190 MADREPORARIA., 
gradually and forking at distances of 6 em. apart. The spikes are 2-3 cm. thick near their 
bases, where they freely fuse together. The living layer may extend some 25 cm. from tip 
to base, where it may bend out as a free explanate sheet. 
The calicles are 1 mm. in diameter at the tips; for 1 em. down the sides they are hardly 
distinguishable, as they open in a pronounced flaky, porous reticulum, the flakes or filaments 
being everywhere smooth and chalky. A few larger pores, however, can be seen to represent 
fossee. Below this, irregular low walls appear which are merely narrow raised portions of the 
flaky reticulum, with the flakes sometimes bent upwards, sometimes expanding horizontally, 
all the flakes with large round pores. Round the bases of the stems, the walls again tend to 
flatten down. The septa are very irregular, most frequently as broad flakes near the wall, but 
centrally tapering to thin points, which meet in twos and threes, uniting with a ring or with 
the parts of a ring, which runs round the fossa. A few smooth, slightly raised points occur 
where the septa meet, and represent pali. The interseptal loculi, sometimes oval, are mostly 
pointed near the wall. The fossa is either very deep and open or early filled with flaky 
reticulum from which an irregular central point rises. 
The section shows an axis of very pronounced flaky reticulum round which both radial 
and concentric elements can be seen, but both of them are so flaky that they form together an 
alveolar tissue, the chambers being arranged in radial and concentric systems separated by 
smooth oval pores. 
There is one large bleached specimen. It again shows the upgrowth of vertical lamellate 
trabecule which, in this case, run out into sharp spikes of great length. Again, also as in 
P. Singapore 7, the lamellate character of the vertical trabeculee has been carried over both 
to the lateral radial trabecule and to the horizontal or concentric elements. 
a. Zool. Dept. 83. 7. 24. 78. 
Group IV.—_INDIAN OCEAN, 
193. Porites Christmas Island (41. (P. Natalis prima.) (Pl. XXVIII. fig. 9.) 
[Flying-fish Cove Flat Reef, coll. C. W. Andrews; British Museum. ] 
Description.—The corallum is apparently massive, with wavy surface. [The only 
specimen is a small chip. | 
The calicles are small, 0°75 to 1 mm. in diameter, shallow but sharply sunk. The walls 
are variable, here thick and reticular, there thin and membranous ; in the latter case, straight 
and not zigzag. The component elements are extremely delicate and thin ; the reticulum is 
