COPEPODS OF THE QENUS CLAUSIDIUM— WILSON. 429 



External specific characters of immature female. — General body 

 form elogate and slender, the exact reverse of that in the mature 

 adult. In the latter the total length is to the width of the cephalo- 

 thorax as 10 to 7, while in this immature female the proportion is 

 nearly as 3 to 1. The cephalothorax is elliptical, the length and 

 width being about the same; the prolongations at the posterior cor- 

 ners take more of a backward and less of a lateral direction. The 

 second and third segments are considerably narrower than the 

 cephalothorax, and are prolonged at their posterior corners similarly. 

 The dorsal shield on the fourth segment is strictly confined to that 

 segment and does not overlap even the fifth segment; it is about the 

 same width as that on the third segment. The fifth and genital 

 segments are uncovered and wholly visible in dorsal view. The 

 fifth segment is contracted anteriorly into a sort of neck and then 

 widened through the bases of the fifth legs. The gential segment is 

 trapezoidal in outline. The abdomen is three-jointed and three 

 times as long as wide; the anal laminae are slender, much longer 

 than wide, and each is armed with two short setae on the outer mar- 

 gin and two much larger ones at the tip, of which the inner is fully 

 twice the length of the outer. 



The first antennae are relatively longer than in the adult, and the 

 other appendages are about the same, except the swimming legs, 

 whose rami have only two joints instead of three. 



Total length, 0.90 mm. Width of carapace, 0.32 mm. 



External specific characters of male. — General body form about 

 halfway between those of the mature and immature females. It is 

 not as slender as the young female and much less thickset than the 

 mature adult. The cephalothorax is nearly orbicular, the same 

 width and length. The second and third segments are considerably 

 narrower and quite short, Their posterior corners and those of the 

 cephalothorax are prolonged backward; those of the third segment 

 reach well beyond the center of the fourth segment and are bluntly 

 rounded. The dorsal plate of the fourth segment is quadrangular, 

 three-fourths as long as wide, and overlapping the bases of the fifth 

 legs. The genital segment is also quadrangular and carries the 

 rudiments of a sixth pair of legs on its lateral margins at about the 

 center; at each posterior corner is a long spine. The abdomen is 

 three-jointed, the last joint irregularly divided. The anal laminae 

 are oblong and like those of the female, but each carries four setae, 

 three at the tip and one on the outer margin. The inner one at the 

 tip is the longest and is fully twice the length of the next in size. 

 The appendages are very similar to those of the female, the chief 

 differences being found in the maxillipeds and first legs, both of 

 which are used as prehensile organs. 



Total length, 0.67 mm. Width of cephalothorax, 0.30 mm. 



