SECT. 3] 



SHALLOW-WATER CARBONATE SEDIMENTS 



655 



The importance of the nature of the skeleton in determining the character 

 of the sediments has long been recognized. Sorby (1879, p. 70) was, perhaps, 

 the first to emphasize the need for considerations that transcend phyletic 



Fig. 1. Representative sheaths and spicules. 



A. Green and red algae that have fragile sheaths of calcium carbonate. Left to right: 

 Galaxaura sp., Udotea sp., three specimens of Penicillus sp. and Ehipocephalus sp. 



B. Negative print of a thin section of Penicillus sp. The large rings are cross-sections of 

 the stem (see Fig. lA), and the smaller ones are cross-sections of the filaments. 



C. Electron micrograph of a shadowed carbon replica of the aragonite needles from the 

 stem of Penicillus sp. The crystals are pseudo -hexagonal twins of aragonite. Crystals 

 similar to these are reported from the sediments west of Andros (Lowenstam, 1955). 



D. A specimen of the common Florida sea fan (alcyonarian), Rhipidigorgia flabellunu 

 and its calcite spicules. 



