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Show 1900.] MARINE F A U N A OE CHRISTMAS ISLAND. 139 Spicules. Strongyles 126 x 8 p, slightly curved in the middle. Oxea 108 x 2*5 p, curved at the centre; also very slender oxea of the same length and shape, probably young forms of the thicker kind. The specimen encrusts a Melina-shel], and is produced at one point into a short, stout, digitate process. There is in the British Museum Collection an unnamed specimen (registered 82.10.17.246) of this species from Marie Louise Island, Ami ran te Group. The species from Providence Island described by Ridley (11. p. 607, pi. liv. fig. i) as " Reniera sp. allied to crateriformis " has spicules of the same shape, but much larger (480 x 2 8 p), and the meshes of the reticulum are multispiculate. Distribution. Amirante Isles ; Christmas Island. PETROSIA EXIGUA, sp. n. (Plate XII. fig. 7 ; Plate XIII. fig. 4.) Sponge forming a hard, thick, nodulated crust. Colour pale grey; surface smooth, and in parts showing an irregular reticulate pattern formed by pore-areas. Oscules 1 to 1*5 m m . in diameter, numerous, some level with surface, others with slightly raised margin. Skeleton formed of slender main lines of fibres passing vertically to the surface and connected at right angles to this plane by closely packed single spicules, so as to form circular or obscurely polygonal tubes about 70 m m . in diameter, the skeletal tubes being much more apparent near the surface and very ill-defined deeper, where the skeleton becomes a dense, confused network. Special dermal skeleton absent. Spicules. Oxea 114 x 5*5 p, curved at the centre, and diminishing to very sharp points. The single specimen is 4 x 4 c.c. in area, and l-5 c.c. in thickness, The salient character of this species lies in the very small size of the spicules, which are less than half the size of those of Petrosia similis Ridley & Dendy (13. p. 9, pi. ii. fig. 10, pi. iii. figs. 3 & 4), a species closely allied to it in other respects ; the spicules are considerably smaller than those of any species with oxeote spicules from this region of the Indo-Pacific. HALICHONDRIA SOLIDA Ridley & Dendy. 1886-7. Halichondria solida Ridley & Dendy (12. p. 326, and 13. p. 4, pi. ii. fig. 5). The specimen, which is white and with an even surface, differs slightly from the type in having the ends ot the oxea sharp-pointed ; the spicules (770 x 22 u) are curved at the centre. Distribution. Amboina; Tahiti ; Christmas Island. HALICHONDRIA SOLIDA var. RUGOSA Ridley & Dendy. The specimen is dark brown and wrinkled in places, as in the type of the variety from Api. There are several oscules, from 1 to 4 mm. across, with conspicuous membranous sphincters. The |