80 "ALBATEOSS" TROPICAL PACIFIC EXPEDITION. 



platform seems to adapt itself to its conditions. Holothurians are found 

 only in deep pools worn out of the flat or protected by masses of boulders. 

 Every boulder is full of boring annelids, mollusks, Crustacea, and sponges, 

 which one would seek in vain on the expo.sed face of the reef flat, but which 

 are abundant in the quieter waters of the lagoon. Echini, Ophiuran.« and 

 Holothurians, which are buried in deep holes on the sea reef, live quietly 

 exposed to full view on the lagoon side. Of Echini, those with massive spines 

 like Heterocentrotus seem to be able to withstand the force of the surf; and 

 they as well as the peculiarly protected Colobocentrotus, the spines of which 

 are merely a close pavement covering the test and strong enough to with- 

 stand the full shock of the largest rollers breaking on the edge of the reef 

 flat, seem to prefer the sea face of the atoll to the lagoon side, where in fact 

 they do not occur.' 



The outer edge (the sea face) of the reef platform is from one to two feet 

 higher than the general level of the platform itself, and in some places even 

 three feet (PI. 51, fig. 2). This forms a wide channel between the outer Nulli- 

 pore edge and the foot of the beach, and in this rudimentary lagoon a strong 

 current is formed by the surf driving the water on to the platform through 

 the channels and gaps left between the Nullipore knolls. On a wide reef 

 platforui a regular barrier reef with a diminutive lagoon would be formed, 

 as at Anu-Anurunga and Hereheretue. This reef lagoon may be compared 

 to the long narrow sinks formed on the lagoon side of the land rim by 

 the throwing up of sand beaches in a line parallel to the land rim. 



We examined about five miles of the lee side of the lagoon to the south 

 of the village, and found it identical in structure with that westward of 

 the village, patches of beach rock cropping out at short intervals for a 

 coujjle of hundred yards, and again separated by coral sand beaches rising 

 to a height of not more than three or four feet. Here and there we came 

 upon an outcrop of old ledge running at right angles to the beach, en- 

 croaching more or less diagonally upon the face of the beach, or upon some 

 outlier of the old ledge, part of the outer platform of the lagoon. 



The underlying old ledge is generally concealed by beach rock or by 



^ I was greatly surprised to find Echinoneus alive on the outer reef platform, buried in cavities of 

 the rock. 



