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 necessary. 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 transverse intersections. But not- 

 withstanding these arguments, 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 in the Sardinian marble are not retained in distinct tubes; but 



VOL. n. i 



