TBEBIUS. 81 



Genus 10. TREBIUS Kroyer, 1838. 



Resembling Lepeophtheirus in its general form and 

 in the absence of lunulas, and also generally in the 

 structure of the body and its appendages, except that 

 the fourth pair of thoracic legs are biramose in both 

 the male and female. The second maxillee tolerably 

 long with the extremity pointed or slightly bifurcated. 

 The first maxillipecls rather stronger than in Lepeo- 

 phtheirus or Oaligus, but the second pair less powerful 

 than those of the two genera mentioned. 



Several species of Trebius have been described, but the one 

 recorded below is the only species represented in the British 

 fauna. 



1. Trebius caudatus Kroyer. 

 (Plate XXII, figs. 1, 2; Plate LIV, figs. 1-11.) 



1838. Trebius caudatus Kroyer. (70) p. 30, pi. i, fig. 4. 

 1850. Trebius caudatus Baird. (4) p. 280, pi. xxxiii, fig. 3. 

 1900. Trebius caudatus T. Scott. (112) p. 155, pi. vi, figs. 20-26. 

 1907. Trebius caudatus C. B. Wilson. (147) p. 681. pi. xv, figs. 11-13 ; 

 pi. xvi. figs. 14-22. 



Female. — Carapace suborbicular, rather longer 

 than broad, and equal to about a third of the entire 

 length of the animal. Frontal plates narrow, without 

 lunulas. Eyes conspicuous and close together. Free 

 thoracic segment small. Genital segment oblong, 

 rather longer than broad, its width equal to about 

 three-fourths of the length, and to about two-thirds 

 the width of the carapace, the lateral margins nearly 

 straight, its posterior end truncated, and the postero- 

 lateral corners rounded and provided with three small 

 but stout marginal spines. Abdomen elongated, 

 narrow, and composed of three segments, its length 

 about equal to that of the carapace, the proximal joint 

 longest, being rather more elongated than the next 

 two combined; the second about twice the length of 

 the third joint, the articulation between the second and 

 third joints not very clearly defined. Caudal rami 

 short and furnished with a few plumose apical seta3. 

 vol. t. 6 



