396 



BULLETIN 15 8, UNITED STATES NATIOISTAL MUSEUM 



Two and often three eye lenses visible near the anterior margin of 

 the head ; first antennae 4-segmented in female, 5-segmented in male ; 

 rami of first three pairs of legs and exopod of fourth pair 3-seg- 

 mented, endopod of fourth pair 2-segmented; fifth legs short in 

 female, with an inner lobe and three apical setae, wholly lacking in 

 the male. 



CYMBASOMA RIGIDUM Thompson 



Figure 244 



Cymhasoma rigidum Thompson, Journ. Linn. See. London, vol. 20, p. 154, pi. 13, 

 1888.— Sars, Crustacea of Norway, vol. 8, p. 21, pis. 10, 11, 1921. 



.Occurrence. — Eeported by Fish as one of the summer forms at 

 Woods Hole blown in from the Gulf Stream during heavy storms. 



Distribution. — Atlantic, near Teneriffe (Thompson) ; Mediterra- 

 nean (Giesbrecht) ; French coast (Claparede) ; British seas (Thomp- 

 son, T. Scott) ; coast of Norway (Sars) ; North 

 Sea (Timm) ; Woods Hole (Fish). 



Color. — Body very transparent anteriorly, 

 but posteriorly yellowish gray, irregularly 

 tinged with dark brown. 



Female. — Cephalic segment dilated at the 

 midline, with convex lateral and ventral mar- 

 gins; three distinct optical lenses visible near 

 the frontal margin; free thoracic segments 

 tapered regularly backward. TJrosome less 

 than one-sixth as long as metasome, 2-seg- 

 mented, the anal segment notched laterally; 

 caudal rami longer than wide, divergent, each 

 armed with three setae. First antennae 

 scarcely one-third as long as cephalic segment, 

 4-segmented, the end segment nearly as long 

 as the other three combined. Fifth legs a little 

 longer than wide, the inner terminal seta much 

 shorter than the other two, which are about 

 equal, the inner margin produced into a nar- 

 row tongue-shaped lobe curved outward. Ova attached to the genital 

 filaments very numerous. Total length, 2.2-2.5 mm. 



Male. — Smaller and stouter than the female; urosome 3-segmented, 

 the anal segment notched laterally as in the female; copulative 

 appendage on the genital segment ending in two large divergent lobes 

 which are sausage-shaped; each caudal ramus with four subequal 

 setae. First antennae longer than in the female, the last segment 

 hinged and terminating in a slender spine ; fifth legs entirely lacking. 

 Total length, 1.5-1.75 mm. 



Remarks. — This species may be distinguished by the three eye 

 lenses near the frontal margin of the head, and by the 2-segmented 



Figure 244. — Cymhasoma 

 rigidum: a. Female, dor- 

 sal ; b, female, fifth leg 



