NULLIPORA. 239 



This coral, which is of a stcny hardness and solidity, occurs in 

 irregularly branched nodulous masses, adhering at first by a small 

 circumscribed basis, but becoming readily detached, and hence 

 generally found free or loose. " It is of very different colours, 

 as red, yellow, greenish, ash-coloured, but seldom white." " It 

 is often shaped liked the kernel of a walnut, often in larger com- 

 pressed masses, sometimes like a bunch of very small grapes, 

 most frequently in tuberculated branches. It is foimd from 

 one inch to three inches long. When it is taken out of the 

 sea, it is covered with a slime. The pores on the surface are 

 very small, so that to see them it is necessary to wipe the shme 

 very clean off, and to use a large magnifier." Ellis. 



I have received from my Ii'ish friends two varieties which de- 

 serve to be distinguished. The one forms circular deeply lob- 

 ed crustaceous masses adhering only in the centre. (PI. XXIV, 

 Fig. 3.) The other is also crustaceous, but the surface, which is 

 vmeven or nodulous, is punctured like the top of a thimble. 

 (Plate XXV. Fig. 2.) 



The mass is occasionally drilled with small circular holes, 

 which are the work of some saxicavous worms. 



Ray tells us that this coral is " dredged out of Falmouth Ha- 

 ven to manure their lands in Cornwall." Mr Thompson informs 

 me that it is dredged up in Bantry bay for the same purpose. Ac- 

 cording to Dr Fleming, it is found in such abundance in Ork- 

 ney " as to warrant the conclusion, that it might be advantage- 

 ously employed for the purposes of agriculture and building, es- 

 pecially as limestone in Orkney is scarce, and generally of bad 

 quality, some of it containing 20 and even 30 per cent, of im- 

 purities." Edin. Phil. Journ. viii. p. 301. 



Of the cathedral of Icolumbkil the cement, says Dr Walker, 

 " like that of other ancient structures, is so strong, that it 

 is easier to break the stones than to force them asunder. It is 

 of lime that has been calcined from sea-shells, and formed in- 



