POLYNESIAN PORITES. 71 
they are variable in size, the larger being 1‘5 mm. in diameter. The wall-ridge is con- 
spicuous, thin, smooth, and very wavy, sometimes zigzag; seen sideways, it is membranous 
and porous, and with ragged edges. The inner synapticular wall is seldom complete, and 
mostly fills in the angular spaces between the circular septal system and the more polygonal 
wall-ridges, but these spaces are not so marked as they would be if the ridges were straighter 
and less flexible. When the inner wall is present its skeletal elements are thin and smooth, 
and with the wall-ridge make a very open large-meshed wall-reticulum. The septa slope 
downwards to make funnel-shaped depressions, and are exceptionally thin and very delicately 
echinulate; they do not always fuse, and consequently the pali, which are delicate jagged 
points, are not well developed. The fossa is fairly deep; its base is occupied by a rather 
close columellar tangle, mostly without central tubercle except in the lateral calicles, in 
which the skeletal elements are coarser and the pali better developed. 
The single specimen of this beautiful form is an oval mass 10 cm. long, 6 cm. across, 
and 6 cm. high. It appears to have become detached owing to the presence of a sponge which 
has burrowed through its substance and made the base rotten. The sponge apparently 
follows through the whole mass close under the living colony, and many oscula open on its 
surface, and always through an individual calicle, that is, never destroying a wall. There 
can hardly be a doubt but that the specimen is closely related, for instance, to No. 5, but 
a comparison of the photographs will show how striking are the differences, which may be due 
to the stimulus of the sponge burrowing beneath the living layer. 
a. Zool. Dept. 1904. 10. 17. 29. 
There is another specimen, b, from the same locality, the calicles of which, in spite of the 
difference in the wall-ridges, have the same sloping septa and of the same character, and on 
this account I propose to class it with this. It may be described as follows :— 
(Pl. XIII. fig. 18.) 
Syn. Porites arenosa (partim) Gardiner (non Esper), Proc. Zool. Soc. (1898) p. 272. 
Description —The corallum is massive, with smooth round top. The lower edges are 
closely adherent, and either bent under or outwards in thin sheets, which encrust and form 
a solid mass with the remains of other corals. 
The calicles vary greatly in size—the largest are 2 mm., the smallest, which occur in 
groups in the slight concavities of the encrusting sheets, are often under 0°75 mm. The 
median ridge is as conspicuous and a little thicker than, but not quite so high as in a; 
it is straight, smooth, and stout; seen from the side, it is a nearly solid membranous 
ridge, level-topped or slightly jagged. The inner synapticular wall is nowhere regularly 
developed, it is most complete in the largest calicles, but for the most part it simply fills 
in the spaces between the angles of the polygonal ridges and the circular septal system. 
When developed it consists of the same smooth skeletal tissue as the wall-ridge. The intra- 
calicular skeleton is like that of a. 
b. Zool. Dept. 1905. 1. 19. 18. 
