Acroperus leucoceplialus. 311 



spines. The eye is large, and the macula nigra is about one- 

 naif as large. The process of the labrum is little developed. 

 The length of the animal is about 0.36 mm. The male was 

 not found. 



No species of this genus has been found in America before. 

 Herrick ('84 p. 118) speaks of a Chydorus-like form seen once 

 by him, which had several young in the brood-cavity. It could 

 therefore scarcely have belonged to this genus where there are 

 only two young. His suggestion that Anchistropus is possibly 

 a male C/iydorus is certainly incorrect. 



Acroperus leucocephalus, Koch. 



Acroperus angustatus, Sars. 



Both of these species appear in my collections, as do 

 forms in some respects intermediate between them. I have long 

 been puzzled by the species of Acroperus found at Madison, as 

 it seemed to approach A. leucocejihalus in some respects and 

 A. angustatus in others. In 1892 I received specimens of the 

 genus from Cambridge and Easthampton, Mass., and collected 

 them in Green lake and at Minocqua and other points in 

 northern Wisconsin. All of the Massachusetts specimens are 

 "typical A. leucoceplialus and entirely resemble the European de- 

 scriptions and figures. Specimens from northern Wisconsin are 

 nearly typical A. angustatus. The form from Green lake ap- 

 pears to have a higher cephalic crest than do those from any 

 other locality. In no case do I find the antennse quite as short 

 as is required by the description of Hellich ('77, p. 80.). 



In lake Gogebic and elsewhere both species occur together. 

 In such cases I have failed to find intermediate forms. 



In the ordinary form at Madison the dorsal line is somewhat 

 arched, the posterior angles are less prominent than in typical 

 angustatus and the antennary setse reach nearly to the posterior 

 edge of the valves. In all these particulars they approach 

 leucocephalus, while in general form and in most other respects 

 they resemble more nearly angustatus and should probably be 

 ranked with that species. 



