Si 



Stat. 1 1 8 (HENSEN vertical net 900 metres to surface). Stat. 141 (HENSEN vertical net 



1500 metres to surface). -- Stat. 203 (HENSEN vertical net 1500 metres to surface). 



The species is very distinct. It is easily identifïed by lts elongate body and greatly 

 produced last thoracic segment. 



The posterior margins of the last thoracic segment reach to near the distal end of the 

 genital segment. The combined length of the abdomen and furca is equal to one-fourth of the 

 total length of the cephalothorax, from the frontal margin to the posterior end of the last thoracic 

 segment. The rami of the rostrum are two-jointed and the apical section is very slender. The 

 antennules are twenty-four-jointed, and extend to the end of the second abdominal segment. 

 The fifth pair of feet is composed of two free joints attached to a basal part. The second joint 

 is furnished with one apical and two subapical spines (Plate XXXIII, fig. 18). 



Length of female 2,68 mm. 



Xanthocalanus agilis is apparently very rare. It has only been recorded from the Gulf 

 of Naples by Giesbrecht. 



Genus Brachycalanus Farran, 1905. 



The members of this genus are separated from the other genera belonging to the family, 

 chieny by the very strong, slightly bifurcate, and highly chitinised rostrum. The mouth organs 

 and swimming feet closely resemble those of Xanthocalanus. 



The genus was represented in the 'Siboga' plankton by a single immature specimen. 



1. Brachycalanus gigas nov. sp. Plate XXXV, figs. 10 — 18. 



Length — 7,2 mm. 



Yiewed from above, the body appears moderately robust, and elongate ovate in outline. 

 The head is distinctly separated from the first thoracic segment, and is furnished with a low 

 median crest. The fourth and fifth thoracic segments are separated. The last thoracic segment 

 is produced to the end of the first abdominal segment, and terminates in distinct points that 

 project outwards from the segment. Seen from the side, the forehead is arched, and the median 

 crest is rounded at the apex. The last thoracic segment is irregularly triangular in outline, with 

 a small point at the apex. The rostrum is robust, of moderate length, and is slightly bilurcate 

 at the upturned apex. No filaments are present. 



The abdomen is composed of five segments. The combined length of the abdomen and 

 furca is contained about three and a halPtimes in the total length of the cephalothorax, from 

 the frontal margin to the base of the genital segment. The abdominal segments are wide, and 

 are comparatively short. The anal segment is very small. The furcal joints are rather longer 

 than broad, and are about twice as long as the anal segment. 



The antennules are composed of twenty-four joints, and extend to the end of the third 

 abdominal segment. The eighteenth joint is faintly divided. 



The antennae, mandibles and maxillae are similar to those of Xanthocalanus. 



The first pair of maxillipedes also resembles that of Xanthocalanus, but the claw-like 



81 



SIBOHA-EXPEDITIF. XXIX a. '! 



