15(; PROF. T. W. EDGEWOETH DAYID, .MR. G. H. HALLIGAN, AND MR. A. E. FIXCKH. 



Of the encrusting types of foraniinifera, Polytrema plamun and P. miniaceum 

 are abundant, as well as several species of Carpenteria. Polytrema planum in 

 particular is quite an important rock builder on the ocean submarine slope of the 

 atoll, as described by Mr. F. Chapman,* 



Mr. CHAPMAxf aptly compares the large white flaky masses of Polytrema p/rt/»r/7?, 

 many specimens of which we dredged from the ocean slope, to the nest of the AA^asp 

 (Vespa). Much of the frothy white material (like spongy plaster-of-paris), AA'hich 

 plays such an imjDortant part in cementing the sand and fine rubble on the ocean slope 

 of the reef down to at least 200 fathoms, has noAv been proved by Mr, Chapman to be 

 Polytrema planum. At the time Ave dredged it Ave mistook it for dead Lithothamnioii. 

 A certain amount of living Litliothamnioa Avas in most cases associated with it. 



In addition to the Cydoclypens, another organism of l)athymetric value is the 

 pteropod. This was sufficiently abui)dant to form a deposit almost amounting to a 

 pteropod ooze at a depth of about 200 fathoms, and occurred in some abundance at 

 a rather less depth, being met Avith in our dredgings from depths of 80 fathoms doAvn 

 to 200 fathoms.j In this case also it might be argued that an absence of pteropod 

 shells from the core at a depth as great as 186 fathoms (the maxinunn depth attained 

 l)y the bore) argues subsidence. With a stationary or rising floor it must be 

 assumed that, if the reef rock of the core Avas formed on the ocean side of the atoll, 

 the pteropods Avere neA-er able to occupy this in numbers sufficient to produce 

 pteropod ooze, and if on the lagoon side, that this part of it had originally the 

 extraordinary depth of 186 fathoms, both of AA'hich are very improbable hypotheses. 



(2) As regards the distribution of LitJiofJianinion, Avhile on tlie Avestern side of 

 the atoll branching and knobby varieties are extremely abundant in the shalloAV 

 Avater from the level of low-AA'ater spring tide to a few feet below tliat level, the 

 encrusting type of Lithotliamnion, Avliich is of universal distribution around the 

 ocean face of the atoll, lives from low-AA^ater spring tides doAvn to at least 200 fathoms. 

 As already stated, it is the great cementing agent wliicli binds together into a 

 compact rock the sand and rubble of tlie ocean slope of the I'eef as well as that of 

 the shores of the lagoon beloAv low Avater. Its red colouring matter (pliycoerythrin ?), 

 as FoL and Sarasin and others liaAe sliown, no doubt ena])les it to liA^e at depths 

 too great for tlie red or yelloAv rays of the solar spectnnn to ])enetrate, and far 

 below the zone of green plants, like Halimeda. Altliougli tliis encrusting Litho- 

 tliamnion is a slow groAA^er, still it Avould be hard to over-estimate its imjjortance as 

 a former of reef rock. The question here suggests itself, is there any evidence derived 

 from an examination of the Lithothamnion of the sul)marine cliff AA'hicli Avould enable 



depth than .30 fathoms, the same species Avas found (jff tlie Tonga Island at 20 fathoms, and on a piece of 

 the reef coral Tvrhinnria from the Mauritius a specimen Avas found, which presunialily liA'cd at the same 

 depth as the coral." — Ed.] 



* 'Linnean Soc. Journ.,' Zoology, vol. 28 (1900-03), pp. 387-39G. 



t Ihvl, p. 391. 



+ Australian Museum, Sydney. Memoir ITT. " Tlie Atoll of Funafuti." By C. Hedlka', p. .549. 



