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Show 758 MR. J. STANLEY GARDINER ON [June 6. The polyps of P. abdita, P. purpurea, and P. fusco-viridis are crowded with commensal zooxanthellae, and from colonies of these species I succeeded in collecting a certain amount of oxygen1. These three species live on the extreme breaking edge of the reef and are exposed at spring-tides for 2 or 3 hours to the sun, though constantly wetted by the spray. They form also large spreading masses as deep as can be seen outside the reef. 1. PRIONASTRCEA ABDITA Ell. & Sol. (Plate XLVH. fig. 4.) Madrepora abdita, Ellis & Solander, Zooph. p. 162, pl. 1. fig. 2 (1786). Prionastrcea abdita, Milne-Edwards & Haime, Cor. ii. p. 514 (1857). Astrcea virens, Dana, Zooph. p. 228, pl. xi. fig. 8 (1848). Prionastrcea profundicella, Milne-Edwards & Haime, Ann. des Sc. Nat. ser. 3, xii. p. 131 (1850), and Cor. ii. p. 131 (1837). I have referred six specimens to this species. All were obtained from the same position on the reef and had their polyps uniformly coloured olive-green. Of these the largest specimen (a) (Pl. X L V I I . fig. 4) appears to be the edge of a massive colony which has been killed in the centre. The specimen is much thicker towards its upper edge, and here its surface is rather irregular, tending to form blunt lobes. The under surface, where visible, is covered with a thick epitheca. Towards the upper edge and on the lobes the calices are more or less polygonal with extremely thin walls, and vary in size up to about 11 m m . in diameter by 8*5 m m . in depth. Towards the lower edge the walls of tbe calices are often 2-3 m m . broad, while the calices vary up to 15 m m . in breadth, but are seldom more than 6 m m . deep. The septa do not vary much in the different calices, from 30-40 generally being present. Of these about 18 are larger than the rest, subequal in size, and fuse with the columella. The septa are usually continuous from calice to calice over the walls, but are very narrow at their upper ends and only slightly exsert. All end at their edges in large, sharp, pointed teeth, which vary enormously, but commonly are much longer in the deeper calices, where the septa merge into the columella. In the shallower calices the larger septa end in large, vertically projecting teeth, which form a very distinct corona round the axial fossa. The columella is situated about 2 m m . below the top of the corona in the shallower calices, and is markedly oval in shape. It is in all the calices formed by an anastomosing mass of trabeculae from the septal edges, and is much more compact in the shallower calices. Of the other specimens, (b) is a small nodule with deep thin-walled calices. The columella is much larger and more compact than in the deep calices of (a), and the general facies of the 1 " The Coral Eeefa of Funafuti, Eotuma, and Fiji, together with some notes on the Structure and Formation of Coral Beefs in general." Proc. Oamb Phil. Soc. vol. ix., 1898. |