arcuicornis mainly by its smaller size and 

 other, minor, differences. The prosome is 2.5 

 to 2.8 times as long as the urosome. The caudal 

 ramus (fig. 14) is longer than wide (63:37). 

 The segments of the fifth leg (fig. 15) have the 

 proportional lengths of 21:29:50, from prox- 

 imal to distal. 



Ctenocalanus vanus Giesbrecht, 1888 

 Occurrence 



Sta. 5. 2 adult females, 1.06 to 1.08 mm. 



Sta. 7. 6 adult females, 1.08 to 1.13 mm. 



Sta. 9. 2 adult females, 1.14 mm. 



Sta. 13. 4 adult females, 1.08 to 1.17 mm. 



Sta. 15. 1 adult female, 1.26 mm. 



Sta. 16. 5 adult females, 1.26 to 1.28 mm.; 



5 fifth copepodid males, 1.26 to 



1.28 mm. 



Remarks 



This species was at all the stations except the 

 southernmost, although not in large numbers. 

 It can easily be recognized by the ctenoid form 

 of the external spines of the third exopodal 

 segment of the third and fourth legs. 



FAMILY AETIDEIDAE 



Aetideus jMcificus Brodsky, 1950 



(Plate 4, figs. 16-24; plate 5, figs. 1-7) 



Occurrence 



Sta. 16. 5 adult females, 2.21 to 2.30 mm. 



A complete description is given here because 

 Brodsky's original description is brief and 

 seems to disagree in some anatomical details 

 with the present specimens. 



Description of Female 



The total lengths of five adult females range 

 from 2.21 to 2.30 mm. The prosome is about 

 3.4 times as long as the urosome. The general 

 outline of the prosome is ovate. In dorsal aspect 

 the greatest width is at the fusion of the ceph- 

 alosome and the fir.st metasomal segment 

 (plate 4, fig. 16). The fir.st metasomal segment 

 is completely fused with the cephalosome. The 

 fourth and fifth metasomal segments are also 

 completely fused. The cephalosome in dorsal 

 aspect is slightly dilated in the oral region and 

 is smoothly rounded anteriorly. The rostrum 



consists of two rather slender processes (plate 

 4. fig. 18). 



The posterolateral corners of the metasome 

 are produced into acute points, which in dorsal 

 view are slightly curved inward (plate 4, fig. 

 19) and in lateral aspect point straight back- 

 ward (plate 4, fig. 20). These points do not 

 reach the distal margin of the genital segment 

 in either dorsal or lateral view. 



The urosome (plate 4, figs. 19-21) is 4-seg- 

 mented; the segments and the caudal rami 

 have the following proportional lengths, from 

 anterior to posterior: 31 :17 :14 :13 :25. The gen- 

 ital segment in dorsal aspect is slightly wider 

 than long (29:25) and has broadly rounded 

 sides. The caudal rami are about 2.5 times as 

 long as broad, and have 4 terminal setae of 

 subequal length, a long ventral seta that is 

 longer than the terminal setae, and a short ex- 

 ternal seta. 



The antennule is 24-segmented ; when it is 

 applied against the body, its last 2 segments 

 extend beyond the end of the caudal ramus 

 (plate 4, fig. 17). 



The endopod of the antenna (plate 4, fig. 22) 

 is slightly shorter than the exopod. The first 

 endopodal segment is styliform and has 2 setae 

 almost at the distal end of the internal margin. 

 The second endopodal segment is about half 

 the length of the first and has 6 -f 1 setae on 

 the external lobe and 8 setae on the internal 

 lobe. The first and .second exopodal segments 

 are incompletely separated; the former has no 

 setae, but the latter has 2 setae proximally 

 along the internal margin and 1 seta at the 

 distal end of that margin. The third to sixth 

 segments are fully separated ; each has a strong 

 seta. The .seventh segment is elongate and has 

 an internal seta at about two-thirds the length 

 of the segment from the proximal end, and 3 

 apical setae. 



The mandibular palp (plate 4, fig. 23) has a 

 squarish basis with 2 setae along the internal 

 margin. The endopod is 2-segmented and has 2 

 setae on the first segment and 9 + 2 on the 

 second. The exopod is 5-segmented ; each of the 

 proximal 4 segments has a strong seta, and the 

 distal segment 2 apical setae. The cutting edge 

 of the mandibular blade (plate 4, fig. 24) is 

 armed with about six groups of teeth and a 

 ba.sal seta. 



542 



U.S. FISH AND WILDLIFE SERVICE 



