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, and is more translucent 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.' 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 aragonite, 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 large 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 75 to 345 mm. (about 345 to yxgy in.). In a few cases, as 
shown by staining, the outermost fibrous zone is absent. 
Some recent spicules of Leptoclinum from Anticosti (Canada), very 
1 W. Meigen. Centralblatt Min., 1901, pp. 577-578. 
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