182 Marsh — Limnetic Crustacea of Green Lake. 



species of Cyclops are brevispinosus, pulchellus and fluviatilis; 

 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 in more northern lakes D. mi?iutus 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. 



In 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- 



