106 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
of the coral are empty or partially filled with a fine calcareous “mud” 
derived from the disintegration of reef organisms. 
In a slightly altered coral it is noticed that-the centers of calcification 
are less marked, and the ‘“‘mud” in the cavities of the coral has been 
partially recrystallized. 
A section of such a coral is seen in Figure 1, from the first terrace of 
Niue at a height of 80 feet. 
Corals have been shown to be built up of aragonite fibers, and the 
spicular character of the skeleton seems to determine the mode of crys- 
Lien ah 
Nive, 80 feet. Section of a reef-forming coral showing spicular structure and 
centers of calcification. Cavities are partly filled with a dark mud which in places 
has altered to crystals of aragonite in crystallographic continuity with the fibers 
of the coral. x 30. 
tallization of the ‘‘mud.”’ Lining the walls of the coral the “mud” is 
seen to have changed to long prismatic crystals of aragonite, deposited 
in crystallographic and optical continuity with the coral fibers. The 
aragonite crystals formed from ‘“‘mud” are generally to be distinguished 
from the clear material deposited immediately from solution, by the 
presence of numerous opaque particles caught up in the crystals. Both 
types are met with in the limestones, but the aragonite deposited di- 
