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AGASSIZ: FIJI ISLANDS AND CORAL REEFS. 119 
row walls to separate them (Plate 30); and finally, near the outermost 
edge begins the belt of Madrepores and Pocillopores (Plates 28, 29) 
which extends into the line of breakers and beyond. ‘There are few of 
the massive type of corals, like Porites, Astrea, and the like, and these 
are of diminutive size. 
The surface of the reef is kept from too rapid wearing by the growth 
of marine algze and of corallines which carpet it. The alge are mainly 
species of Udotea, of Caulerpa, and of Turbinaria, which in sheltered 
pools grow to a considerable size. One finds also on the surface an occa- 
sional large black Holothurian, a blue Linckia, a green Goniaster, a Mu- 
reena darting from a pool, a large Cancer trying to hide under a shelving 
piece of coral, or a Squilla, or a few specimens of small fish of a brilliant 
blue color. There are few Mollusks under the negro-heads and dead 
masses of coral scattered upon the reef; the lower surface of these is 
often carpeted with many species of brilliantly colored Sponges, in which 
small Mollusks, Crustacea, and Annelids find refuge. The coral masses 
themselves are perforated in all directions by boring Mollusks and 
Annelids. 
The substratum of the eastern and the western reef is made up of 
elevated limestone, extending from the shore close to the outer edge of 
the reef, independently of the thin crust of corals which grows upon the 
outer edge of the reef flat. So it is also with the musbroom-shaped heads 
which are found upon the outer edge of the reef flats of Vatu Leile, Mango, 
and other islands consisting wholly or in part of older limestones. They 
are all composed of the same rock as that of the shores and of the sub- 
structure. The outer sea face of the reef flats is steep in all cases, irre- 
spective of the increase due to the corals growing upon them, or of the 
character of the underlying rock, which is only an unimportant factor 
in determining the thickness of the sea face edge of the outer reef 
flats. 
We also examined the outer face of the barrier reef forming the east- 
ern edge of the passage leading into Suva Harbor. This reef flat, like 
that of the western side of the passage, is entirely made up of ancient 
elevated limestone planed down by the sea to a level flat, with small shal- 
low pools scattered over the surface. Negro-heads and fragments of the 
ancient limestone are scattered over considerable areas on the reef flat ; 
many of them have been thrown up by the sea from the undermining of 
the outer edge, others have been torn off from the reef flat itself, and both 
are gradually wearing away, leaving fragments of branches of corals and 
of coral heads scattered over the reef. The belt of the reef flat close to 
