1 882.] and Adjacent Waters. 641 



Claus, but from this the description on another page will serve 

 easily to distinguish it. 



This is the only Cyclops which I have yet noticed in Lake 

 Michigan, and is certainly far the most abundant species. 



Of the many species of Cladocera occurring here, I have 

 selected but three for especial comment. The first of these, Lcp- 

 todora hyalina Lillj.(Pl. ix Fig. 3), which occurs also in Europe, 

 is a most interesting creature. When in its native element it is 

 almost perfectly transparent, and consequently invisible— a true 

 microscopic ghost— a fact associated by Professor Weissmann 

 with its predaceous habit and feeble locomotive power. To the 

 little Cyclops host it must indeed be a dreadful and mysterious 

 enemy. Concealed by its transparency, it need not lurk in ob- 

 scure hiding places, like grosser robbers, but can wing its way 

 unnoticed among its prey. 



The common Daphnids of the lake are, however, almost 

 equally transparent, and as these are not at all carnivorous, we 

 must either suppose that they have developed independently the 

 same peculiarity for a directly opposite purpose— that of self-pro- 

 tection— or else we must conclude that there is something in the 

 conditions of life here which tends to render the bodies of all 

 entomostraca transparent. 



A single mutilated specimen of Leptodora was dredged by 

 Professor S. I. Smith in Lake Superior; it has been found in 

 both ends of Lake Michigan, and I have also collected it in the 

 Illinois river and the small lakes adjacent, and in a muddy pond in 

 Northern Illinois only half a mile across and twenty feet in depth. 



A careful comparison of my specimens with the descriptions 

 and figures of Lilljeborg and Weissmann, leaves no room for 

 doubt that they belong to the European species. 



This is likewise the case with the remarkable JHolopedium 

 gibberum Zaddach (PL ix Figs. 12-15) found as yet only in Grand 

 Traverse bay, where it occurred not rarely with Epischura, Diap- 

 tomus, Cyclops and Daphnia hyalina. In this animal the bivalve 

 shell has undergone a truly monstrous development, the brood 

 cavity on the back being elevated to a height greater, when filled 

 w ith young, than that of the remainder of the animal. On the 

 other "and, the lateral valves of the shell are so shortened that 

 they do not completely cover the branchial feet. For the protec- 

 t'on of the creature and its young, and partly also, according to 



