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THE VOYAGE OF H.M.S. CHALLENGER. 
typical Rhabdammina abyssorum. The material employed for its construction is a fine 
light-coloured sand of even grain, the general appearance of which, under a magnifying 
power of 60 diameters, is shown in PL XXII. fig. 6. The superficial texture, as displayed 
in this figure, exhibits a strong contrast to that of the comparatively roughly built test of 
Rhabdammina abyssorum, represented in figs. 12 and 13 of the previous plate. The walls, 
though moderately firm, are neither so hard nor so compactly built as those of the 
type, owing to the comparative absence of the highly-coloured cement which binds 
together the coarse sand employed by the latter species. 
The two arms are not always of the same diameter, and they vary a good deal in 
contour in different specimens. Sometimes they taper a little towards the oral 
extremities, but as frequently preserve a nearly even width, and occasionally become 
slightly wider near the ends. The apertures and the test in their immediate 
vicinity, are generally tinged reddish-brown. 
Rhabdammina linearis is known as far north as the mouth of the Hardanger Fiord, 
Norway, 126 fathoms (Mr. Norman’s collection), and has been noted at three other points 
in the North Atlantic, namely, off the west coast of Ireland, 816 fathoms, off Sombrero 
Island, West Indies, 450 fathoms, and off Culebra Island, West Indies, 390 fathoms. It 
occurs at two Stations in the South Atlantic,— off Pernambuco, 675 fathoms, and east of 
Buenos Ayres, 1900 fathoms ; and at two in the South Pacific, — east of New Zealand, 1100 
fathoms, and off Amboyna, 1425 fathoms. 
Rhabdammina cornuta, H. B. Brady (PI. XXII. figs. 11-13). 
Astrorhiza cornuta , Brady, 1879, Quart. Journ. Micr. Sci., vol. xix. N. S.,p. 43, pi. iv. figs. 14, 15. 
Rhabdammina cornuta , Id., 1882, Proc. Boy. Soc. Edin., vol. xi. p. 714. 
Test free, asymmetrical, outspread or rounded ; consisting of a central wider tube 
(sometimes shortened so as to form an irregular inflated chamber), with lateral arms or 
branches at irregular intervals, not radiating from one point ; branches either terminating 
in rounded apertures, or furnished with protruding simple or bifurcating chitinous tubes, 
which serve the same purpose. Walls composed of coarse sand-grains, firmly cemented 
together ; exterior very rough. Length of the larger specimens about jth inch (6 mm.). 
This species was originally assigned, with some reservation, to the genus Astrorhiza, 
owing to the resemblance in contour of the larger specimens to the branching variety of 
Astrorhiza arenaria ; but its minute structure accords so much better with the general 
characters of Rhabdammina that it has been transferred to its present position. In point 
of fact it bears something like the same relation to the typical Rhabdammina abyssorum 
that the branching specimens of Astrorhiza arenaria bear to the regularly stellate 
forms. 
