58 



on the contrary their radii of one star are frequently seen to run into 

 and connect with those of the adjoining stars; whereas in the silicious 

 specimen, the figures of the stars, though polygonal, possess a con- 

 siderable degree of regularity, the consequence of being restricted by 

 perpendicular lamellae disposed in their interstices. These lamellae are 

 indeed not always discoverable; they are however frequently suffi- 

 ciently so, to determine indisputably their existence. This is particu- 

 larly the case on the reverse of the rpecimen: the stars may be there 

 seen evidently circumscribed by the perpendicular lamellae ; and a 

 net-like plexus may be seen proceeding from their sides in such 

 a manner as to produce a stellated appearance, but totally different 

 from that which is displayed on the other surface of the specimen. 

 For the purpose of making this difference better understood, a re- 

 presentation of a part of this surface is given, Plate VI. Fig. 13, to 

 allow of a comparison with the appearance of the upper surface at 

 Fig. 12. 



The ascertaining of the cause of this remarkable difference neces- 

 sarily became highly desirable ; but it was not until after the examina- 

 tion and comparison of various specimens that any reasonable con- 

 jecture could be formed respecting this curious difference. The first 

 circumstance which seemed to lead to an elucidation, was the co- 

 existence of the polygonal sides of the tube and of the reticular 

 plexus; these being both evident on the inferior, and both disappear- 

 ing on the upper part of the specimen. Farther examination made 

 it also appear that a reddish brown colour, and a slight degree of 

 transparency, existed in those parts of the superior surface corres- 

 ponding with those parts which, on the inferior surface, were white and 

 opaque. A considerable change, it was therefore evident, had taken 

 place in the organized matter from which the stone had derived its 

 characteristic configurations. The nature of this change was, after a 

 little consideration, ascertained : the parts of the original coral, which 

 still remained in the inferior part, had evidently been removed from 



