24 CORAL-REEFS. 



increase of depth at some points, and the circumstance 

 of the line having been cut, as if rubbed, when between 

 500 and 600 fathoms were out, indicate the probable 

 existence of submarine cliffs. 



On the margin of the reefs, close within the line where 

 the upper surface of the Pontes and of the Millepora is 

 dead, three species of Nullipora flourish. One grows in 

 thin sheets, like a lichen on old trees ; the second in stony 

 knobs, as thick as a man's finger, radiating from a common 

 centre ; and the third, which is less common, in a moss-like 

 reticulation of thin, but perfectly rigid branches. 1 The 

 three species occur either separately or mingled together; 

 and they form by their successive growth a layer two or 

 three feet in thickness, which in some cases is hard, but 

 where formed of the lichen-like kind, readily yields an 

 impression to the hammer: the surface is of a reddish 

 colour. These Nulliporae, although able to exist above 

 the limit of true corals, seem to require to be bathed during 

 the greater part of each tide by breaking water, for they are 

 not found in any abundance in the protected hollows on the 

 back part of the reef, where they might be immersed either 

 during the whole or an equal proportional time of each tide. 

 It is remarkable that organic productions of such extreme 

 simplicity, for the Nulliporae 2 undoubtedly belong to one of 



1 This last species is of a beautiful bright peach-blossom colour. Its 

 branches are about as thick as crow-quills ; they are slightly flattened 

 and knobbe 1 at the extremities. The extremities only are alive and 

 brightly coloured. The two other species are of a dirty purplish-white. 

 The second species is extremely hard ; its short knob-like, branches are 

 cylindrical, and do not grow thicker at their extremities. 



2 The xTullipores belong to the Corallinaceae, a group of the sub-class 

 Carpophyccce and the class Algse. Their distinctive characteristic is 

 the encrustation of the thalli with calcium carbonate, hence their 

 re?emblanc2 to the true corals. — Ed. 



