THE PERCY SLADEN TRUST EXPEDITION. 97 
small specimen of Goniolithon Fosliei attached to a piece of coral, together with Litho- 
thamnion simulans, small and not well-developed (partly uncertain) crusts of Goniolithon 
myriocarpon, Lithophyllum Yendoi f. mahéica, and a very young specimen of Zitho- 
phyllum affine (i.e. L. Kaiseri (pallescens), cf. below). 
On comparing the area in question with other parts of the Indian Ocean we find a 
close correspondence particularly with the Maldives, the only area in this ocean which 
was formerly comparatively well known, owing to the collections made by Mr. Stanley 
Gardiner in the years 1899-1900. "This correspondence chiefly concerns the three reef- 
building species mentioned above, viz., Lithophyllum onkodes, L. craspedium, and 
Goniolithon frutescens—the fourth species, Lithophyllum Gardineri, having hitherto 
only been found in the Chagos and at Coetivy. 
As is observed below, the forms of Goniolithon frutescens occurring in the area in 
question are more consistent with forms of the same species from the Maldives than the 
forms of the species occurring at the Ellice Islands (Funafuti) in the Pacific Ocean. 
The same is the case with Lithophyllum onkodes and L. craspedium, which are also in 
structure fully consistent with specimens of the same species from the Maldives, whereas, 
in this respect particularly, the specimens from the area in question and some of those 
collected at the Maldives are in part a little diverging from the specimens picked up at 
the Ellice Islands (cf. Lithoph. Mald. & Laccad. p. 468). 
Thus in rather great areas of the Indian Ocean these three or four species are shown to 
be important reef-builders in the littoral region and in the uppermost part of the sublittoral 
region. Besides, also Lithophyllum Kaiseri ( pallescens) and other species here and there 
contribute to the formation of reefs, but apparently in far less degree than the species 
mentioned. Farther down in depths of up to about 60 fathoms, Lithothamnion indicum 
as well as L. australe seem to be rather essential builders of new strata in places suitable 
for them, partly alone, partly gregariously, and here and there associated with other 
species in smaller number. | 
To judge from our present knowledge of the Lithothamnia within the tropics, it seems 
that in places where these algz occur in such abundance and so large masses that atolls 
With surface-reefs are practically entirely formed by them, as, e. g., in the Chagos, or in 
Places where they are of essential importance for the consolidation of corals into true 
reefs the number of species is frequently small, but each of them appearing in an 
enormous number of individuals. Besides in the Chagos, this is the case at Coetivy, at 
certain places in the Maldives, and at the Ellice Islands (Funafuti), as well as the Gilbert 
Islands in the Pacific Ocean. In banks which are formed of freely-developed branching 
Specimens in the lowermost part of the littoral region, as, e. g., at Haingsisi in the Malay 
Archipelago *, or, more frequently, at different depths in the sublittoral region, one species 
will predominate +. On the other hand, in places where the Lithothamnia do not appear 
in such quantities, on exposed as well as on protected and suitable localities, a larger 
Variety of species is often found. 
* Cf. A. Weber van Bosse in ‘Siboga’ Exp. n. lxi. livr. 18. p. 4. 
T In boreal or arctie areas such banks are often formed of only one species. 
