Acroperus leucocephalus. 
311 
spines. The eye is large, and the macula nigra is about one- 
hialf 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 
h>y 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 Ancliistropus is possibly 
a male Ghydorus is certainly incorrect. 
Acroperus leucocephalus, Koch. 
Acroperus angustatus, Sars. 
Both of these species appear in my collections, as do 
lorms 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. leucocephalus 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 G-reen lake and at Minocqua and other points in 
northern Wisconsin. All of the Massachusetts specimens are 
Typical A. leucocephalus and entirely resemble the European de¬ 
scriptions and figures. Specimens from northern Wisconsin are 
nearly typical A. angustatus. The form from G-reen lake ap¬ 
pears to have a higher cephalic crest than do those from any 
other locality. In no case do I find the antennae 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 
ungustatus and the antennary setae 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. 
