MUSEUM OF COMPARATIVE ZOOLOGY. 97 



The structure of the branchiae themselves, in this and in the two following 

 species as well, is very different from that in Penceus, or any of the Penaeidae 

 described in this paper. The branchiae are pinnate in form, and each pinna is 

 a complete phyllobranchia ; that is, they are compound phyllobranchiae, while 

 those of Penceus are compound trichobranchiae. The structure is more like 

 that in Sicyonia (judging by Bate's description of the branchiae of that genus) 

 than that in Penceus. 



The first pair of thoracic legs are subchelate, and the dactyli of the external 

 maxillipeds and the propodi of the first, second, and third pairs of legs are 

 multiarticulate, as in the next species, the articulations being more conspicuous 

 than in that species. These structural characters of the thoracic legs are, how- 

 ever, undoubtedly characteristic of all the species of the genus. 



[ Sergestes robustus, sp. nov. 



Plate XVI. Figs. 5-8 b . 



Male. — The carapax is strongly compressed, the breadth being considerably 

 more than the height at the base of the antennae, but much less than the greatest 

 height posteriorly, which is rather more than twice that at the base of the an- 

 tennae. The dorsum is broadly rounded to the base of the rostrum, which rises 

 rather abruptly from the dorsum, is very thin, acutely triangular, and extends 

 a little forward of the truncated middle lobe of the ophthalmic somite. 



The eyestalhs to the tips of the eyes are about two fifths as long as the 

 antennal scales, and the diameter of the eye itself about half the length. The 

 peduncle of the antennula is about a fifth longer than the antennal scale, the first 

 segment scarcely half as long as the antennal scale, and the second and third 

 successively a little shorter ; all the segments are very stout, the diameter in the 

 second and third being equal to more than half the length. The proximal seg- 

 ment of the upper or major flagellum is scarcely more than a fourth as long as 

 the distal segment of the peduncle, and scarcely longer than the proximal seg- 

 ment of the lower flagellum, which is modified as in the allied species. The 

 antennal scale (Fig. 7) is about half as long as the carapax along the dorsal 

 line, about a third as broad as long, and much broader at the tip than in the 

 allied species. 



The oral appendages do not differ essentially from the oral appendages of 

 P. Frisii and arcticus as figured by Kroyer. 



The external maxillipeds reach by the tips of the antennal scales fully the 

 length of their dactyli, and are about as stout as the third pair of legs : all five 

 segments of the endopod are approximately equal in length though the dactylus 

 is slightly shorter than the others, and all are armed with very slender spines ; 

 the dactylus is slender and multiarticulate, being composed of about five 

 segments, and tipped with two or three spines. The legs of the first pair fall a 

 little short of the tips of the antennal scales : the merus is about twice as long 

 as the carpus and about as long as the propodus, which is very slender, com- 



vol. x. — no. 1. 7 



y 



