58 
on the contrary the 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 specimen ; 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 corre- 
sponding 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 
