1896.] OF THE GEITOS SBEGBSTES. 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 huudred figures) ; 

 but seeing now that at least during some years I shall be very 

 much engaged with other worlc, while authors continue to describe 

 larvaj 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 jielagic Crustacea ; it will be rather 

 toilsome, but very roniuncrative, as at tlio 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 Scrgestes is nowgenerallj' referred to a separate family, 

 the Sergestidce. To this also the following genera have been 

 transferred]: Sciacaris, Bate ; Petalidium, Bate: Acetes, H. M.-Edw. ; 

 and Leiwifei; Vaugh. Thoraps. On Sciacaris and PeUdidium 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 tliese few remarlcs I desire to offer my sincere 

 thanks to Prof. Dr. K. Brandt (Kiel) and Q eheimrath Prof. Br. 11. 

 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 Eagusa and Lesina in the Adriatic. 



ii. The Ilisionj of the Genus. 



As 0. 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 



