North American Centropagidce. 227 



ing character in the Copepoda. If this be taken as a basis 

 of classification here, Osphranticum, with its 2 3 -segmented 

 antennae, would form a group by itself, while Limnocalanus, 

 Diaptomus, and Epischura, with their 2 5 -segmented antennae 

 would constitute another group. 



Osphranticum seems to be the most primitive of the Amer- 

 ican Centropagidce, the fifth legs especially being less differ- 

 entiated than in any of the other genera. This is particularly 

 true of the female, in which all of the legs are biramose, each 

 ramus consisting of three segments. In the male the left 

 fifth leg is similar to the preceding legs, but the right one has 

 a two-segmented outer ramus, the second segment being ap- 

 parently formed by the coalescence of the second and third 

 segments. In both sexes the inner rami of all the pairs of 

 legs are alike. 



Limnocalanus apparently approaches most closely to Os- 

 phranticum, the fifth pair of legs of the female being very 

 similar in general structure to those of Osphranticum, as is 

 perhaps most strikingly illustrated in the case of L. macru- 

 rits Sars and . labronectum Forbes. In Limnocalanus both 

 rami of the four anterior pairs of legs in both sexes are three- 

 segmented and but slightly modified, as are also those of the 

 fifth pair of legs of the female; but in the latter the second 

 segment of the outer ramus is produced on the inner margin 

 into a hook-like process, as in Osphranticum. The inner 

 rami of the fifth pair of legs of the male are still three- 

 segmented and similar to those of the preceding legs, but the 

 outer rami are modified and are two-, or indistinctly three-, 

 segmented. 



Diaptomus is perhaps next in respect to modification. 

 In this genus all the legs are biramose, but the first pair con- 

 sists of a three-segmented outer, and a two-segmented inner, 

 ramus. The following three pairs have both rami three-seg- 

 mented. In the female the fifth pair of legs has a two- or, 

 more rarely, three-segmented outer ramus, and a one- or, 

 occasionally, two-segmented inner ramus. In the male the 

 inner ramus is one- or, rarely, two- segmented, while the outer 

 ramus of the right leg is three-segmented and that of the left 

 leg two- segmented. 



