OF THE ENGLISH LAKE DISTRICT. 
445 
specimen was found in Bassenthwaite in October 1921^ it is certainly not 
established there. 
It would be o£ interest to ascertain if there are any constant differences 
between the Bosminas of the different lakes, but this can only be done satis- 
factorily by'measurements of large numbers of specimens carried out on the 
system laid down by Burckhardt, and my material does not admit of such 
treatment, since the necessary number of well-preserved specimens is not 
available from all the lakes at the same seasons. Since there is very great 
variability within the population of a lake with regard to length of antenna 
and of mucro, the selection of single individuals for measurement or 
illustration is likely to be misleading, but it is not entirely without value, 
Table 10. — Measurements of single individuals of 
JSosmina obtusirosti'is. 
Date. 
Lengtli. 
Shell. 
Length, j Width. 
Length 
of 
antenna. 
Length 
of 
mucro. 
Windermere 
21. vii. 
•63 
743 784 
310 
81 
Esthwaite 
3. ix. 
•6 
726 ^ 71i' 
328 
96 
Uerweiitwater .... 
17. V. 
■64 
703 ' 750 
390 
93 
ITllswater 
1. vii. 
■65 
753 , 753 
430 
92 
Hawes Water .... 
11. vii. 
■6 
716 783 
333 
66 
Coniston 
0. vii. 
■68 
706 647 
404 
110 ! 
Ennerdale 
28. ix. 
■66 
712 788 
393 
45 ; 
Wastwater 
30. ix. 
■92 
717 706 
543 
152 
Cruuimook 
1-2. vii. 
■89 
707 752 
505 
123 
Flo litem Tarn . . . . 
27. ix. 
•52 
711 7II 
327 
38 
Goats Water 
3.x. 
■53 
754 717 
490 
19 
and a table of measurements of individuals chosen as being so far as could 
be judged typical of the population is here given. The example given from 
Esthwaite is above and that from Ennerdale below the average size, but 
typical of foim. This table shows that the Bosminas of the two primitive 
lai%es Grummock and Wastwater are not only very much larger than all 
others, but have longer mucro and antenna (text-fig. 3, D). On the other 
hand, those from Ennerdale, which might be expected to be similar to those 
from Wastwater, are, in fact, conspicuous for their very short antenna and 
mucro and are not particularly large. Herein is another strong point of 
difference between these two apparently similar lakes. In the remainder 
of the lakes the differences shown between the Bosminas are not, I think, 
outside the limits of their ordinary variation within a single population. 
The specimens from Floutern Tarn and Goats Water are not only the 
siniiHest, but have ihe shortest mucro. 
