54 GEOL. AND NAT. HIST. SURVEY OF MINNESOTA. 



tion of the inner branches of the fifth feet in both sexes is especially 

 remarkable in view of the three-jointed inner rami of all the other 

 feet. The fifth foot in the male is exceedingly like that of Drepanojms^ 

 pedinatus Brady, while those of the female are on the same numerical 

 plan; in that genus, however, the male antenna is not geniculate and 

 the first foot has the inner ramus one jointed. The genus Encliwta is a 

 somewhat miscellaneous assemblage, with some species resembling 

 Fseiido dicq)tomus. In some respects Centropages is likewise similar. 



GENUS DIAPTOMUS Westwood. 1836. 



Ci/dopsina — Milne-Edwards. 

 Glaucea — Koch. 

 Poniie — Ouchakoft'. 



Cephalothorax composed of seven segments, of which the cephalic 

 two are more or less closely fused to form the head. Abdomen nar- 

 row and shorter than the thorax, with three obvious segments in the 

 female and four in the male, the last bearing two short stylets. Each 

 caudal stylet is armed with five sub equal plumose setre and one 

 smaller internal bristle. Antennae of the first pair long, 25 jointed; 

 in the male geniculate between the eighteenth and nineteenth joint 

 and variously thickened and armed, the penultimate segment often 

 provided with a curved hook. Second antennae (antennules) two- 

 branched; the exterior branch seven-jointed and bearing apically sev- 

 eral long setfe, inner ramus shorter and two jointed. The mandibles 

 armed with about nine acute teeth and provided with a hi- ramose 

 palp. The maxilke of the first pair are very short and armed with 

 closely set setee. The second maxilhe, or maxillipeds, are long and 

 directed cephalad, seven-jointed. The first pair of swimming feet has 

 a two-jointed inner ramus while each of the three pairs following has 

 both rami three jointed. The fifth pair of feet are dissimilar between 

 the sexes and furnish the most important specific characters. In the 

 female the two feet are alike and the inner ramus is reduced or rudi- 

 mentary, often one jointed. The external ramus develops a strong 

 claw from the penultimate joint, while the small apical segment bears 

 one or more small spines. The fifth feet of the male are dissimilar, 

 five jointed, the inner rami rudimentary, the right limb having a long 

 apical claw on the outer branch. 



The species are generally slender and graceful, the antennte often 

 exceeding the body in length. Some forms are brilliantly colored 

 while most are pellucid. Perhajs most of the species are found in 

 open lakes, but a number are restricted to swampy localities and 

 others are limited to early spring or late fall and stand in such close 

 relations to other species as to strongly suggest a sort of heterogenesi& 

 like that we have demonstrated in Cyclops. 



