364 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



structure destroyed. They are found, for example, in sections of the 

 talus, where one would expect alteration to take place readily from the 

 fragmentary and therefore porous nature of the rock. It is probable 

 from this that tunicates took a more important part in the composition 

 •of the reefs than appears at first sight, owing to the ease with which 

 aragonite organisms are obliterated. The value of tunicates as rock- 

 formers may be estimated in a particular case by finding the fraction of 

 the surface area of a section of the rock covered by the spicules. 

 Choosing a section in which the tunicates were especially abundant, 

 they were found to form approximately 2 per cent of the whole mass. 

 In most of the slides they form a smaller proportion than this, and about 

 .5 per cent may be taken as the average for the Niue rocks. 



In the Niue specimens the spicules are circular in sections and show a 

 clear outer zone of radiating fibres surrounding a dark granular area. 

 In many specimens the centre of the granular portion has a distinctly 

 radiating structure, aud is more temslucent than the remainder of the 

 central area, but in others the granular area extends to the centre. The 

 composition of the spicules is shown to be aragonite, by staining them 

 with cobalt nitrate. 1 By this method, in which a polished slice of rock 

 is boiled with ordinary cobalt nitrate solution for some time, aragonite 

 is stained pink, but calcite is not affected, unless the boiling has been 

 continued for a very long time, when it may stain blue. The granular 

 portion of the spicule stains pink readily, but the outer zone stains only 

 on long boiling. This shows that the whole is aragouite, but that there 

 is some difference between the zones, probably due to a difference in the 

 .state of aggregation. 



From the fact that the sections are invariably circular, the spicules 

 must be spherical. In a few cases the surface is seen, and appears tuber- 

 culated, the bases of the relatively lai-ge but low tubercles being in close 

 contact. 



The spicules vary very greatly in size, as is shown by the figures, which 

 are drawn to the same scale. Many of the smallest sections show only 

 the outer fibrous zone, and are, therefore, probably tangential sections 

 of large spicules. But others, such as the one figured, show the central 

 granulated area occupying the same proportion of the whole as in the 

 larger forms, and these must be small spicules. The diameter varies in 

 length from fa to Tihs mm - ( an out ^^ to ysVff in.). In a few cases, as 

 shown by staining, the outermost fibrous zone is absent. 



Some recent spicules of Leptoclinum from Anticosti (Canada), very 

 i W. Meigen. Centralblatt Min., 1901, pp. 577-578. 



