524 GILBERT C. BOURNE. 



that at some little distance below the surface the coenenchymal 

 tubes are bounded by trabeculse, which run at 90° to the 

 surface of the colony. Each trabecula is triradiate in trans- 

 verse section^ and is seen in longitudinal section through its 

 centre to be composed of crystalline fibres diverging upwards 

 and outwards from a centre of calcification. In any given 

 transverse section the fibres emerge at the surface of the 

 section, and thus a series of concentric lines is formed around 

 each centre of calcification^ which are exactly similar to the 

 ''growth-lines" described by Mrs. Gordon in various Madre- 

 porarian sections. It is quite clear, from what has gone 

 before, that these concentric striations in Heliopora are 

 not the expression of layers of calcified cells which have 

 been added to the thickness of the corailum. I shall hope to 

 show that the same is the case in the Madreporaria. 



Owing to the comparative coarseness of its component crys- 

 tals, there is very little doubt as to the cause of the concentric 

 markings in the Helioporan skeleton, and for the same reason 

 the markings themselves are not so delicate, and betray their 

 origin more clearly than do those of Madreporaria. 



Professor Sollas kindly determined the specific gravity of 

 the skeleton of Heliopora for me, and found it to be 2*82, 

 ealcite being 2'7 and aragonite 2"9. The skeleton of Heliopora 

 therefore appears to be composed of aragonite, and not of 

 ealcite, as are the spicules of other Alcyonaria. The diflPerence 

 between the specific gravities of the Helioporan skeleton and 

 aragonite is to be accounted for by the presence of a consider- 

 able amount of organic matter in the former. This organic 

 matter, as I showed in my previous paper (4), is associated 

 with the blue colouring matter characteristic of the species. 

 The coralla of Euphyllia and Madrepora are, by Professor 

 SoUas's determination of specific gravities, of 2*77 and 2*78 

 respectively, thus standing nearly halfway between ealcite and 

 aragonite. In this case, too, the difference between the coral 

 fragments and aragonite is probably due to the fragments being 

 full of organic matter, particularly of the filaments of a parasitic 

 boring fungus, and a boring sponge of the genus Clione. 



