272 THE VOYAGE OF H.M.S. CHALLENGED. 



five somites of the pleon subequal in length and dorsally smooth. The sixth equals in 

 length the four preceding somites, is dorsally smooth and produced posteriorly to a small 

 tooth, as is also the postero-inferior angle of the lateral margins. 



Telson about two-thirds the length of the sixth somite. 



The ophthalmopoda are short, long-ovate, and reach but little beyond the extremity 

 of the rostrum. 



First pair of antennae having the peduncle subequal in length to the carapace. First 

 joint nearly twice as long as the ophthalmopoda ; second about two-thirds the length of 

 the first, and the third about half the length of the second, supporting two rlagella that 

 are shorter than their peduncle. The longer branch has only five or six articuli, of 

 which the basal is the longest and the terminal the shortest. The shorter branch has 

 three or four articuli that are tolerably robust at the base, and three or four that 

 suddenly become slender at the distal extremity. 



The second pair of antennae has the flagellum slender and about as long as the 

 carapace, and carries a scaphocerite that equals in length the peduncle of the first pair. 



The anterior three pairs of pereiopoda gradually lengthen posteriorly. 



Fig. 47. — Pencctcs gracilis, Dana. 



The fifth or posterior pair is slender. 



The pleopoda are all single-branched ; the first pair is straight, rigid, and carries in the 

 median line of the ventral surface of the somite a protuberance that is furnished with 

 three or four long hairs. The second pair is more flexible and has a similar but narrower 

 protuberance at the base crowned with long hairs. The succeeding pairs are long, 

 flexible, gradually lessening posteriorly, and are furnished in the median line at their 

 base with a protuberance, produced to a sharp tooth, which becomes gradually less 

 important posteriorly. The terminal pair, which goes to form the outer plates of the 

 rhipidura, are broad, foliaceous and as long as the telson. 



Length, 10 mm. Dana's specimen is given as 8 or 9 lines, or about double the 

 length of the Challenger specimens. 



Habitat. — The New Hebrides, and Port Jackson, Australia, are the places at which 

 specimens corresponding with this species were obtained by the Challenger. 



Dana's specimen was taken in the " Sooloo Sea, twenty-five miles east of Penay. 

 Collected Jan. 27, 1842. Some seaweed was seen floating by during the day." 



