968 DK. H. J. HANSEN ON CRUSTACEANS [DeC. 1 , 



times longer and its proximal half considerably more incrassated, 

 near and on the apex with some long setse and without the trace 

 of any chela ; on trl.^ a feeble beginning to a chela is found. The 

 relative length and the structure of mxp.' and trl.'-trl.^ differ 

 very mnch from that found in Serf/estes. The branchia) are very 

 interesting. A rudiment belonging to mxp." I do not dare to 

 interpret : above mxp.' and trl.'-trl.^ a small pleurobranchia and a 

 plate are present ; tlie plate above mxp.^ is a little larger than the 

 branchia, and the plates are much increasing in size from before 

 backwards, so that the plate above trl.^ is 3-4 times larger than 

 the branchia ; above trl.^ a rudimentai'y branchia. The abdomen 

 is rather clumsy, dorsally smooth ; the ext. br. of urp. with the 

 exterior margin naked in c. J-^ of the whole length, as the well- 

 developed spine is situated near the distal end. 



The smallest specimen examined is 4*9 mm. long, and differs 

 from the described stage in several particulars of not much import- 

 ance — a somewhat different shape of the rather short rostrum, 

 a well-developed supra-ocular spine, trl.* and trl." 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 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 ; but we must call to our remembrance that the branchial 

 plates or lamellae may be much altered during the further gi-owth, for 

 instance they may be proportionally much reduced in size (cfr. the 

 curious reduction of the branchial laraelliB in S. henseni (Ortm.) 

 during its development from a Mastic/opus 6*2 mm. in length to 

 the adult form). Unfortunately the legs and the uropods in 

 Petalidium are quite unknown. The species, which must receive 

 the name of P. olesum (Kr.), is decidedly distinct from P.folia- 

 ceum, Bate. 



ix. Geographical and Bathymetrical 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 Q reenland ; 

 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 alUed 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, 



