108 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
probably consisting of aragonite. It will be noticed that the “mud” 
lining the coral walls has crystallized in prismatic needles of aragonite 
deposited in optical continuity with the fibers of the coral. 
The above description represents the appearances of the less altered 
reef-forming corals. The more commonly occurring constituents of the 
limestones are, however, compacted “coral sand,” detrital fragments of 
organisms, and a certain amount of rubble rock. A section of a com- 
pacted “coral sand” is composed of organic fragments from the reef. 
Fig. 3. 
Curistmas Isianp, No. 372. (After boiling with cobalt nitrate.) The arago- 
nite organisms, corals, gastropods, and Halimeda, as well as the aragonite crystals 
in the coral cavities, are stained pink. The calcite organisms, Orbitolites, echinid 
spines, and Lithothamnion, together with the fibrous calcite cement, are quite un- 
stained. X 30. 
As a rule, it shows aragonitic organisms such as coral fragments, Halli- 
meda, gastropods, etc., together with organisms having calcite skeletons, 
such as most of the Foraminifera, echinid spines, Lithothamnion, ete. 
The organisms are often fragmentary, and are cemented together by 
a matrix consisting either of “mud” or of calcite. The calcite may exist 
in two forms. The more usual is a somewhat clear granular mosaic of 
crystals, but frequently it coats the organisms with an investing layer 
of long fibrous crystals. A good example of the latter is seen in Figure 
