50 BRITISH PARASITIC COPEPODA. 



Caligus rapax is apparently widely distributed in the North 

 Sea and North Atlantic, and is, according to C. B. Wilson, 

 " the most common species of the genus on the north-eastern 

 coast of the United States, having been taken from more than 

 twenty-five different kinds of fish by many collectors working 

 in the interests of the United States Fish Commission " 

 (' Proc. U. S. National Museum/ vol. xxviii, p. 571). 



A certain amount of variation occurs in the size of different 

 specimens of this species, and also in the proportional dimen- 

 sions of the different parts. The colour is also somewhat 

 variable; it seems to change to some extent with the colour 

 of the fish. 



4. Caligus centrodonti Baird. 

 (Plate V, figs. 1-3 ; Plate XVIII, figs. 1-3.) 



1850. Caligus centrodonti Baird. (4) p. 272, pi. xxxii, figs. 6, 7. 

 1863. Caligus abbreviatus Kroyer. (71) p. 61, pi. iii, figs. 3 a-h. 

 1905. Caligus abbreviatus T. Scott. (116) p. 109, pi. v, figs. 1-6. 

 1905. Caligus centrodonti C. B. Wilson. (145) p. 652, pi. xxvii, figs. 

 333, 344. 



Female. Length of carapace about equal to the width, 

 and to two-thirds of the entire length of the animal, 

 widest posteriorly, and becoming considerably narrower 

 towards the front. Frontal plates large, lunulaa pro- 

 minent. Free thoracic segment very short. Genital 

 segment short and subquadrangular in outline, the 

 width equal to about one and a half times the length, 

 and fully half as wide as the carapace ; greatest width 

 near the anterior end ; lateral margins obliquely 

 rounded, posterior margin slightly concave. Abdomen 

 very short, small, uniarticulate, and scarcely reaching 

 beyond the lateral lobes of the genital segment. 

 Caudal rami also very short, with four or five short 

 seta3 round their distal ends. Antennules moderately 

 short. Sternal fork stout, with tolerably stout and 

 slightly divergent rami. Maxillae and maxillipeds and 

 also the first three pairs of thoracic legs resembling 

 those of the species already described; the fourth 

 pair elongated, the basal joint moderately stout and 

 furnished with a lobulate process on its upper aspect ; 

 the single two-jointed ramus somewhat slender ; the 



