604 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxviii. 



slender terminal claw but with no spine on the posterior margin of the 

 basal joint. 



First maxilla? lacking; second pair stout and broad. First maxilli- 

 peds with basal joint about three fourths the length of the terminal; 

 second pair very stout, basal joint subcorneal in shape, without protu- 

 berances, the terminal claw less than half the length of the basal joint. 

 Furca simple, the branches divergent, straight and acuminate. Fourth 

 legs three-jointed with four simple spines, no teeth on the spines nor 

 sheathing seta? at their bases. 



Total length 9.7 mm. Length of carapace 4.9 mm.; width of same 

 3.4 mm. 



Color not given. 



{thymni, the generic name of its host.) 



Male. — Carapace elliptical, about one-eighth longer than wide, not 

 narrowed much anteriorly. Frontal plates small, without an emargi- 

 nation at the center and very narrow; lunules small and semicircular. 

 Free segment like that of the female. Genital segment nearly quad- 

 rate, the sides somewhat convex, the posterior margin almost squarely 

 truncate, with the posterior angles scarcely projecting at all, but each 

 armed with a bunch of setae representing the fifth leg of that side. 



Abdomen a little shorter than the genital segment, almost as wide 

 as long, two-jointed, the basal joint only about half the terminal as in 

 the female. Anal lamina? as before, but the plumose seta? much 

 longer, as long as the entire abdomen. 



The appendages show the usual increase in the size of the second 

 antenna? and second maxillipeds. The former have a*stout branch on 

 the inner margin of the terminal joint near its center. The fourth 

 legs are longer and more slender than those of the female, and the 

 spines which the}^ carry are longer and weaker. 



This species was obtained from the oceanic bonito (Gymnosarda 

 pelamis Linnasus; Tkynnus pelamys Cuvier and Valenciennes), and is 

 the fourth species of Caligus to be recorded from that host. The 

 other three species are pelamydis, productus, and bonito. It will be 

 instructive to review here the distinguishing characters of these four 

 species. 



They differ, first, in size, and may be arranged in an ascending 

 series, pelamydis, 3-4 mm. long; productus, 5-6.5 mm.; bonito, 7-8 

 mm., and thymni, 9-10 mm. 



Again, they differ considerably in color, although unfortunately the 

 color of thymni has never been given. Pelamydis is a light yellowish 

 white, the genital segment almost pure white without pigment of any 

 sort. Productus is a light dirt-brown, everywhere transparent, except 

 the egg strings; while bonito is a very transparent white faintty tinged 

 with yellow, and with a large spot of rusty-brown pigment in the pos- 

 terior portion of the lateral area on cither side, like a large lateral eye. 



