194 CEYLON PEARL OYSTER REPORT. 



2 '4 millims.. length of genital segment l'i)f> inillims., length of abdomen <V92 milliin., 

 length of egg-sacs 4'2 millims. 



SUB-FAMILY: TREEING. 

 Trebius exilis,* n. sp. Plate II., figs. 20 to 33. 



Female. Carapace ovate, one-seventh longer than wide, contracted anteriorly, and 

 well arched. Transverse grooves separating the cephalic and thoracic portions of the 

 lateral areas situated far forward, leaving the thoracic portion much the longer of the 

 two. Eyes small, purplish red, and some little distance from the anterior margin. 

 Frontal plates better developed than either T. caudatus or T. tenuifurcatus, but still 

 less than half the width of the carapace. 



Third thorax segment only a trifle wider than the fourth and considerably shorter. 

 It projects backward, however, nearly its whole length beyond the lateral lobes of the 

 carapace, just as the thoracic area does in some of the Caliginse (Caligus rapax, 

 C. rnjimaculatus, &c.). Fourth segment considerably longer than the third, and 

 widened through the bases of the fourth legs more than in either of the other 

 two species, giving it a spindle shape. 



Genital segment almost a perfect ellipse, the only deviation being anteriorly, where 

 it is contracted into a short and narrow neck before joining the fourth segment. It 

 is more than three-fifths the size of the carapace and shows no spines or processes 

 at the posterior corners. The egg-strings are about the same width as the abdomen, 

 but are from two and a half to three times its length, thus contrasting sharply with 

 those of T. caudatus which are but a trifle longer than the abdomen. The eggs are 

 of medium thickness, 40 to 50 in each string. 



The abdomen, even including the anal laminae, is one-half shorter than the genital 

 segment instead of one-half longer as in T. caudatus. It is also made up of a single 

 joint and is of the same diameter throughout. The anal laminae are elongate, more 

 than twice as long as wide, well separated at the base, but convergent toward the 

 tips, where each carries four good-sized plumose setae. As in T. caudatus, the outer 

 seta is the shortest, the inner one next in length, while the two middle ones are 

 considerably longer. 



Of the appendages the first antennae are relatively much longer than in T. caudatus, 

 the basal joint is stouter and more heavily armed with plumose setae, while the 

 terminal joint is slender, not enlarged at the tip, and stands out prominently. The 

 second antennae are large and stout ; the terminal claw is wider at the base than in 

 T. caudatus and is relatively as long. But the abrupt bend is at the centre instead 

 of near the tip, and this makes the claw appear shorter. There is also a long and 

 slender hair on the inner margin of the claw near its base (fig. 22). 



* Exilic, slender, beautiful. 



