Marsh — Limnetic Crustacea of Green Lake. 193 



ably caused by some differences in the development of food sup- 



There are only two species of Diaptomus found in Green Lake,. 

 — D. minutus and D. sicilis. In the counting no distinction 

 was made in regard to these species, but a slide was prepared 

 from each collection and examined under the compound micro- 

 scope and thus a rough idea obtained of the relative abundance 

 of the two forms. During Sept. and Oct. D. minutus was much 

 more abundant. In Sept. very few of D. sicilis were found. Dur- 

 ing October and November the relative number of D. sicilis in- 

 creases, and in the winter months the collections were almost 

 entirely of D. sicilis. 



In 1894 I first found D. sicilis in the collection of Sept. 28. 

 In 1895 it first appeared Oct. 5, and 1896 on Oct. 6. Although 

 I did not find this species in the summer months while I was 

 making my serial collections, I do not think that it was probably 

 entirely absent from the lake; for in 1890 and 1891 I found it in 

 summer collections, although I did not find it in 1892. (Marsh, 

 '93 p. 198.) I find, on looking over my notes of 1890 and 1891 

 that it was not numerous in those years, and I presume that it 

 occurs in the summer months, but only in very small numbers. 

 A reexamination of my notes on the Michigan copepods shows 

 that the same thing holds true there. In the collections made 

 by Professor Reighard in April, in Lake Michigan, D. sicilis 

 was always present, while in the summer collections in the 

 Great Lakes and Lake Michigan, D. minutus was the more com- 

 mon form, as I have already noted in my paper on Michigan 

 copepods, and D. sicilis occurs rather infrequently. In 

 April and May D. minutus is entirely lacking in Green Lake, 

 but appears again in June. 



Inasmuch as it is claimed by some that some copepods show a 

 seasonal dimorphism, one might raise the question whether we 

 did not here have a case of that kind. I do not think that 

 this is so, although I have not now material to fortify my be- 

 lief. 



The Diaptomi are found at all depths, but in the deeper strata 

 only in small numbers. There were very few hauls in which I 

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