no. 1404. PARASITIC COPEPODS—CALIGIDM- WILSON. 605 



The genital segment in pelamydis is nearly orbicular, only a trifle 

 longer than wide (one-sixteenth), not narrowed into a neck anteriorly 

 and only slightly emarginate posteriorly. 



In product xs and honito this segment is elliptical, narrowed into a 

 very marked neck anteriorly and with stout lobes prolonged far back- 

 ward posteriorly on either side of the abdomen. 



In thymni it is trapezoidal, widest posteriorly, with straight sides 

 and short, narrow posterior lobes. 



The abdomen is two-jointed in each of the species, but in pelamydis 

 the basal joint is three times the length of the terminal and only one- 

 third as wide as the genital segment. 



In honito the basal joint is about twice the length of the terminal 

 and the abdomen is two-fifths the width of the genital segment. In 

 productus the terminal joint is twice the length of the basal and the 

 abdomen is one-third the width of the genital segment. In thymni the 

 terminal joint is also twice the basal, but the abdomen is fully half 

 the width of the genital segment. In the structure of the abdomen, 

 therefore, the species arrange themselves in two pairs, a large and a 

 small species in each pair, which correspond quite closely. 



The fourth legs in pelamydis are four-jointed with five spines, the 

 last two joints very short and squat, bringing the five spines close 

 together in a row. In productus these legs are three-jointed with 

 four spines, the two terminal joints long and slender, the last two 

 spines toothed along their outer margins. In honito the legs are of 

 medium length, three-jointed with five spines, none of which are 

 toothed. In thymni they are long and slender, four-jointed with five 

 spines, none of which are toothed. All the authors who have hitherto 

 noticed productus are agreed that the first swimming legs have no 

 plumose seta3 on the posterior border of the terminal joint. 



The male of pelamydis is unknown, but in the males of the other 

 three species there is even greater differences than in the females. In 

 prod/uctus the free segment of the male is a little narrower than the 

 genital segment, and the fourth legs do not reach its posterior margin. 

 The terminal joint of the abdomen is half as long again as the basal 

 and the anal papilla 1 are small. In the male honito the free segment 

 is a little wider than the genital segment and the fourth legs reach 

 considerably beyond its posterior margin. The two abdomen joints 

 are about equal and the anal lamina? are veiy large. 



In thymni the free segment is about half the width of the genital 

 segment and the fourth legs just reach its posterior margin. The 

 abdomen is as wide as it is long with the terminal joint nearly twice 

 the length of the basal. These four species are thus well distin- 

 guished in both sexes, but more clearly in the males, since there is not 

 as much specific variation in this sex. 



Proc. X. M. vol. xxviii— 04 39 



