26 
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 J/onaczlla, as the males were 
unknown when Professor G. O. Sars described it. The preliminary description of the female, 
without figures, by that author, is insufficient 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 A‘TIDEIDAE. 
Genus /Etideus Brady, 1883. 
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 originally 
described by Professor Brapy, upon which the genus was founded in 1883. Professor G. O. Sars 
regards the species described by Brapy, to be identical with the Calanoid described by Borck in 
1872, as Pseudocalanus 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. <tideus gresbrechtz Cleve. Plate 1V, Figs. .1—13. 
Aitideus armatus Giesbrecht, 1893, p. 213, pls. 2, 14, 36. 
“Etideus armatus T. Scott, 1893 (pars), p. 70. 
Aitideus armatus Giesbrecht & Schmeil, 1898 (pars), p. 31. 
Aitideus armatus Giesbrecht, 1903, p. 200. 
Atideus armatus Wolfenden, 1903, p. 266. 
itideus armatus Thompson & Scott, 1903 (pars), p. 244. 
Aitideus gtesbrechtt Cleve, 1904, p. 185. 
Mitideus giesbrechtt Sars, 1905 (a), p. 3. 
tideus giesbrechtt Farran, 1908, p. 20. 
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, I specimen. 
There is no doubt, I think, that this is the species GirsprecuT considered to be identical 
with Brapy’s 4%tideus 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 
36 
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