430 MR. R. GURNBY ON THE CRUSTACEAN PLANKTON 
I possess a few specimens of L. grimaldii from the Caspian Sea which 
were given to me by the late Canon Norman some ye:irs ago, and by the 
kindness of Dr. C. Juday I have had the opportunity of examining 
L. macrurus from Green Lake, Lake Erie, and Lake Canandaigua (one of 
the Finger Lakes east of Lake Ontario). Dr. S. Ekman also has been good 
enough to send me specimens from Lakes Malaren and Lisjon in Sweden. 
Measurements of these races are given in Table G. 
So far as the numerical expression of the head-form goes, the Ennerdale 
race cannot be compared with any of the Baltic races, since in none of them 
is so great a head-height accompanied by so low a crown-level. The only 
race with similar head-height (Lake Mjosen) has a dorsal swelling rising 
above the horizontal dorsal line. On the other hand, the high, evenly- 
rounded dorsal outline rather closely resembles Ekman's figure of the race 
from Unden in Sweden, though that race has a crown-height according to 
his measurements of only — 6. It seems that in respect of this character the 
Ennerdale race stands somewhat apart, but that it must have been evolved 
from the f/rinialdii-type at a comparatively remote period. 
The North American races differ but little in head-form, and conform more 
closely to the Baltic type. Both have a posterior more or less horizontal part 
with an anterior descending slope, and agree very closely in appearance with 
the Malaren form, though the measurements do not agree very well with 
those given for that race (PI. 23. fig. 6). But, as compared with the Enner- 
dale race (PI. 23. fig. 1), specimens from North America certainly approach 
far more nearly to the grimaldii type, and are consequently to be regarded as 
of more recent origin. 
This conclusion is strengthened by a consideration of the other characters 
in which the two forms differ. 
(1) The true grimaldii form of the Arctic Ocean (examples from Jana 
River) measure over 3 mm., while the somewhat modified form of the Baltic 
may also attain 3 mm. (Grulf of Finland). It is, on the other hand, roughly 
true of the Baltic area that the oldest fresh-water races are the smallest 
(Sommen 1-87 mm., Unden 1'72 mm.). Small size is therefore to some 
extent evidence of prolonged sojourn in fresh water. In respect of this 
character the Ennerdale race (2"18 mm.) holds amove or less middle position. 
The race from Lake Erie, on the other hand, is one of the largest known 
(2'86 mm.), greatly exceeding any other fresh-water race and exceeded only 
by those of North Siberia and the Grulf of Finland in salt water. The; 
Green Lake form, though similar in structare, is but little larger than that 
of the Ennerdale race. 
(2) The antenna in the true grimaldii form reaches to the base of the furca 
or even further (PI. 23. fig. 5), whereas in typical L. macrurus it does not 
extend further than the first abdominal segment. In this respect the Enner- 
dale race belongs to the extreme fresh-water tj'pe, while those from North 
