Al 
Six specimens apparently identical with this species, were obtained from the following 
deep water samples of plankton. 
Stat. 141, HENSEN vertical net, 1500 metres to surface, 2 specimens. 
Stat. 143, HENSEN vertical net, 1000 metres to surface, 2 specimens. 
Stat. 148, HENSEN vertical net, 1000 metres to surface, 2 specimens. 
- ~The specimens agree in almost every detail, with the description and figures given by 
Sars. The minor points of difference are: — The external margins of the rostrum are more 
concave, and the excavation between the rami is much narrower than is shewn by Sars’ figure. 
The specimens are also smaller, being 3,5 mm. instead of 4,4 mm. as given by Sars. 
The distribution of this species appears to be very limited. It has hitherto only been 
observed in the North Atlantic Ocean. I have recently been able to compare specimens of the 
northern type, sent by my father, which he obtained in plankton collected in the deep water 
of the Farée Channel, by the Scottish International Fisheries Cruiser ‘Goldseeker’. I do not 
see any material difference between the North Atlantic form, and the ‘Siboga’ specimens. 
Genus Chiridius Giesbrecht, 1893. 
I follow Girsprecut’s definition of this genus, which was established for the reception of a 
Calanoid without a trace of a rostrum, and having the last thoracic segment produced into spines. 
Sars in the first portion of his volume on the Calanoida ‘Crustacea of Norway’, included 
two species possessing a distinct rostrum, under this genus. One of the species, however, was 
removed to the genus Gazdzus, on the publication of the supplementary part of the Calanoida. 
G. P. Farran (1908) follows Sars in including Euchaeta armata Boeck, under the genus 
Chiridius, and says, ‘The fact that GirsprecuT originally defined the genus as not having a 
‘rostrum should not in itself have any weight’. This is surely a mistaken view, and if pursued 
to any extent, would lead to endless confusion in all systems of classification. I regard the 
original definition, so far as the rostrum is concerned, as final, and think that Awchaeta armata 
Boeck, ought rightly to be excluded. WoLFENDEN (1903) has already done this, and has established 
a new genus, Pseudactideus, for its reception, which is the most satisfactory solution. 
Three species, apparently all distinct, agreeing with Gresprecut’s definition, were found 
in the plankton collected during the traverse of the ‘Siboga’. In the absence of males, one has 
considerable doubt of course, as to whether the various forms ought to be regarded as really 
distinct, or only varieties. All the females examined, had the endopodite of the second pair of 
swimming feet composed of a single joint with a faint thickening, indicating where a joint may 
have been at some time. 
1. Chiridius popper Giesbrecht. Plate XI, figs. 10—17. 
Chiridius popper Giesbrecht, 1893, p. 224, pls. 14 & 36. 
Chiridius popper Giesbrecht & Schmeil, 1898, p. 33. 
Chiridius popper Cleve, 1904, p. 187. 
Chiridius popper Sars, 1905(a), p. 3. 
Chiridius popper van Breemen, 1908, p. 34, fig. 36. 
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SIBOGA-EXPEDITIE XXIX@. 6 
