ART. 15 COPEPOD CRUSTACEANS OF CHESAPEAKE BAY WILSON 47 



detailed description is here given of both sexes, with figures of the 

 appendages to supply the omitted details. 



Specific characters of feTnale. — Body of the usual slender form but 

 relatively stouter than in any species except rohusta and hehes, the 

 width of the anterior portion being nearly half its length. Rostrum 

 considerably shorter than that of similis and rohusta, pointed and 

 curved over ventrally so as to be wholly invisible in dorsal view. 

 Head more or less completely fused with the first thoracic segment, 

 the dividing suture often practically invisible. Head itself half the 

 length of the anterior body; first thoracic segment as wide as the 

 head, the second, third, and fourth segments tapering regularly 

 backward, the fourth one a trifle more than half the width of the 

 first. Fifth segment abruptly contracted into a narrow neck an- 

 teriorly, swollen through the bases of the fifth legs to twice the 

 width of the neck, and contracted again posteriorly. Genital seg- 

 ment swollen anteriorly and tapered posteriorly, as long as the en- 

 tire abdomen, with the transverse suture just in front of the center. 

 Abdomen of three segments, the terminal or anal one the shortest 

 and often indistinguishably fused with the one in front of it. 

 Caudal rami longer than this anal segment, more than twice as long 

 as wide, each tipped with four setae. The second seta from the out- 

 side is the longest and is curved inward across the base of the one 

 next to it, which is second in length. 



First antennae slender and, when reflexed, reaching the posterior 

 margin of the second thoracic segment; composed of about 11 seg- 

 ments, but the grooving of the basal portion is so faintly defined as 

 to be very uncertain. Second antennae 2-jointed, the tenninal joint 

 a little shorter than the basal and tipped with a tuft of six plumose 

 setae, with three others on the outer margin nearer the proximal end. 

 First maxillae with the masticatory lobe large and tipped with two 

 very stout setalike appendages, and on the outer margin a small 

 jointed knob carrying spines; proximal lobe recurved, ending in 

 three long setae, with two other shorter ones on the outer margin. 

 Second maxillae 5-jointed, basal joint with three setae and a knob 

 bearing three other setae; second with two setae; third joint with 

 two large setae and a third much smaller one; fourth joint with a 

 single seta on each distal corner; fifth joint tipped with two setae. 

 The large setae are branched rather than plumose, the branches scat- 

 tered and much too large for plumes. Maxillipeds 4-jointed, basal 

 joint with one large and two small setae; second joint with two 

 large setae; third joint with two of moderate size; last joint with one 

 large and a smaller one. Swimming legs like those of other species 

 of Oithona, the exopod of the first pair with 1.1.3 outer-edge spines,, 

 and the exopod of the fourth pair with 1.1.2 outer-edge spines. 



