Nos. 620-621] THE HAWAIIAN CORAL BEEFS 



421 



at varying distances from the shore. In any ease they 

 distinctly indicate, by their horizontal banding of algal 

 and hydroid life, the ranges of high and low tide. The 

 rocks are either of consolidated reef coral or of black 

 basaltic lava; tufa, rocks, and sedimentary coral sand- 

 stone are infrequent. Some groups of marine organisms 

 show strong preference for the coral rock, others for the 

 lava rock. The rocks may be in somewhat protected 

 situation or may be exposed to the full force of the surf. 

 The following genera contain alga? which are representa- 

 tive of the kinds that withstand the constant battering of 

 the waves: Gymnogongrus, Codium, Ealiseris, Aspara- 

 gopsis, Dictyota, Gelid i-um, Ahnfeldtia, Porphyra. The 

 controlling factor in the alga-flora of the partially sub- 

 merged rocks seems to be the circulation of pure, well- 

 oxygenated sea water. Rocks in stagnant or impure 

 water support a scanty flora as compared with those of 

 the surf-swept localities. 



3. Pools.— Beyond the rock litter, although sometimes 

 interspersed by it, lies the zone characterized by numerous 

 pools or pockets. These cup-like depressions in the 

 lagoon floor vary in size from little pockets two or three 

 feet in depth and diameter to large pools twenty or thirty 

 feet in depth and diameter. In wading or paddling over 

 the reef, the pools are easily distinguished by the darker 

 tint of their waters as contrasted with that of the shallow 

 lagoon. These pools in the floor of the lagoon are not to 

 be confused with the tidal pools, that lie along the beaches, 

 and are entirely detached at low tide. The lagoon pools 

 are inhabited by a great variety of alga? and animals that 

 prefer these shadowy havens to the exposure of the 

 shallows or the outer reef. The bottom of the pool may 

 be covered with clear coral sand, or coral debris, or 

 masses of living coral; its alga-flora will depend upon its 

 depth and the resultant intensity of illumination. 



The following are typical alga genera that have repre- 

 sentatives in the lagoon pools: Corallina, Pcysnnnclia, 

 Grateloupia, Ceramium, Amansui, Polysiphonia, Chori- 



