NO. 1490. A NEW COSTA RICAN AMPHIPOD—STEBBING. 503 



the fifth apparently quite coalesced with the .sixth and consisting of a 

 narrow triangular piece h^ing- alongside the oblong fourth joint. The 

 sixth joint proper has an oblong trunk about twice as long as broad, 

 with the hind margin distally produced into a slender nearly straight 

 thumb about two-thirds the length of the trunk. The finger, some- 

 what stronger than the thumb, when closed overlaps the pointed apex 

 of the thumb with its own curved and ])lunt apex, and brings its 

 bulging middle part into contact with the spinulose distal half of the 

 thuml), leaving an irregularly oval gap between the proximal con- 

 fronting edges of this true chela. It will be noticed in the enlarged 

 figures of the second gnathopod of Orchestia darvHnii that the pro- 

 ^wrtions and outline of the trunk are quite different from those of the 

 present species, and also that the finger in closing upon the thumb 

 there presents an arrangement which is scarcely more than subchelate. 

 The second gnathopod of the female is not exceptional. The fourth 



Fig. 2. — Orchestia costaricana, female. 



joint has a pellucid boss, the fifth a similar median projection of the 

 free hind margin, and the sixth is broadly produced beyond the exceed- 

 ingly small chela-forming finger of this delicate structure. The pera3- 

 opods in both sexes are slender, the first longer than the second, 

 of which the finger is not constricted; the third much shorter than the 

 fourth; and the fourth than the fifth. The second joint of the third 

 pera?opod is rather narrowly ov^al; that of the fourth larger but similar; 

 that of the fifth again larger, but also much broader proximally than 

 distally, with the lower margin .straight. In this pair and to a less 

 degree in the third and fourth pairs the sixth joint is considerably longer 

 than either of the two preceding joints. The branchia? are of the 

 usual slender character, sofne of them lobulate. 



The pleopods are very narrow. The first uropods have the rami 

 nearly equal to the peduncle. In the much shorter second uropods 

 the rami are equal one to the othei' and to the peduncle. The third 



