xvii 
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 
