106 Illinois State Laboratory of Natwral History. 



subequal. Second pair of maxillae short and thick ; third 

 pair elongate, directed forward, 7 -segmented, and provided 

 with short setae. The eight anterior swimming feet biramose, 

 the inner ramus of the first pair 2-, of the following pairs 3-seg- 

 mented. The last pair of feet different from the rest, 5-seg- 

 mented, the second segment armed within with a small 

 appendage or rudiment of an inner ramus ; in the female 

 short, equal, the last segment very short and rudimentary, 

 the penultimate always produced into a strong hook curved 

 inward ; in the male the right foot subchelate, the last article 

 formed into a very long movable hook. Eye single."* 



The following remarks on the genus are from de Guerne 

 and Kichard's "Revision " : 



"The genus Diaptomus, known at a very early date, was 

 for a long time confounded with Cyclops. Clearly distin- 

 guished much later, it contained for a very long time only a 

 few recognized species, and even these were insufficiently 

 denned. Since their study has been taken up with more 

 attention other forms have been distinguished, and the num- 

 ber of species now exceeds forty, and further explorations will 

 undoubtedly bring others to light.! 



" If we attempt to arrange characters in the order of their 

 importance from a systematic point of view, we must say 

 in the beginning that they are furnished almost wholly by 

 the males. Except in certain cases the isolated females are 

 difficult to determine. They are, however, rarely met with 

 alone, and collections commonly contain, whatever the season, 

 both sexes together. 



" Among the characters furnished by the male, the greatest 

 importance must be assigned to those of the fifth pair of feet. 

 The length of the inner rami, composed of one or two seg- 

 ments, varies considerably (minutus to castor). As for the 

 outer rami, the last article of the left foot should be examined 

 first. It sometimes has the form of a kind of forceps and 

 sometimes terminates in a sort of cushion bearing two short 

 obtuse spines, which perhaps represent the branches of the 



*Translated from the Latin diagnosis of de Guerne ami Richard's "Revision des 

 Calanides <1 I'.uu Douce. ' pp. 9 and Id. 



■jsixty-tivenow. many having been added since the publication of the '-Revision." 



