34 Illinois State Laboratory of Natural History. 



are deepest near the distal end and thence diminish in hoth 

 directions. The segment itself is deeply excavated within and 

 the plate merely completes its usual outline, while in ( '. 

 leuckarti the segment is but slightly excavated and the broad 

 plate projects far beyond the outline of the segment. 



In C. edax the last two segments are equal, while in 

 C. leuckarti the sixteenth segment is a fourth longer than 

 the last. 



The two pointed teeth of the connecting lamella of the 

 fourth feet, which are characteristic of C. leuckarti, may 

 sometimes be present in ('. edax, though they are in this 

 species not so sharp as in the other and are placed farther 

 apart. 



The transverse ridges of the outer maxillipeds, which 

 Schmeil describes as characteristic of C. leuckarti, are usually 

 absent in ('. edax and when present are quite inconspicuous. 

 They begin, as in C. leuckarti, at the proximal end of the seg- 

 ment, but soon fade away. 



The abdominal stylets in ( '. edax are more divergent than 

 in C. leuckarti, and are inserted farther apart. 



The fifth feet (PL IX., Fig. 3) are markedly different. In 

 C. edax the two seta- of the distal segment are parallel and 

 the surfaces to which they are attached are at right angles to 

 the long axis of the segment. In ('. leuckarti the surface of 

 attachment of the lower seta is at an angle of about forty-five 

 degrees to the long axis of the segment. Furthermore, the 

 whole distal segment is broader in ( '. edax. 



There are differences in general proportions and appear- 

 ances which make it very easy for one well acquainted with 

 these species to distinguish them at a glance, but these 

 differences can only be demonstrated by a long series of 

 measurements. The structure of the receptaculum scminis 

 is identical in the two species, so far as 1 can tell; but 

 though the species are undeniably closely related, I think 

 that on account of the above-mentioned differences they 

 should be kept distinct. 



Cyclops edax may possibly be the same as ( '. leeuwenhoekii 

 Hoek, which Schmeil has made synonymous with ( '. leuckarti. 



