568 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxvm. 



This distribution shows two facts very pIainl} T . First, the menhaden 

 is evidently the host of the species, the presence of a single specimen 

 on the blue-fish being easily explained by the fact that the latter feeds 

 upon the menhaden. And then such a large percentage taken in the 

 tow indicates that this species is very active, as much so as C. rapax, 

 although its hosts are nowhere near as numerous as those of the latter 

 species. 



There is also a strong suggestion that these parasites are not likel} T 

 to change their depth very materially; those which frequent surface 

 fish stick to the surface even when they are swimming about freely. 

 And probably the same can be said of those which are parasitic on fish 

 that frequent the botttom. 



This species is not very abundant, as the paucity of specimens clearl} T 

 shows. As they were all taken in the latter part of August and the 

 first of September, this period is probably the height of the breeding 

 season. 



The species can be recognized easily by the sharp angles at the 

 posterior corners of the carapace and the general trapezoidal appear- 

 ance of the latter. The diagnosis can then be verified by an examina- 

 tion of the claws at the tips of the first swimming legs. 



CALIGUS RAPAX Milne Edwards. 

 Plate VII, figs. 3, 7, 9, 10, 18, 22, 23, 26, 32, 36, 46, 47, 48, 49, 50 in the text. 



Caligus elongatus Nordmann, 1832, p. 24. 



Caligus elongatus Kroyer, 1837, p. 201. 



Caligus rapax Milne Edwards, 1840, p. 453, pi. xxxvin, figs. 9-12. 



Caligus elongatus Milne Edwards, 1840, p. 454. 



Caligus leptochilus Frey and Leuckart, 1847, p. 165. 



Caligus rapax Baird, 1850, p. 270, pi. xxxu, figs. 2 and 3. — White, 1850, p. 

 119. — Steenstrup and Lutken, 1861, p. 359, pi. n, fig. 4. — Kroyer, 1863, 

 p. 71.— Olsson, 1868, p. 8.— Macintosh, 1874, p. 262.— Smith, 1874, p. 575.— 

 Richiardi, 1880, p. 148.— Bassett-Smith, 1896, p. 156; 1899, p. 448.— Brian, 

 1898, p. 10, pi. ii, fig. 6; 1899, p. 2.— T. Scott, 1900, p. 148, pi. v, figs. 13-19. 



Female. — Frontal plates wide; frontal margin slightly rounded; 

 lunules large, orbicular, and projecting considerably. Carapace ovate, 

 longer than wide; posterior sinuses narrow, of medium depth, and 

 with approximately parallel sides; thoracic area broad, three-fifths of 

 the entire width, well rounded, and projecting posteriorly about the 

 depth of the sinuses bej^ond the lobes; lobes narrow, short, slightly 

 wider at the tips (fig. 79). 



Free thorax segment very short, only three-tenths as wide as the 

 carapace. Genital segment large, more or less quadrilateral, with 

 rounded corners, slightly wider than long, seven-tenths the width of 

 the carapace. Abdomen unsegmented, varying considerabty in length 

 (from 0.5 to 0.8 the length of the genital segment), about half as wide 

 as the genital segment; anal sinus scarcely perceptible; anal laminae of 



