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THE AMERICAN NATURALIST [Vol. XLV1 



essentially, but not exactly, as were tbose of the type 

 which I had already studied. I was much pleased to thus 

 substantiate, at least in a general way, on this American 

 material, Rousselet 's judgment on the distinction between 

 the trophi of these two species. I will add that a score 

 of culture experiments started with single individuals 

 of the two types fully confirmed their distinctness. De- 

 spite their very close resemblance, I reared from one set 

 of the delicate saccates the humped amphora type with 

 which I was so familiar; while parallel cultures, with 

 identical conditions as to food and temperature, pro 

 duced no modification in the A. It right irelli other than a 

 slight increase in size. 



I therefore reached the conclusion that, delicate as are 

 the differences which separate the saccate form of A. 

 amphora from the invariably saccate A. brightwelli, they 

 were none the less sharply demarcated. Ignoring other 

 features, it seemed perfectly safe to trust the one char- 

 acter of the absence or presence of the larger inner tooth 

 on the trophi. 



Imagine my surprise, then, when, upon visiting an 

 entirely different locality— Custer County, South Dakota 

 — I discovered an Asplanchna in countless numbers 

 which completely upset this distinction and introduced 

 me to a seemingly new type of variation within the genus. 



It was in the charming little mountain lake (or rather 

 reservoir, for the original site contained a mere pool 

 which has now been increased to a depth of 80 feet by an 

 artificial dam) called Sylvan Lake, that I came upon the 

 rotifer in question. The lake was indeed swarming with 

 rotifers of different species, which constituted the 

 majority of its plankton. Monarch of them all, and 

 profiting greatly by its superior size and ingesting power, 

 was a superb Asplanchna, Aside from a slight excess 

 in size every outward character indicated A. brightwelli 

 Moreover, I had found the species in the very nick of 

 time, for both males and resting eggs were copiously 

 present. Among very large numbers of these which I 



