758 MR. J. STANLEY QABDiiirER OTf [June 6. 



The polyps of P. ahdita, P. lourpurea, and P. fusco-viridis are 

 crowded with commensal zooxanthellse, and from colonies of these 

 species I succeeded in collecting a certaiQ amount of oxygen^. 

 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. Prionastk^a abdixa BU. & Sol. (Plate XLVII. fig. 4.) 

 Madrepora ahdita, Ellis & Solan der, Zooph. p. 162, pi. 1, fig. 2 



(1786). 



Prionastrcea ahdita, Milne-Edwards & Haime, Cor. ii. p. 514 



(1857). 



Astrcea virens, Dana, Zooph. p. 228, pi. 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 XLVII. 

 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 mm. in diameter by 8*5 mm. in depth. Towards the 

 lower edge the walls of the calices are often 2-3 mm. broad, while 

 the calices vary up to 15 mm. in breadth, but are seldom more 

 than 6 mm. 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 

 shghtly 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 mm. 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 

 trabecule from the septal edges, and is much more compact in the 

 shallower calices. 



Of the other specimens, (b) ia 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 Eeefs of Funafuti, Eotuma, and Fiji, together with some notes 

 on the Structure and Formation of Coral Eeefs in general." Proc, Oamb. 

 Phil. Soc. vol. ix., 1898, 



