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Show 54 DR. II. J. HANSEN ON CRUSTACEANS [Jan. 20, antenn. ped. = peduncle of the antennulae ; mxp.2-mxp.3 = the second and third pair of maxillipeds ; trl.1-trl.s = the first to the fifth pair of trunk-legs ; br. = the first, b r .^ th e second branchia above the same legs ; ext. br. of urp. = external branch of the uropods. I. Notes on the Species of Petalidium and Sergestes. ct. P e ta lid ium Bate. To this genus Bate has referred only one species. It will be convenient to deal with the characters of the genus together with those of the species. P et. foliaceum Bate, pp. 348-50, pi. lx. (Plate X I. figs. \a-\ g.) Bate mentions five specimens from two stations : Stat. 146, lat. 46° 46' S., long. 45° 31' E., 1375 fathoms; and Stat. 159, lat. 47° 25' S., long. 130° 22' E., 2150 fathoms. All these specimens are at present in the Museum, but even Bate's text and his figure of the entire animal show that most of the appendages presenting specific characters are wanting or have been mutilated. Bate writes (p. 349): " The great distinction between this genus and Sergestes exists in the form, character, and arrangement of the branchial plumes, which consist of a series of plates and cylindrical filaments, situated side by side in a series of rows at right angles to the stalk. There is but one plume to each of the live anterior somites of the pereion, the posterior two somites having none ; between some of the somites is a large foliaceous j)late." But this description is difficult to understand: his figure 3, representing the branchiae, is defective, and his tabular view (p. 349) is wrong. He was of opinion that the foliaceous plates- of which he had seen only three-were pleurobranchiae, while the " branchial plumes" were arthrobranchiae; but this is incorrect: they are decidedly pleurobranchiae as in Sergestes. Petal, foliaceum Bate difiers from all species of Sergestes in one cjuite unimportant feature, that no trace of branchiae is found above trl.4, and in one important character, viz., the structure of the pleurobranchial plumes. This structure is very interesting (fig. 1 e). The most developed branchiae are, as usual, those above trl.1 and trl.2; each of these consists of an anterior and a posterior half, and each half of five (to six) transverse rows of branchial plates, generally five or six in each row, and these plates (some of the lowest excepted) are directed upwards. The pleurobranchiae above mxp.3 and especially trl.3 are less developed, with a lower number of transverse rows, and partially with a lower number of plates in the rows. The pleurobranchial plumes in Petalidium look very different from those in Sergestes ; the real differences are : a much lower number of rows, a much lower number of plates in the rows, and that the plates are much larger, curved upwards, and look much more independent. Behind the upper part of each of these four pleurobranchiae originates a pleurobranchial lamella (/.), |
