169 
convexity is sometimes so trifling, that the edges very nearly approximate. 
In this case, the disks necessarily exceed in number those which fill the 
same space, but whose convexity, and, of course, whose central thick- 
ness is more considerable. The size of the disks is not always the 
same, even in the same column ; nor is the shaft with which they are 
united always of the same form, it being in some circular, and in 
others, pentagonal or stelliform. A variety of appearances also are 
observable in these fossils, dependent on the absence of the shaft, or 
the absence of the whole or a part of the disks. The size of the 
columns, and of course, of the cavity in the stone in which they are 
contained, varies from the eighth of an inch to nearly two inches in 
width ; the size of the cavity in which the column is contained, it 
must be, however, remarked, always exceeds, and sometimes very 
considerably so, the size of the column. The number of disks in a 
column is also very various : sometimes not exceeding three or four, 
and at other times amounting to upwards of thirty. The ends of the 
several cavities containing these columns are striated, in the same 
manner as the disks themselves, and have the columns attached to 
their centres ; the vestiges of the broken shaft being observable, when 
the column has, by accidental injury, been removed. The sides of 
these cavities are also marked by concentric rings, which, as has been 
observed of the disks, are, in some specimens, disposed much more 
closely together than in others. On the sides of these cavities, and 
between these rings, very small projections and depressions are also 
sometimes observable. The substance of which these stones are 
formed have generally been supposed to be of a silicious nature, but 
Mr. Walch supposes them to be a hard and compact star; acknow- 
ledging, however, that those specimens which are obtained from Hut- 
tenrode are of a hardness nearly equal to quartz : a circumstance 
which he attributes to the ferruginous particles with which they are 
penetrated. They are most commonly of a deep brown colour ; evi- 
VOL. II. z 
