of ike Fishery Board for Scotland. 



365 



and the termination of the vertebral column, continuing what had only 

 begun in the previous sub-families, has assumed an axial form and 

 become diphycercal. The distribution of this sub-family is similar to 

 that of the tropical forms of the two preceding sub-families, and it is 

 difficult to say from which of these two the Cynoglossinae have been 

 derived, probably from both. 



Thus far it has been possible to pass over in review the various 

 groups of the flat-fishes which are found between the Arctic Seas and 

 the tropics. It has been assumed that their primitive home was in 

 the former regions, and that the various changes in structure which 

 present themselves as we pass from the one end to the other are 

 specialisations or adaptations to special habits and habitats. Since the 

 series from the least to the most specialised is fairly well complete, we 

 might think that all the forms that could occur must lie somewhere 

 within the groups which have already been described. In reality, how- 

 ever, we may have little more than half of the story, and the few forms 

 yet known from the temperate regions of the southern hemisphere 

 make this very probable. Passing over the aberrant Lepidopsetta 

 (Ifancopsetta) of South America, which is at present an isolated " fact," 

 we have the Solei-Pleuronectinse of the southern Australasian fauna, 

 which display some remarkable transitional characters. They have 

 been recognised by Jordan, even from the external characters, as having 

 certain resemblances to the Pleuronectinse, and this resemblance is still 

 further increased by the fact that they have the same type of olfactory 

 organ. In other characters — structure of the ventral fins and of the 

 abdominal vertebrae — they resemble Platophrys of the Turbot-group and 

 the Achirince amongst the Soles. 



One may readily understand how the asymmetry of their ventral fins 

 may have arisen, on the principle aforementioned that this asymmetry 

 is chiefly a persistent inheritance from a tropical ancestor, but their 

 having the type of olfactory organ which is found only in the temperate 

 and Arctic regions of the northern hemisphere is harder to explain. 

 There seem to be but two possibilities — either that some of the 

 species which have made their way from the northern hemisphere 

 through the tropics have retained this form of olfactory organ through- 

 out, or that the olfactory organ has again changed in the temperate 

 regions of the southern hemisphere from the rosette-shaped to the 

 lamellar form, just as it changed in the reverse direction in the northern 

 hemisphere.* Sufficient knowledge has not yet been gained to enable 

 one to say definitely which of these possibilities is the correct one. On 

 the former we should expect to find intermediate forms which have the 

 lamellar form of olfactory organ, in the regions between Australia and 

 the China Sea. If these are not to be found, we should be inclined to 

 believe that these forms show a specialisation away from the Sole-type ; 

 i.e., in an environment similar to that of the temperate regions of the 

 northern hemisphere, that the Sole type is gradually changing to the 

 Pleuronectid. In such a case — and Brachypleura of the same regions 

 is another example of the same process — we are led to think of Murray's 

 hypothesis of the bipolar origin of the oceanic fauna (41), and its more 

 recent discussion by Herdman (24) and Thompson (53). If it is possible 

 for the structures of the flat-fishes so to alter as they pass from the 

 tropics to the Antarctic circle that they reassume in many ways a similar 

 appearance and similar structures to those in similar regions of the 

 northern hemisphere, then it is not difficult to see how a false appear- 

 ance of identity may arise between the forms of the Arctic and 

 Antarctic seas. 



* See footnote p. 361. 



