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



ing, or is confluent with the margin. The second pair of antennae has a long and wide 

 scaphocerite, strengthened on the outer side by a ridge that terminates in a sharp 

 tooth, near the distal extremity, and it carries a long and slender flagellum. The siagon 

 or mandible carries a two-jointed synaphipod that does not reach beyond the second 

 joint of the peduncle of the second pair of antennae. 



The first pair of gnathopoda is subpediform and carries a long and slender basecphysis. 

 The second pair is very long, slender, and carries a very long basecphysis. 



The anterior three pairs of pereiopoda are long and slender, and the dactyli are long. 

 The posterior two pairs of pereiopoda are slender, but not quite so long as the preceding. 



The pleopoda are long, slender, and unequally branched. 



In its external appearance this genus approximates to Pen&us, but it may readily be 

 distinguished by the character of the ophthalmopod, the rudimentary state of the pro- 

 sartema, and the condition of the stylocerite of the first pair of antennae, as well as 

 by the difference in the character of the branchiae. In this genus a podobranchial plume 

 is attached to each of the five mastigobranchiae, and two arthrobranchial plumes are 

 attached to the antepenultimate pair of pereiopoda. 



In these two latter characters the branchial arrangement approximates to that of the 

 genus Benthesicymus, from which it differs, first, in having no mastigobranchia attached 

 to the penultimate pair of pereiopoda, and, second, in a character that might be thought 

 to possess only specific or even only varietal value, but which in reality is important : 

 in Benthesicymus the mastigobranchial plates increase in size posteriorly, and the last is 

 the longest, and is a very important appendage, and all the podobranchial plumes are 

 large and well developed, and the pleurobranchiae, although more important posteriorly 

 than anteriorly, are all large and useful organs, whereas in Hemipenseus the pleuro- 

 branchiae are all small and feeble organs, except the posterior pair, and the podo- 

 branchiae are also small, those of the third pair of pereiopoda being very feeble, and 

 the mastigobranchiae are absent from the last, and exist only as rudimentary lobes on 

 the penultimate pair. 



The arrangement may be tabulated in the following formula : — 



Pleurobranchias, 

 Arthrobrancbiae, 

 Podobranchije, 

 Mastigobranchiae, 



The structure of the branchiae also differs from that of Benthesicymus, and corresponds 

 more with that of Pcnams, each plume consisting of a central stalk which carries a 

 series of club-shaped filaments, each branch curling over so that the extremities of 

 opposite branches approximate, and the whole plume forms an obliquely truncated or 

 pen-shaped tube. 



