224 Part III. — Eighteenth Annual Report 



Table VII. 



Averages for abdominal (MJ and caudal (M 2 ) vertebrce ; M 2 .P.F., probable 

 fluctuation of M 2 . 



Region. 



ISO. or 

 Specimens. 



Mi- 



M 2 . 



M 2 .P.F. 



Heligoland, 



308 



12-91 







30-12 



29-95 —30-29 



Helder, 



200 



12-91 



29.995 



29-845—30-145 



Lowestoft, .... 



170 



12-94 



30-106 



29-936—30-276 



Grimsby, .... 



120 



12-94 



30-07 



29-87 -30-27 



Aberdeen, .... 



254 



12-96 



30-03 



29-905—30-155 



Sol way Firth, .... 



197 



12-90 



29-95 



29-81 —30-09 



Baltic, 



47 



12-96 



29-47 



29-07 -29-87 



What has been said concerning the specimens from Helder applies 

 with slightly greater force to those from the Solway Firth. The average 

 is certainly smaller than that of the North Sea, and in such an important 

 character as the vertebrae such a difference has considerable importance. 

 It will be necessary to wait, however, until other characters have been 

 examined before drawing definite conclusions. 



For the specimens from the Baltic there can be little doubt of the 

 great difference in the average. Although the fluctuations overlap 

 with some of the other groups, the small number of specimens, 

 as well as the large standard deviation, tend to show their 

 distinctness from the North Sea groups. If the number of specimens 

 were increased with a consequent diminution of the standard deviation 

 the fluctuations would cease to overlap. As it is, the total fluctuation is 

 •9, whilst the greatest in the other groups is '36. Again, Duncker (I.e.) 

 found that the average for the Baltic was 42 '3, which, if allowance be 

 made for the comparatively small number of specimens in both cases, 

 shows that the average is practically constant. He also gives the averages 

 for the Kattegat as 43 0 and for Heligoland as 42*9, which shows that 

 these groups are also fairly constant. 



It is fair to conclude, therefore, that the average number of vertebrae 

 remains the same for the whole North Sea and the Kattegat, that the 

 Solway Firth group is perhaps different in this respect, and that the 

 Baltic group is certainly different. 



These differences lead to the consideration of what is known concerning 

 the changes in vertebrge. Heincke shows that for the herring there is a 

 difference between the groups of summer- and winter-spawners in regard 

 to this character, the former having a smaller number of vertebrae on the 

 average than the latter. Again, the American writers have shown in 

 detail what Gunther and others had previously remarked, that in each 

 of the chief groups of flat-fishes the numbers of vertebrae decrease as we 

 pass from the northerly to the southerly species. These remarkable facts 

 would seem to show that temperature has something to do with the num- 

 ber of vertebrae. In the plaice it has been mentioned that there is no 

 such extended spawning-period in the North Sea as there is for the 

 herring. Further, it is probable that the spawning-period in the northerly 

 North Sea is at its maximum a little later than that of the southerly 

 North Sea. Hence, the difference in the temperature between these two 



