of the Fishery Board for Scotland 



225 



regions at the spawning-periods may be very small, and thus we can 

 understand the uniformity of the average number of vertebrae over the 

 North Sea. And not only the vertebrae but the fin-rays, whose num- 

 bers are correlated with those of the vertebrae, are uniform over the 

 whole North Sea, as will presently be shown. 



The temperature is not the only influence, however, that seems to 

 affect the vertebrae. The Baltic herring, like the plaice of that region, 

 have a smaller average-number of vertebrae than the North Sea herring. 

 Similarly, the herring from the Zuyder See resemble those of the Baltic 

 in this character, and it has been remarked that the plaice from this 

 region (Helder) are perhaps distinct from the North Sea. The average 

 number of vertebrae and fin-rays in dorsel and anal mis is somewhat 

 smaller than in those of the North Sea plaice. The difference between 

 these regions and the North Sea is probably more one of salinity than of 

 temperature. On the other hand, again, with the possible exception of 

 the lemon-sole, the flounder is the nearest ally to the plaice in North Sea 

 waters and has probably been derived from it. The essential difference 

 between them lies in the vertebrae and fin -rays, which are, on the average, 

 less numerous in the flounder than in the plaice. The young of the 

 flounder, as is well known,* enter the brackish waters at the stage when 

 the vertebrae and fin-rays are forming, and proceed thence into the fresh- 

 water streams. On the other side we have the lemon-sole, which has 

 apparently been derived from the plaice also, and differs from it mainly 

 in the greater number of vertebrae and fin-rays. The young of this form 

 are not found with those of the plaice and flounder, but seem to seek the 

 deeper waters, where the salinity is greater and the temperature probably 

 lower. The pole-dab (P. cynoglossus) shows a further advance in the 

 same direction. 



It appears, therefore, that salinity as well as temperature has been one 

 of the factors leading to the differentiation of these races and species. 

 Another factor has also to be taken into account, but the subject is too 

 large and complex to be discussed here, and a consideration of the facts 

 is postponed to a future period. 



Fin-rays. — In the two following Tables, IX. and X., are set forth the 

 averages of all the fins, the standard-deviations, and the fluctuations of 

 the averages. As the full discussion of all that is contained in these 

 Tables, as well as in those of the vertebrae, would lead to the considera- 

 tion of many fine points interesting by themselves but not connected 

 with the scope and object of the present, paper, attention will be directed 

 only to the general and obvious differences and resemblances. 



Considering, in the first place, the dorsal and anal fins, reference must 

 be made to the supposed differences which Cunningham found in these 

 characters in plaice from Plymouth and the North Sea. As already 

 mentioned, this observer did not calculate the averages of the observed 

 frequencies, much less the probability of truth in his results, but trusted 

 himself to general reasoning. The following Table gives the averages 

 for the number of fin-rays found by Cunningham in the plaice 

 for the different regions. The males and females are included together 

 for the reasons stated in the section on sex-variability. 



* Vide Kyle, H. M., Sixteenth Hep., Scot. Fish. Board, pp. 225-246. 



[Tablb. 



