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Show 56 DR. II. J. HANSEN ON CRUSTACEANS [Jan. 20, described; the posterior branchiae to a large degree are destroyed, and as to the other features, I refer to Bate's description (p. 429). 1 cannot say with certainty whether the animal is a very young specimen of P.foliaceum Bate, or belongs to an unknown form; the rostrum deviates considerably from the type of P.foliaceum, but its shape presents a stage between those in the adult and in a larva, nearly agreeing with that in a larval form mentioned below. In my earlier paper on Sergestes I described S. obesus Kr. ( = /S. sanguineus Chun). I stated that it was a larva, a Masti-gopus, and added (p. 968): " It is easily seen that this species cannot remain in the genus Sergestes, but whether it should be referred to Petalidium Bate, or a new genus should be established for its reception, is impossible to decide with certainty. The branchial plates recall the plates found in Petalidium, and therefore I provisionally transfer it to that genus. . . . " I had not seen any adult Petalidium, but suggested (p. 967) that the branchial plumes interpreted by Bate as arthrobrancliiae are in reality pleurobranchiae. I can now decide that S. obesus Kr. is the Mastigojms of Petalidium: the branchiae prove i t ; besides, trl.3 in the Mastigopus is exceedingly elongate, with the basal part very thick, and in the adult form the basal joint of the same pair (fig. 1 e) is exceedingly thick, very much thicker than those of trl.1 and trl.2. Finally, the shape of the rostrum of S. obesus Kr. agrees nearly with that of the young Petalidium from Stat. 137 just mentioned. I described the branchiae of >$. obesus Kr., stating that in the largest specimen a rudimentary branchia was found above trl.4; in the largest but one of the specimens I have now looked for this branchia, and found it. Finally, I stated that P. obesum (Kr.) is " decidedly distinct from P. foliaceum Bate," and I maintain this opinion, founding it on the fact that P. foli-aceum does not possess any branchia above trl.4; and it is very improbable that such a branchia begins to develop itself in the Mastigopus and disappears again in the adult. b. S ergestes II. M.-Edw. S erg. intermedius Bate, p. 383 (no figure). Bate mentions one specimen, 5 mm. in length, from the " China Sea, off Luzon." The specimen has been preserved ; according to the rostrum, the eye-stalks, and the ext. br. of urp., it belongs to " Serg. brachyorrhos Kr.," which is the youngest Mastigopus of S. edwardsii Kr. (P. Z. S. 1896, pp. 963-64). S e rg . p r eh en s ilis Bate, p. 385, pi. lxxi. (Plate X I. figs 4 a 4 6.) Bate has examined one specimen from Stat. 236, lat. 34° 58' X., long. 13&° 29' E., off Japan, 775 fathoms. The type, an adult male, has been preserved; it differs from all other species known to me. It will be useful to redescribe its most essential characters, and two new figures are given (PI. X I. figs. 4 a, 4 b). The |