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Only one specimen of this peculiar Calanoid was 'observed in all the 'Siboga' material. 

 It was found in a Hensen vertical net collection, from 1500 metres to the surface at Station 203. 



This species is only doubtfully included in the genus Monacilla, as the males were 

 unknown when Professor G. O. Sars described it. The preliminary description of the female, 

 without fio-ures, by that author, is insufficiënt to enable one to decide whether the male now 

 described really belongs to this genus or not. I was at first inclined to regard it as the male 

 of Oxycalanus, but its very small rostrum apparently separates it from that genus. 



5. Family ^Etideidae. 

 Genus jEtideus Brady, 1S83. 



The females of this genus are easily recognised by the very strong and bifurcate rostrum, 

 by the highly arched forehead when seen in lateral view, and by the well pronounced spiniform 

 projections of the last thoracic segment. 



A considerable amount of doubt has arisen as to the identity of the species originallv 

 described by Professor Brady, upon which the genus was founded in 1883. Professor G. O. Sars 

 regards the species described by Brady, to be identical with the Calanoid described by Boeck in 

 1872, as Pseudocalamts armatus. This view is probably partly correct, and will be dealt with later. 



The genus was well represented in the plankton collected during the traverse of the 

 'Siboga', and one had no difficulty in separating the material into the three apparently distinct 

 species described below. 



1. d5.tideusgiesbrech.ti Cleve. Plate IV, Figs. 1 — 13. 



Aitideus armatus Giesbrecht, 1893, p. 213, pis. 2, 14, 36. 



Aitideus armatus T. Scott, 1893 (pars), p. 70. 



Aitideus armatus Giesbrecht & Schmeil, 1898 (pars), p. 31. 



Aitideus armatus Giesbrecht, 1903, p. 200. 



Aitideus armatus Wolfenden, 1903, p. 266. 



Aitideus armatus Thompson & Scott, 1903 (pars), p. 244. 



Aitideus giesbreciiti Cleve, 1904, p. 185. 



Aitideus giesbreciiti Sars, 1905 (a), p. 3. 



Aitideus gitsbrechti Farran, 1908, p. 29. 



This characteristic member of the genus was found in nine of the collections, and was 

 apparently the most common type in the area investigated by the 'Siboga'. The following are 

 the stations where it was noted. 



Stat. 118, 4 specimens. — Stat. 128, 2 specimens. — Stat. 141, 10 specimens. — Stat. 142, 

 5 specimens. -- Stat. 143, 10 specimens. — Stat. 185, 10 specimens. — Stat. 203 (1500 metres 

 to surface), 8 specimens. - - Stat. 220 (vertical net) 1 specimen — Stat. 276, 1 specimen. 



There is no doubt, I think, that this is the species Giesbrecht considered to be identical 

 with Brady's s'Etideus armatus, and is therefore the same Calanoid that was raised to specific 

 rank by Cleve in 1904. It is a very characteristic form and readily recognised. The forehead 



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