iieiv Parasitic Copepods on Fish. 361 



border of the apron extending over the upper part of tlie 

 genital segment; the paddle-joints are placed some distance 

 apart, the outer having four short plumose bristles on the 

 inner border and three simple hairs on the outer, the penulti- 

 mate joint having a long ciliated hair on the inner border 

 and a siiort one on the outer side. Hamulus posterior rather 

 long and slightly curved. 



Posterior thoracic segment very large (about one third as 

 long as broad), giving off the fourth perceopods, which are 

 three-jointed and extremely long, extending as far as the 

 caudal plates ; the first joint is long and muscular, the second 

 one third as long, giving off on its inner border a single 

 slender curved claw nearly equal to the length of the terminal 

 joint ; this last ends in a pectiniform edge and three curved 

 elongate claws placed close together near it, each of these 

 having fine dentations at their bases. 



Genital segment heart-shaped, about one third the length 

 of the carapace and rather less broad. 



Abdomen indistinctly two-jointed, equalling in length the 

 last segment, the first joint Being of a narrow oblong shape ; 

 the second is rather longer, broadening at its extremity, where 

 it gives off the two caudal, plates ; these are very large, with 

 narrow pedicles and square-cut extremities; the inner border 

 runs nearly parallel to the outer and is covered with long 

 ciHa ; these plates terminate in three long, straight, stout^ 

 plumose bristles and a shorter one on the outer border ; the 

 stout terminal bristles and caudal plates are pigmented blue ; 

 the egg-sacs are long and of a green colour in life. 



Length 5 millim. 



Male. — This differs from the female in being rather 

 smaller, tlie carapace is narrower, the genital segment is 

 oblong, the abdomen broader and more distinctly two-jointed, 

 and the caudal plates are oval in shape : of the organs the 

 jjosterior antennae are smaller, terminating in a very short 

 hook; Xh.Q hamulus anterior is longer and more robust; the 

 second maxillipeds have a thicker basal joint, with a double 

 tooth on its inner border, and the terminal claw-joint also has 

 a minute tooth near its extremity. 



Length 4*5 millim. 



Caligus rohustus, sp. n. (PI. XI. figs, 1, 2.) 

 A large number of specimens of both sexes were found of 

 this species freely moving about, generally on the inner side 

 of the operculum or on the bony gill-rays of various species 

 of Caranx and Thynnus, viz. T. macroiAerus, C. affi,nis, and 

 C. djedaha at Trincomaloe, and C. Rottleri at Aden. 



