XVll 



velope ; others penetrate to the interior of the axe, 

 and live on its substance however solid it may be, till 

 at last perishing, its fragments are cast upon the 

 shore in a state scarcely recognizable, and at the 

 mercy of the elements finally reduced to calcareous 

 sand. 



FORMS. 



Their forms are too various to admit a general de- 

 scription, as may be seen in the following outline. 



The Celluliferous Polypidoms appear in aggrega- 

 tions of isolated cells, placed on the surface of ma- 

 rine bodies ; or else cells so united as to form, by their 

 adherence to each other, a thin crust on the surface 

 of the Thalassiophytes or testaceous Molluscas ; and 

 frequently appear in leafy and diversified expan- 

 sions : sometimes the cells are placed on stems, like 

 leaves on their branches ; at other times these cells 

 appear in the form of very long branching or simple 

 tubes, separated from each other in their upper part, 

 and united in their lower to form stems, as well as a 

 footing or root by which they adhere to a solid base. 



In the Calciferous Polypidoms there are also great 

 disparities ; some of them ramify like shrubs, whilst 

 others divide in numberless dichotoma, or imitate a 



Cor, c 



