becoming pale in the herbarium, and chalky-white if exposed to the elements. 

 Substance stony, calcareous ; the tissues containing much lime. 



Specimens of this species were collected by Dr. Mueller 

 cluriug Mr. Gregory's expedition to North Australia; but our 

 figure is drawn from one gathered by myself at the Eriendly 

 Islands in the Pacific. It is a characteristic and common plant 

 on coral rocks within the tropics of both hemispheres, growing 

 on the upper surface of the reef, where the coral is dead and 

 covered with sand and debris. The matted roots sink deep in 

 the sand, and take a firm grip on the indurated matter below. 



A second species of Halimeda {H. macroloha, Dene.) occurs in 

 Western Australia ; and no doubt //. Optmtia will yet be found 

 on the northern or north-western coast. H. incrassata is readily 

 known from both these by its thicker substance, and much nar- 

 rower articulations ; the ends of the branches often extending 

 like fingers or toes. 



Fig. 1. Halimeda incrassata, — the natural size. 



