DEPARTMENT OF MARINE BIOLOGY. 183 



gory, three were selected for study and comparison, viz, those at Mount 

 Tahara and Arue in the Arue district and one near Nivee in the Papenu 

 district. Of the latter, the very interesting combination reef near Atiue in 

 the Punaauia district was chosen. 



The northwest point of Tahara Mountain projects, above and below sea- 

 level, cliffed and bare of reef, showing only a fairly broad wave-bench beaten 

 by the waves at times of high water. Around the point on the southwest 

 side of the upper cliffs, a small, more or less triangular reef has been formed 

 of beach-rock, made up of lava pebbles and sand, cemented together chiefly 

 by the incrusting nullipores. From this conglomerate beach-rock the reef 

 is extending seaward by a nullipore-coral rim forming a projecting shelf -like 

 belt and followed shorewards by a Sargassum belt and by a Turbinaria belt of 

 some width. These belts, and even the reefs farther in, are honeycombed 

 below, so that the water spurts up from surface openings many yards in 

 from the reef rim. This seems a good example of an incipient reef. It is 

 the simplest reef observed in Tahiti. 



The reefs along the "iron-bound" coast of Papenu near Nivee are similar to 

 that of Tahara, but more extensive. They show the same construction of a 

 beginning of beach-rock extended by an overhanging nullipore-coral rim. 

 They have been formed in an area inclosed between projecting points of colum- 

 nar lava-rock. 



Out from the village of Arue is an isolated quadrangular reeflet and not 

 far from the small triangular reef at Tahara Mountain. It projects from the 

 midst of a beach of coarse sand, from which it is separated by a shallow, 

 narrow channel. In structure and growth the Arue reef is like those at 

 Tahara Mountain and Nivee. It is larger, not connected, so far as now visible, 

 with any rock mass, and seemingly independent. It is bordered oceanward 

 by a well-developed overhanging nullipore belt, within which is a Sargassum 

 belt, a Turbinaria belt, a belt of short, fuzzy algae, and finally the center is bar- 

 ren and broken up into irregular crests of lines of beach-lava pebbles incorpo- 

 rated within the reef-rock, forming a reef conglomerate. 



These three reefs seem to have been built up by the cementing action of 

 nullipores (lithothamnia) forming a core of pebbles, and sand, shells, and 

 possibly even corals agglomerated together, and are now extending through 

 the agency of the nullipores and corals. The outward face of these exposed 

 incipient fringing reefs shows some resemblance to that of the barrier reefs, 

 but the nullipore belt overhangs a steep slope in these reefs, while, on the 

 barrier reef, the nullipore belt slopes gently both ways from the ridge-line 

 and does not in any way overhang. 



The barrier reefs of the coast of Tahiti are usually separated from the 

 shore by a lagoon moat whose depth normally approaches 18 to 20 fathoms. 

 The inner side of the lagoon moat is formed by a fringing reef (not of the 

 exposed type), whose lagoon-moat side is vertical, at least above. In some 

 cases the lagoon-moat, while yet distinct, is less than 18 fathoms, occasionally 

 only 3 to 4 fathoms in depth. Sometimes there is no distinct lagoon-moat 

 discernible, at least in spots, and there seems to be a fusion of fringing and 

 barrier reefs. Such a case was examined near the village of Atiue, in the 

 Punaauia district. The reef lies north of the pass which runs in toward the 

 mouth of the Punaruu River. The outer portion of the reef is clearly barrier 



