NO. 1573. PARASITIC COPEPODS— WILSON. 353 



Genital segment considerably larger than the carapace, evenly 

 rounded anteriorly, but almost squarely truncated posteriorly, with 

 short and acute spines at the corners and a wide median incision. 

 Fifth legs some distance from the margin on the ventral surface. 

 Abdomen small and entirely hidden in dorsal view; anal laminsB 

 also small, with very short and nonplumose spines. Terminal joints 

 of the second maxillipeds enlarged and fleshy, reniform, with a rough 

 scaly surface. Legs all biramose; rami of third and fourth pairs 

 minute and rudimentary. Egg-strings narrow and nmch longer than 

 the body. 



Male. — Carapace, including the posterior lobes, elliptical, slightly 

 longer than wide, narrowed anteriorly; posterior lobes long and nar- 

 row; posterior margin between the lobes nearly straight. Eyes three 

 in a row, the median one much smaller than the others. Free thorax 

 segments about the same length, diminishing regidarly in width. 

 Genital segment small, sub quadrangular, with the fifth legs very 

 prominent at the posterior corners. Abdomen large, one-jointed; 

 anal laminae large and armed with long plumose setae. Second 

 antennae larger than in the female, but with the adhesion pad much 

 reduced in size. Second maxillipeds with a stout, curved, terminal 

 claw shutting down against a pair of corrugated knobs as in Pandarus. 



Swimming legs all biramose, rami of fourth pair indistinctly seg- 

 mented, of the other pairs two-jointed. 



{Perissopus, Trepiffffoc;, more than the regular mimber or size, and 

 TTovs, foot, alluding to the enlarged second maxillipeds.) 



KEY TO THE SPECIES. 



a. First pair of dorsal thorax plates bilobed ; second pair not meeting at the mid- 

 line; third pair fused (Chlamys) incisus Van Beneden, 1892, p. 354. 



a. None of the thorax plates bilobed; second and third pairs meeting at the mid- 

 line, but not fused b. 



b. Carapace about half the size of the genital segment; its posterior lobes 

 narrow, long, and overlapping the first dorsal plates; rami of third legs 

 fused into a semicircular plate. 



dentatus Steenstrup and Lutken,'^1861, p. 353. 



b. Carapace more than four-fifths as large as the genital segment, its posterior 



lobes very short and wide; rami of third legs distinct and similar to those 



of the third pair communis Rathbun, 1887, p. 354. 



This genus Perissopus was established by Steenstrup and Liitken 

 in 1861 for a species which they named P. dentatus, and for which 

 they gave the following genus diagnosis: 



Cephalothorax undivided; abdominal segments free, carrying four or six dorsal 

 plates; genital segment enlarged and a little widened, covering the short tail and the 

 caudal stylets; abdominal feet destitute of plumose setse, rami of first and second pairs 

 (at least in P. dentatus) two-jointed, the other rami not joiiited, very rudimentary. 

 Male unknown. 



Proc. N. M. vol. xxxiii— 07 23 



