COPEPODS OF THE WOODS HOLE REGION 291 



Genus D'ARCYTHOMPSONIA T. Scott, 1906 



Body slender and subc3dindrical, being some^vhat compressed ; head 

 fused with the first segment and shorter than the rest of the meta- 

 some : rostrum small, not defined at its base. Urosome as long as. or 

 longer than, the metasome; genital segment distinctly divided: 

 abdomen 3-segmented, the segments equal in length; caudal rami 

 small, each with a single elongated seta. First antennae 6- or 

 T-segmented; exopod of second antenna minute, 1-segmented; distal 

 segment of endopod armed with coai-se spines. Exopods of first four 

 pairs of legs 3-segmented, endopods 2-segmented, the apical setae of 

 both rami much elongated ; the two segments of each fifth leg fused 

 into a small lamina, with an outer process at the base, but no separa- 

 tion of distal segment and inner expansion. One species found here. 



D'ARCYTHOMPSONIA PAKVA, new species 



Plate 18 



Occurrence. — Both sexes were obtained from one of the brackish 

 ponds on Chappaquiddick Island, and a single female from another 

 of the same ponds. One female has been chosen as the type of the 

 new species and given U.S.X.M. Xo. 59761. 



Color. — Body as transparent and colorless as glass with no trace of 

 pigment ; no eye visible. 



Female. — Body subcylindrical, slightly compressed laterally, eight 

 times as long as wide and tapered posteriorly. Head fused with 

 the first segment, the resulting cephalothorax two-fifths as long as 

 the metasome ; rostral projection small and poorly defined at its base. 

 Metasome passing insensibly into urosome ; the latter, including the 

 caudal rami, as long as the former : genital segment distinctly divided 

 at about its center; the three abdominal segments about the same 

 length and width ; anal segment rounded on the ventral surface and 

 abruptly depressed at the anal plate on the dorsal surface near its 

 posterior end. Caudal rami twice as long as wide, swollen in the 

 center and narrowed at either end, each tipped with a single stout 

 seta two-thirds as long as the urosome, with a very short seta on 

 either side; there are also several setae on the swollen central part 

 of each ramus. 



The first antennae are short, stout at the base, and regularly 

 tapered, 6-segmented, rather sparsely setose; the fourth segment has 

 a stout aesthetask reaching beyond the tip of the antenna. Second 

 antenna stout, composed of three segments, the exopod replaced by a 

 single spine on the ventral surface of the second segment; the end 

 segment has a single long spine at its tip, four much shorter ones on 

 the outer margin, and a single small one at the inner distal corner; 



