110 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
stable form calcite has resulted either in the gradual conversion of the 
aragonite organisms into granular calcite without loss of the external 
boundaries, or, as frequently happens, this change is attended by the 
complete disintegration and obliteration of the organisms. : 
Figure 4 is in illustration of the first alternative. 
It is a transverse section of a reef-forming coral from the third terrace 
of Makatea, and shows a coral whose cavity was originally filled with 
“mud.” The stereoplasm has been replaced by a mosaic of almost clear 
Fie. 5. 
Manco, 370 feet. A fragmental limestone partially recrystallized. Most of the 
matrix is “mud,” but some has changed to calcite. Calcite organisms, such as 
Lithothamnion and echinid spines, remain unaltered; but aragonite organisms are 
represented by clear calcite and dirt lines round their external boundaries,  X 30. 
calcite. The external wall of the coral can now be traced only by a thin 
dirt line, which possibly represents the impurities and organic matter ex- 
truded during the process of recrystallization. This must often be a 
very gradual process, because the tubes of boring alge which are 
commonly found traversing the substance of the coral are occasionally 
preserved after recrystallization. It will be noticed that the “mud” 
within the coral cavity has shared in the recrystallization, and now 
consists of a mosaic of muddy crystals. The form of these crystals is 
