330 BULLETIN 158, tTKlTED STATES NATIONAL MUSEUM 



an unusual combination ; fifth legs 3-segmented, basal segment short 

 and unarmed, second segment with one outer seta, third segment 

 with two subequal elongate apical setae. Total length, 1.2-1.3 mm. 



Male. — Body relatively stout, cephalic segment broad, its width 

 to its length as 7 : 10, narrowed and slightly emarginate anteriorly 

 and rather squarely truncated posteriorly ; second, third, and fourth 

 segments wide and produced at their posterior corners; fifth seg- 

 ment abruptly narrowed to half the width of the fqurtli. Urosome, 

 including the caudal rami, less than half the length of the meta- 

 some; genital segment about the same width as the fifth segment, 

 with convex sides. Abdomen 4-segmented, the segments diminish- 

 ing in length but not in width backward; caudal rami two and a 

 half times as long as wide, the outer seta at tlie center of the outer 

 margin; three terminal setae, the middle one the longest and longer 

 than the urosome plus the caudal rami. First antennae 16-seg- 

 mented, but some of the joints are very indistinct, the terminal por- 

 tion made up of two equal segments moderately elongated. Second 

 antennae, mouth parts, and swimming legs like those of the female ; 

 fifth legs 3-segmented, the apical setae very long and slender. Total 

 length, 0.T5-0.9 mm. 



Remarks. — This is the only " Cyclops " in the area whose fifth 

 legs are 3-segTnented, and it can be identified by this character alone. 

 It seems to prefer shallow water full of vegetation rather than 

 deeper and clearer locations. 



Genus MESOCYCLOPS G. O. Sars, 1914 



Body cyclopoid, metasome swollen and depressed, urosome slen- 

 der; genital segment elongated and but little dilated anteriorly; 

 caudal rami of moderate length and slightly divergent. First an- 

 tennae iT-segmented ; second antennae slender and 4-segmented, with 

 elongate setae; rami of first four pairs of legs 3-segmented, end 

 segment of exopod with two outer spines, two apical and two 

 inner setae in the first legs, one apical and two outer spines, one apical 

 and three inner setae in the other three pairs of legs ; fifth legs very 

 small and 2-segmented, basal segment with one, distal segment with 

 two subequal setae. One species found here. 



MESOCYCLOPS OBSOLETUS (Koch) 

 FiGUEB 197 



Cyclops obsoletus Koch, Deutschlands Crustaceen, Myriapoden und Arachniden, 



pt. 21, pi. 5, 1838. 

 Mesocyclops obsoletus Sars, Crustacea of Norway, vol. 6, p. 58, pi. 35, 1914. 



Occurrence. — Found in abundance in Bournes Pond, Falmouth; 

 two of the small ponds on the road from Woods Hole to Falmouth ; 

 the Mill Pond at Woods Hole ; the lily pond north of Nobska Light 



