28 MADREPORARIA. 
GEOGRAPHICAL ARRANGEMENT OF THE 
INDO-PACIFIC FORMS. 
Group I.—_POLYNESIA. 
SOCIETY ISLANDS. 
1. Porites Society Islands (gl, (P. Sociorwm prima.) (Pl. I. figs. 1, 2; 
Pl. XIII. fig. 1.) 
[Tahiti,* coll. H.M.S. ‘Challenger’; British Museum. ] 
Syn. Porites arenosa (partim) Quelch (non Esper), Chall. Rep. xvi. (1886) p. 183. 
Description.—The specimens of this coral encrusted a foliate Mycediwm by closely 
enveloping its vertical edges. They adhered as swollen rounded knobs, which tended to rise 
and divide into flat-topped lobes. Their lower encrusting edges were nowhere free. 
The calicles are 1 mm. in diameter. The walls show three variations: (1) on the top, 
where growth is rapid, as a light, open, foaming reticulum, somewhat flaky, but with very large 
irregular pores, some of which seem to run deep into the corallum (fig. 1); (2) on the sides 
and basal parts, the walls are solid looking, their flakes being more pronounced and flattened 
down, with smaller pores, but often showing the same penetration into the corallum (fig. 2) ; 
(3) in valleys between the lobes, where the calicles are smaller; and wherever they are crowded 
together the walls are thin, and seem to form membranous or flat trabecular ridges, often 
arranged in a zigzag (see fig. 1 in the left-hand lower corner), The individual trabecule end as 
frosted knobs. The flat, thick wall (No. 2) sometimes has a very conspicuous ring of large 
frosted granules; where the wall is quite smooth it is possible that the specimen has been 
rubbed, and the frosted granules broken off. The septa are very perforate, and consequently 
irregular along their upper edges; the typical formula is everywhere seen. There are five, six, 
or eight pali, forming the three most usual symmetrical patterns (see Introduction, p. 19, 
tig. 3, F, C, B). The pali frequently rise from a light columellar ring, and there is mostly 
a visible central tubercle. Both pali and tubercle end in frosted knobs, large or small, 
according to the position on the corallum. The interseptal loculi are round and open—except 
in the more solid basal parts of the corallum. 
There are two very fine specimens of this Porites, and a fragment in spirit. The smaller 
* Most probably from the Papeete reefs (see specimen c). 
