38 MADREPORARIA. 
cut off by the formation of concentric synapticule which thicken the wall. The septa are 
thin and lamellate, but wavy and bent, though not so much so as to spoil the radial symmetry. 
The pali form complete circles conspicuous to the naked eye (ef. fig. 3, B, p.19). The principal 
pali are often V-shaped plates. The central tubercle is mostly flattened in the directive plane, 
and is supported on radial plates which seem to be usually joined to the fused pairs of septa. 
All the skeletal elements gradually thicken and frost over down the sides of the stock, and 
the pali become very conspicuous as small compact groups rising in the centre of very shallow 
polygonal areas. 
There is again only one specimen. Its growth-form is interesting compared with the 
pear-shaped growths of the preceding two forms, which differ in being attached to the tips of 
branches of other corals. The more important differences relate to the details of calicle 
structure, and in spite of the close superficial resemblances between this, the next, and the two 
preceding forms, can be easily seen on a close comparison of the figures and descriptions. 
Compare the observations under the next heading. 
a. Zool. Dept. 1904, 10. 17. 5. 
12. Porites Tonga Islands qoy7, (Pl. II. figs. 3, 4.) 
{Tongatabu, coll. H.M.S. ‘Challenger’; British Museum.] 
Syn. Porites arenosa (partim) Quelch (non Esper *), Chall. Rep. xvi. (1886) p. 183. 
Description.—The corallum is only a fragment, but is apparently a vertical segment of an 
erect pear-shaped stock, like those of the last three forms. 
The calicles are quite flush with the surface, a little over 1 mm. in diameter, and sub- 
circular, The walls on the top are an open reticulum without median ridge, very uneven in 
thickness, sometimes allowing communication between adjacent calicles, and with the skeletal 
elements uniformly thin and membranous, but so porous as to be almost filamentous. The 
septa are still thinner but slightly more lamellate, and with very ragged tops and edges; they 
are in a complete formula (fig. 3, B). The pali are so minute as to be invisible to the naked 
eye. The central tubercle, which rises from a columellar tangle without a ring, is often 
flattened in the directive plane. All the skeletal elements are slightly echinulate. The pali 
remain invisible even in the calicles near the lower edges of the stock, where all the elements 
are immensely thickened. The vertical section shows a regular radiating expansion of 
trabecule from the centre of the base. The outermost bend outwards and hang down all 
round the base. 
This is the last specimen of this remarkable group, which is united by the similarity in 
growth-form, by the extreme delicacy of texture, and by the general type of calicle structure. 
But the more closely the four corals are examined, the more marked are their differences. We have 
no means of ascertaining what their real affinities are: we can only describe them as so many 
* For P. arenosa see the forms from unknown localities, Part II. 
