Yol. 62.] HALIMEDA AS A REEF-FORMING ORGANISM. 



'09 



Agardh ( = H. macroloba, Harvey non Decne.). The measurements 

 of the peripheral cells also compare more closely with the latter 

 species, as they average 36 fx in diameter. 



There are a fair number of gastropoda and some occasional fora- 

 miniferal shells. Noteworthy among the latter is a large form 

 of Carpentaria, calling to mind C. rhaphidodendron, although more 

 depressed in its habit of growth. One of the joints of Halimeda is 

 completely surrounded by an encrusting Gypsina, like G. inlia y rens 

 (Schultze), but more regular, and with smaller chambers. 



The nature of the calcareous cement in this rock is very interesting. 

 Both calcite and aragonite are represented, and are found respectively 

 in optical continuity upon the prismatic or other crystalline structure 

 of the original organisms. Thus the Halimeda exhibits a growth or 

 fringe (in sectional view) of acicular aragonite-crystals, as do also the 

 shells of gastropods, while the Carpentaria and lamellibranch-shells 

 are coated with calcite. A secondary inhlliDg of the interspaces 

 between the organic particles, where it has taken place, is in the 

 form of a calcitic mosaic. The microscopical appearance of the 

 cementing-material which binds the constituents of the above 

 limestone is comparable in many respects with that seen in fig. 28 of 

 Dr. Cullis's ' Report on the Mineralogical Changes observed in the 

 Cores of the Funafuti Borings.' x (PI. LI, fig. 1.) 



The following analysis was made on selected material, not on 

 bulk, as it was desired to arrive at the composition of the 

 Halimeda- Qon<$titxi%rLt, uncontaminated by infiltered extraneous 

 matter. The joints were, therefore, separately detached, and 

 carefully scraped free of surface-stain before powdering. 



H (below 110° C.) "0-34 



HlO (above 110°)....; 1-66 



Insol. +Si0 2 006 



Al o 3 -f-Fe.,0 3 (faint trace MuO) 044 



Cat) : 54-44 



MgO 0-43 



CO. 4239 



P 2 0. 025 



Alkalies slight trace 



Sulphate nil 



Chloride nil 



Organic matter nil 



100-01 



H (below 110° C.) 0-34 



H^O (above 110° C.) 1-66 



Insol. +Si0 o 0-06 



Al 2 3 +Fe,0 3 o-44 



Calcium-carbonate 95'35 



Magnesium-carbonate 0-89 1 



Tribasic calcium-phosphate . . 0'55 

 Lime (probably mineral com- 

 bination) 0'72 



100-01 



IV. Concluding Observations. 



That which apparently detracts from the importance of Halimeda 

 as a reef-forming agent is its greater readiness to decay, as compared 

 with Lithothamnion, corals, or foraminifera. That this decay is 

 more apparent than real, may be recognized through the observations 

 made by the members of the Funafuti Exploring Expedition. The 

 borings into the lagoon proved that the Halimeda retained its 



1 ' The Atoll of Funafuti ' 1904, p. 397. 



