57 
is rendered much more beautiful by having a slight tinge of red ; and 
in others, its appearance is pleasingly varied, by its having acquired a 
greater degree of transparency. In some specimens it passes from the 
light ashen-grey into a greyish black, and gaining at the same time a 
still more considerable degree of transparency, its transition into flint 
is completed. 
It is obvious that the configurations with which the different sur- 
faces of this fossil are adorned proceed from some coralline body ; 
which, like the madrepore in the marble just described, is composed 
of stars closely ranged. The form of the stars, in this fossil, varies 
very much, so that it is not easy to discover two exactly similar : a 
disposition to the pentagon appears, however, to be most prevailing. 
This dissimilarity of figure and of arrangement is equally observable 
in the plates or rays of which the stars are formed. These plates 
originate nearly in the centre of each star, and by their several divi- 
sions and bifurcations fill up the polygonal area allotted to each star : 
the rays belonging to each star being in general disposed in six pencils 
or fasciculi, each bundle terminating in four rays or points. But it is 
so difficult to convey an idea, in words, of the manner in which the plates 
divaricate, as to render a reference to the representations of this fossil 
the more necessar\ . Plate VI. Fig. 12 and 13. By an examination of 
these, the similarity of the mode in which the separation of the plates 
takes place in this fossil and in the marble last figured may be per- 
ceived. It is necessary also to notice another agreement between these 
fossils. As in the madrepore from which the marble originated, the 
longitudinal plates are not intersected by any transverse septa, so it 
also appears that the madrepore, thus invested and impregnated with 
silicious matter is also without any tranverse intersections. But not- 
withstanding these agreements, the differences between these two fossils 
are, in other respects, such, as to plainly manifest that the madrepores 
of which they are constituted, differed materially. The stars of the 
coral .11 the Sardinian marble are not retained in distinct tubes ; but 
VOL. II. 
I 
