169 



convexity is sometimes so trifling, that the edges very nearly approximate. 

 In this case, the disks necessarily exceed in number those which till 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 co- 

 lumns, and of course, of the cavity in the stone in which they are con- 

 tained, 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 r 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 spar; 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- 



\ 7 OL. II. 2 



