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



biramose. The anterior pair is much the longest, while the posterior is the shortest. 

 The outer plates of the rhipidura are armed on the outer margin with a small tooth, 

 distant from the extremity about one-third the length of the plate ; the margin beyond 

 gradually tapers to the apex and is fringed with ciliated hairs. 



Observations.- — There are some points of difference between the Challenger specimen 

 and the description and figure of Kroyer. The rostrum in the former is conspicuous for 

 so small an animal, whereas Kroyer both figures and describes it as rudimentary. The 

 ophthalmopod agrees with Kroyer's description, but differs from his figure in not having 

 the eye symmetrically implanted on the stalk. The sixth somite is shorter than in 

 Kroyer's figure, and although the telson in a general way resembles that given by him, it 

 differs in minute details. Kroyer represents the telson as terminating in two long spine- 

 like horns curving towards one another at the apex, but in the Challenger specimen the 

 telson is cleft at the extremity into two lateral lobes, each of which terminates in a 

 spine that distally curves towards the other, and besides these, there is on the inner 

 side of each another small spine ; there are also two others distally situated, one on 

 each side of the telson (3z). 



These are points that are not of any great value in specific definition and may be 

 only features of immature growth. They are probably just those details that vary most 

 in specimens from distant localities and lead to greater or less permanent changes of 

 character. 



Our specimens were taken in the South Pacific, and those of Kroyer within the 

 Arctic zone. 



Sergestes oculatus, Kroyer (PI. LXX1V. fig. 1). 



Sergestes oculatus, Kj-oyer, Monograph. Fremstilling af Krajb. Sergestes, pp. 27, 61, Tab. iii. 

 fig. 5, a-f. 



" Rostrum rudimentary. 



" Ophthalmopoda longer than the first two joints of the peduncle of the first pair of 

 antennae, extending to about the extremity of the second, fungiform and supported 

 on a long and slender pedicle. Ophthalmus short, somewhat dilated, and about one- 

 fifth the length of the pedicle. 



" First pair of antennae having the peduncle one-fourth shorter than the carapace ; the 

 third joint as long as the first and one-fourth longer than the second. 



" Second pair of antennae having the scaphocerite four times longer than the last thick 

 joint of the peduncle. 



" Pereiopoda having the fourth pair scarcely natatorial. Sixth somite of the pleon 

 one-fifth the entire length of the animal, as long as the fourth and fifth or as the first 

 and second somites united ; nearly twice as long as broad, and three times longer than 

 the telson. 



