fins {Microchirus), and the disappearance 

 of the pectoral of the left or eyeless side (Monochirus) . Accompanying 

 these external changes is an internal change in the structure of the 

 abdominal vertebra?, which is found also in the transitional forms of the 

 Hippoglosso-rhombina^ a nd Ithombinse. The posterior abdominal vertebrae 

 bear haemapophyses,a nd the firct interhsemal spine decreases greatly in size. 

 In these respects, therefore, as well as in the form of the olfactory organ, 

 the Soleinse display affinities to the Citharus and Turbot-groups, whilst in 

 the structure of the mouth, position of the eyes, and symmetry of the 

 ventral fins they are more nearly related to the Plaice-group. It is thus 

 impossible to draw a hard and fast line between the separate groups, 

 although the probability is in favour of this group of Soles having sprung 

 from the Plaice-group. The similarity of the olfactory organ and of the 

 abdominal vertebrae in Citharus, Platophrys, and the Soleinae is the 

 sign of a certain amount of similarity in the habits and surroundings of 

 these genera, whilst the other characters seem to be the persistent 

 inheritance from the more northern Plaice-group. If this were not the 

 case, we should expect that these characters — the position of the eyes 

 and of the ventrals— would also have changed. The forms which are 

 allied to the plaice group are distributed chiefly in the tropical and 

 sub-temperate zones, in the Indian Ocean (Synaptura), and Mediter- 

 ranean, but a few species range as far north as the Scandinavian coasts 

 (Solea vulgaris). 



On the other hand, the American Soles seem to have sprung from the 

 Hippoglosso-rhombinae. The ventral fins are here asymmetrical, and the 

 eyes are on the left side. They do not seem to range to the northward 

 as the European Soles do, but towards the south into the tropical 

 regions. As in the latter sub-fami]y, the pectorals in the Achirinse 

 (American Soles) gradually degenerate until they finally disappear. 



The specialisation of structures which is found in the Soleinae and 

 Achirinse reaches the culminating point in the Cynoglossinse. The 

 pectorals and one ventral disappear, the eyes have become very small, 



