1896.] OF THE GENUS SERGESTES. 937 
Several years ago, when trying to determine the very rich 
material of pelagic forms (among them also the type specimens 
of the 15 species described by Kroyer) preserved in the Zoological 
Museum of the University in Copenhagen, I discovered the value 
of numerous species, but I had no mind to write any preliminary 
note on the question. Since then I have not had the time necessary 
for working out a monograph (requiring some hundred figures) ; 
but seeing now that at least during some years I shall be very 
much engaged with other work, while authors continue to describe 
larve as well-established new species, I have thought it convenient 
to write this communication. A monograph will, nevertheless, be 
extremely desirable, for of most species and larval stages new full, 
and accurate descriptions and new figures, much better than the 
existing ones, must be worked out. Many of the described forms 
it is impossible to recognize with certainty without a re-examina- 
tion of the type specimens. A monograph must also be based 
upon the investigation of the collections in the few museums 
which possess rich material of pelagic Crustacea; it will be rather 
toilsome, but very remunerative, as at the present time it is 
scarcely possible within any other group of Decapoda to elucidate 
a large portion of the development of almost two-thirds of the 
species. 
The genus Sergestes is now generally referred to a separate family, 
the Sergestide. To this also the following genera have been 
transferred: Sciacaris, Bate ; Petalidium, Bate; Acetes, H. M.-Edw.; 
and Leucifer, Vaugh. Thomps. On Sciacaris and Petalidium some 
remarks will be communicated in the following pages; the two 
other genera I must omit on this occasion, though much addition 
to our knowledge could be given. Leucifer has been treated at 
great length by Bate, who admits only 2 species, but 4 species are 
preserved in our museums. Of Acetes 2 species are known (one 
of which has not been examined since 1837), but we possess 
6 species, the distinctive characters of which are very curious; it 
is, however, impossible to give a good idea of the species of these 
two genera without a considerable number of figures. 
Before concluding these few remarks I desire to offer my sincere 
thanks to Prof. Dr. K. Brandt (Kiel) and Geheimrath Prof. Dr. R. 
Leuckart (Leipzig), who lent me two type specimens, and espe- 
cially to Geheimrath Prof. Dr. V. Hensen (Kiel), who lent me 
examples of 4 Plankton species, and Prof. Dr. C. Chun (Breslau), 
who, on my request for the loan of type specimens of two species, 
favoured me with his whole finely preserved material collected by 
himself, chiefly with a closure-net, “Schliessnetz,” at the Canary 
Islands and at Ragusa and Lesina in the Adriatic. 
i. The History of the Genus. 
As C. Spence Bate and A. Ortmann, the last two authors who 
have given an apparently but not really complete enumeration of 
the known species, have overlooked several publications, and as 
