42 THE VOYAGE OF H.M.S. CHALLENGER. 



female and 2 mm. in the male ; the third segment is a little smaller than the second in 

 the female and about the same size in the male ; the two last thoracic segments are nearly 

 the same size, the anterior being a trifle the larger, and measuring about one half the 

 diameter of the preceding segment. 



On the under surface the sterna of the first two thoracic segments are, as in all other 

 species, divided by sutures into three portions, which resemble in every particular those 

 of Serolis cornuta ; the posterior thoracic segments are not so fused together as they often 

 are ; the sixth is quite distinct from the following ones, and is not divided into a central 

 piece and two wedge-shaped lateral pieces as it is in Serdlis necera and other species ; a 

 deep groove separates the seventh from the eighth, but it does not extend as far as the 

 suture which marks the boundary between the epimera and sterna. 



Abdomen. — The first abdominal epimera are long, and reach beyond the end of the 

 caudal shield in the male, in the female they do not reach quite so far as the end of the 

 caudal shield ; the second epimera are short, as in Serolis necera, and only extend for a 

 very short distance down the lateral margin of the caudal shield ; they are not distinctly 

 longer in the male than in the female. 



The sterna of the abdominal segments differ in the two sexes as already said ; in the 

 male the jjosterior margin is straight, and the two ends project backwards as short spines; 

 the third segment has, in addition, a short median spine ; in the female the median portion 

 of all three is produced into a short broad spine. 



The caudal shield is hexagonal in outline ; the posterior end projects as a short spine ; 

 there is a distinct median keel and two lateral keels which start from the base of a strong- 

 blunt spine situated in the middle line at the anterior end, and terminate in two short 

 spines j)laced some way in front of the attachment of the uropoda; a transverse ridge, 

 which is prolonged backwards into three short spines, of which the median larger one is 

 upon the longitudinal carina, traverses the caudal shield ; viewed in profile, the caudal 

 shield appears to consist of two portions bounded by this transverse ridge ; the anterior 

 part, which possibly corresponds to the three fused terminal segments of the body, over- 

 laps the posterior portion or telson ; on the other hand it is possible, as suggested Ij}- 

 Studer, that the two obliquely running longitudinal keels mark the boundary between 

 the terminal segment of the body and the telson. 



The two pairs of antenncB are of about the same length. The anterior pair 

 have a short proximal joint with fine hairs upon the upper surface ; the two succeed- 

 ing joints are elongated and somewhat curved ; the posterior surfece is furnished with 

 fine hairs ; the fourth joint is short and oval ; the filament is made up of about twenty- 

 three joints, of which the proximal ones are short, but gradually increase in length towards 

 the distal extremity. The second pair of antennae consist, as usual, of five joints and a 

 terminal filament ; the joints increase in length towards the distal end of the autennfe, 

 the last joint being the longest and considerably narrower than the rest, which are about 



