678 PROCEEDINGS OF THE NATIONAL MUSEUM vol. xxxi. 



ANALYTICAL KEY TO SPECIES. 



1. Abdomen distinctly shorter than the genital segment; fiirca three or four times as 

 long as wide exilis Wilson, 1906, p. 678. 



1. Abdomen much longer than the genital segment; furca only twice as long as 

 wide 2 



2. Carapace wider than long, semilunar; furca with slender elongate branches and 

 small foramen; abdomen of female two-jointed, joints equal. 



tenuifurcatuf! Rathbun, 1887, p. 679. 



2. Carapace longer than wide, elliptical; furca with short, stout branches and very 

 large foramen; abdomen of female three-jointed, joints diminishing in size 

 toward the tip caudatus Kroyer, 1838, p. 681 



TREBIUS EXILIS Wilson. 



Plate XV, figs. 1 to 7; figs. 2 and 3, pp. 675 and 676. 



Trebius exilis Wilsox, 1906, p. 194, pi. ii, figs. 20-33. 



Female. — Carapace ovate, one-seventh longer than wide, narrowed 

 anteriorly, and well arched. Transverse grooves separating- the 

 cephalic and thoracic portions of the lateral areas situated far for- 

 ward, leaving the thoracic portion much the longer of the two. Eyes 

 small, purplish red, and one-third the distance from the anterior mar- 

 gin. Frontal, plates better developed than in either of the following 

 species, but still less than half the width of the carapace. Third 

 thorax segment but a little wider than the fourth and considerably 

 shorter; fourth segment strongly widened between the bases of the 

 fourth legs. Genital segment almost a perfect ellipse, but contracted 

 anteriorl}^ into a narrow neck where it joins the fourth segment. It 

 is more than three-fifths the size of the carapace, and shows neither 

 spines nor legs at the posterior corners in dorsal view. 



Egg strings about the same width as the abdomen, but from two 

 and a half to three times its length, thus contrasting sharply with 

 those of caudatus. Eggs of medium thickness, -iO to 50 in each string. 



Abdomen, even including the anal laminae, at least one-half shorter 

 than the genital segment; made up of a single joint, and of the same 

 diameter throughout. Anal laminaB elongate, more than twice as long 

 as wide, each armed with 4 long plumose setae. 



Second antennse large and stout; the terminal claw wider at the base 

 than in caudattis and relatively as long. But the abrupt bend is at the 

 center instead of near the tip, and this makes the claw appear shorter. 



First maxillffi straight, small, and weak, the tip not much longer 

 than the enlarged base, and the whole appendage fused to the ventral 

 surface of the carapace. Second pair two-jointed, the basal joint fused 

 to the carapace and carrying at its center near the terminal joint a 

 good-sized rudimentar}" exopod. The terminal joint (endopod) elongate- 

 triangular and extending for half its length beyond the tip of the 

 mouth tube. This endopod is bluntl}" pointed without an}' trace of 



