28 PROCEEDINGS OF THE NATIONAL MUSEUM vol.82 



An attempt has been made herein to provide a synopsis that would 

 make it possible to separate the species without necessarily referring 

 to the original descriptions. 



The treatment of Pseudodia'ptoinus in the present paper must be 

 recognized as a preliminary study. Much of the published material 

 is very imperfect and possibly contains many mistakes. Thirteen 

 of the species have been described only once. Pseudodiaptomus 

 pelagicus Herrick, which is the type of the genus and was described 

 in 1884, has not been seen since that time; inasmuch as Herrick's 

 figures were poor and his description brief, it leaves a little doubt 

 as to the generic characteristics, to say nothing of the specific 

 distinctions. 



Unfortunately, the writer has had material of only part of the 

 species. It is very desirable that the genus should be monographed 

 after a thorough study of material of all the species. As this is not 

 likely to be done for a long time, this preliminary synopsis may be of 

 value. 



Description. — ^Last two thoracic segments coalescent. The female 

 abdomen is composed of three or four segments; in most cases of 

 four. The first antennae are composed of 20 to 22 segments; the 

 terminal portion of the right antenna of the male is composed of two 

 segments, three in P. liichviard. 



The exopod of the second antenna is composed, according to 

 various authors, of two to seven segments. Herrick, in his original 

 figure, shows three. Most authors state four. The structure is seen 

 in Plate 15, Figure 5; Plate 16, Figure 4; Plate 17, Figure 3; and 

 Plate 18, Figure 3; the first segment is short, the second long, 

 followed hj two or more imperfectly separated segments and then a 

 segment about equal in length to the second. It is these short central 

 segments that have caused the differences of opinion. Herrick does 

 not recognize them at all. Cleve figures four in aurivilli^ making a 

 total of seven. Apparently in coronatios there may be distinguished 

 more or less clearly three segments. The first four swimming feet 

 are biramose, each ramus consisting of three segments. 



The fifth feet of the female have no endopods; in P. hessei and 

 P. stuMmanni there are swellings of the inner margin of the second 

 segment of the basipod which, perhaps, are indications of a rudi- 

 mentary endopod. The exopod consists of two segments ; Giesbrecht 

 and Schmeil (1898) give "two or three segments." There is, in some 

 species, an apparent separation of a third segment, but a study of the 

 genus indicates that this is a modification of the form of the terminal 

 hook. The typical armature of the terminal segment is a hook with 

 a spine branching from its base and an inner and outer spine. These 

 spines vary in length and form. 



