182 Marsh—Limnetic Crustacea of Green Lake. 
species of Cyclops are brevispinosus , pulchellus and flumatilis; 
Epischura lacustris and Limnocalanus macrurus are commonly 
present, and Diaptomus is represented by D. sicilis and D. minu¬ 
tus: D. Ashlandi , is, so far as my observations go, confined 
to the Great Lakes and bodies of water in immediate connection 
with them. 
The distinction thus made in regard to the distribution of 
Diaptomus is not without exception by any means, and I think 
that m more northern lakes D. minutus is found more abundantly 
in shallow lakes than it is in the region that has been more es¬ 
pecially the subject of my studies. Inasmuch as minutus is 
found in great abundance in Greenland and Iceland, I presume 
that the real cause of its greater abundance in the deeper lakes 
of our latitude is not the depth of the water, but the low tem¬ 
perature which is coincident with depth. 
Iu general, we may say that depth rather than extent of sur¬ 
face controls the character of the crustacean fauna. This is 
strikingly shown in a comparison of Green Lake with Lake 
Winnebago. Lake Winnebago is situated about twenty-five 
miles from Green Lake, and is about twenty-eight miles long by 
eight to ten miles broad. Through its whole extent it is very 
shallow, being for the most part from ten to thirty feet in depth. 
Its crustacean fauna consists of those species characteristic of shal¬ 
low lakes, being very different from that of Green Lake. The same 
thing is noticed in comparing the fauna of Lake Mendota, as deter¬ 
mined by Professor Birge, with that of Green Lake, Mendota fall¬ 
ing distinctly into the class of shallow lakes. What depth may be 
considered as characterizing deep lakes, it is difficult to state 
with certainty, and I suppose it is doubtful if an exact limit 
can be fixed, but I think it is about forty meters. Lake Men¬ 
dota, according to the soundings of Professor Birge, has a max¬ 
imum depth of twenty-two meters. Lake Geneva is a little over 
forty meters in depth, and, judging from the collections of Pro¬ 
fessor Forbes, is somewhat intermediate in the character of its 
fauna between the shallow and deep lakes. Lake St. Clair is 
apparently an exception to this classification, as, although it is 
shallow, it has also the fauna of the deep lakes. This is easily 
explained, however, if we remember, as stated in my former re- 
