206 MADREPORARIA. 
whether fixation to the pearl shell in any way modifies the characters, nor whether local races 
always attached to these shells may not have come into existence. We have then nothing 
tc do but to group the specimens according to their resemblances. They show three clear 
variations. 
206. Porites Ceylon (g2)10, (P. Ceylonica decima.) (Pl. XXXI. fig. 2.) 
[Ramesvaram, Pearl Bank, coll. E. Thurston ; British Museum. ] 
Description.—The corallum completely covers the shell, often, indeed, overhangs it with 
steep, bulging sides. It is 2-3 cm. thick, and forms a smooth or faintly wavy convex mass, 
often with one long, blunt ridge running across it, but not through the centre. 
The calicles are polygonal, nearly flush with the surface, faintly pitted, slightly over 
1 mm. in diameter. The walls are either thick, continuous, wavy threads, or here and there 
tending to divide up into the irregular, concentrically flattened tips of the trabeculae. Within 
this is an inner ring, which may be either a thick thread separated by large pores from the 
true wall-thread, or else flaky, showing all degrees of flakiness until down the steep sides of the 
stock the whole wall may consist of broad, flat flakes. The septa appear short when starting 
from the synapticular ring, but most of them come from the wall, and they all join a large, 
nearly complete, columellar ring round the fossa. They are smooth and symmetrically 
arranged, but angular and bent. ‘There is mostly a ring of septal granules rising to the height 
of the wall from the points where the septa cross the synapticular ring, and also a ring of 
pali not quite so high, and very frequently only the five principals. They are angular granules, 
and may be conspicuous to the naked eye. The fossa is either very deep, or the columellar 
ring fills up with cross bars, from which a small flattened tubercle may arise. The columellar 
ring, the synapticular ring, and the wall form three concentric rings, the first-named being 
most nearly round. ‘The interseptal loculi are small, and with uneven edges. 
The colour is a warm, dark buff. 
There are three specimens, having calicle skeletons of the same general construction, 
which seems to show that they are closely allied. Specimen a has thick, thread-like walls 
and thread-like synapticular ring, with open fossa, though here and there the wall is 
thinner, and tends to divide into trabecular points. Very thin, discontinuous walls, with 
inner wall breaking up into flakes or knobs, is the characteristic of specimen 6. In ¢ the wall- 
thread seems to be thicker, and tends, with the synapticular wall, to be a stout, regular 
reticulum with regular oval pores. On all the specimens the tendency of the synapticular walls 
to be broad, flaky shelves, is seen on the steep sides. 
a, 6, c (in box). Zool. Dept. 90. 6. 12. 15. 
In parts of a, where the wall points have been broken off, they can be seen hollowed out 
by an alga. This presence of the alga on the very tips of the skeleton nearest to the surface 
has already been noted. 
