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hooked, spiniform projection of the forehead when viewed from the side. It is not always 

 possible, however, to make out the hooked projection of the forehead. It is easily telescoped 

 and some care is necessary in identification. 



Haloptilus spiniccps has apparently hitherto only been recorded from the Mediterranean 

 and from the Atlantic Ocean. 



16. Family Arietellidae. 

 Genus ArietelluS Giesbrecht, 1893. 



The members of this genus can be recognised by the rather tumid body, and by the 

 characters of the fifth pair of feet of both sexes. 



The female fifth pair is represented by two free joints attached to a basal part. The 

 male fifth pair is well developed and prehensile. The exopodite of each foot is three-jointed 

 and asymmetrical. The last joint of the right exopodite is lamelliform, and the apex is rounded. 

 The last joint of the left exopodite bears two apical claw-like spines. The endopodite of each 

 foot is lamelliform, asymmetrical and apparently two-jointed. The basiopodite is two-jointed. 



The characters that separate the various members of the genus depend almost entirely 

 on the form of the body, the length of the frontal spine, the shape of the last thoracic segment, 

 and the length of the furcal joints. Obvious differences can be noticed, however, in the structure 

 of the fifth pair of feet of both sexes. 



Three species were obtained from the plankton collected by the 'Siboga' in the Malay 

 Archipelago. 



1. Arietellus setosus Giesbrecht. Plate XLIV, figs. 8 — 13. 



Arietellus setosus Giesbrecht, 1893, p. 415, pis. 29 & 39. 



Arietellus setosus Giesbrecht & Schmeil, 1898, p. 124. 



Arietellus setosus Sars, 1905 (£), p. 5. 



Arietellus setosus Esterly, 1905, p. 1S9, fig. 42. 



Arietellus setosus van Breemen, 1908, p. 139, fig. 158. 



One adult and one immature male belonging to this species, were found in the plankton 

 collected with the Hensen vertical net at the following stations. 



Stat. 141 (1500 metres to surface). — Stat. 203 (1500 metres to surface). 



Length of adult male 4 mm. 



Arietellus setosus can readily be separated from the other members of the genus by 

 the short and slightly hooked frontal spine, and by the strong spiniform projections of the last 

 thoracic segment. The apical spines on the exopodite of the left fifth foot of the male are of 

 moderate length. The outer one is longer than the third joint. It has a distinct angular bend 

 near the middle. Esterly's figure of the female fifth foot, shews that the second free joint is 

 nearly as long as the first joint. The terminal spine is as long as the second joint. In the 



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