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 sete 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 much from that found in Sergestes. The branchiz 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 ; the 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 rudimentary branchia. The abdomen 
is rather clumsy, dorsally smooth; the ext. br. of urp. with the 
exterior margin naked in e. 4} 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 
branchiz 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 Jamellz 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 lamelle in S. henseni (Ortm.) 
during its development from a Mastigopus 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. obesum (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 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, 
