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Show 968 DR. H. J. HANSEN ON CRUSTACEANS [Dec. 1, times longer and its proximal half considerably more incrassated, near and on the apex with some long setae and without the trace of any chela ; on trl.2 a feeble beginning to a chela is found. The relative length and the structure of mxp.3 and trl.'-trl.3 differ very much from that found in Sergestes. The branchiae are very interesting. A rudiment belonging to mxp.2 I do not dare to interpret: above mxp.3 and trL'-trl.8 a small pleurobranchia and a plate are present; the plate above mxp.3 is a little larger than the branchia, and the plates are much increasing in size from before backwards, so that the plate above trl.3 is 3-4 times larger than the branchia ; above trl.4 a rudimentary branchia. The abdomen is rather clumsy, dorsally smooth; the ext. br. of urp. with the exterior margin naked in c. |i of the whole length, as the well-developed spine is situated near the distal end. The smallest specimen examined is 4*9 m m . long, and differs from the described stage in several particulars of not much importance- a somewhat different shape of the rather short rostrum, a well-developed supra-ocular spine, trl.4 and trl.5 only buds, the branchiae not yet developed, a short dorsal spine on the fifth and sixth abdominal segments, the spine on the ext. br. of urp. still nearer to the apex, &c. It is easily seen that this species cannot remain in tbe genus Sergestes, but whether it should be referred to Petalidlum, 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 Petalldlum, and therefore I provisionally transfer it to that genus ; but we must call to our remembrance that the branchial plates or lamellae may be much altered during the further growth, for instance they may be proportionally much reduced in size (cfr. the curious reduction of the branchial lamellae in S. henseni (Ortm.) during its development from a Mastigopus 6-2 m m . in length to the adult form). Dnfortunately the legs and the uropods in Petalldlum are quite unknown. The species, which must receive the name of P. obesum (Kr.), is decidedly distinct from P.folla-ceum, Bate. ix. Geographical and Bathymetrlcal Distribution. With one single exception all the species of Sergestes are only found in the tropical and subtropical seas, in the Atlantic reaching northward about to lat. 42°-43° N . The exception is S. arcticus, Kr., which ranges to the seas at the southern part of Greenland; but being distributed to the Mediterranean, and even to lat. 38° S., it is in reality no arctic species but a deep-sea form, with the centre of distribution in all probability towards the northern tropic or the Equator, and notwithstanding going c. 20° more northward than the other allied species. The limits of the geographical range of the species are still very imperfectly known. Above I have mentioned that some of Bate's localities for S. atlanticus, M.-Edw., were uncertain, and that Bate's, |