25 
at right angles those parallel to the general conical surface, and likewise originat- 
ing from each joint. As therefore, the first lines of separation generate a conical 
surface pointing upwards, so this second set generate a conical surface point- 
ing downwards, and thus truncate, or rather excavate, by cutting into them 
the apices of the first cones. When the base is in such a state that the 
lamine of which it is formed are disposed to separate (PL. 11. fig. 15.) its de- 
tached pieces, often present this excavated conical form, altogether resembling 
models of the crater of a volcano. These second lines of separation do not, 
however, extend to the exterior circumference of the base, except near its sum~ 
mit, but terminate in its interior, and are not all of equal lengths, but occur 
in sets, of which the lowest line is always longest, and those which succeed 
it, in proceeding upwards, gradually diminish for a series of from six to ten 
joints, until followed by the longest line of the set next above. Such are the 
appearances ; the explanation I am inclined to offer of them is as follows. Tlie 
muscular membranes which are interposed between the columnar joints, may 
very probably have carried on them the vessels whence the exudation of the 
calcareous matter forming the laminz of the base proceeded ; these membranes 
may have continued to extend themselves in the direction marked by these 
second lines. The original existence of such an interposed membrane will 
readily account for the separation which, as we have seen, still takes place in 
this direction, in many specimens. And as we have already seen reason to be- 
lieve that the exudation of the calcareous matter did not proceed continuously, 
but was rapidly thrown out at certain periods; so we may suppose all the lines 
of a single set to have been formed by the extension of the membranes during 
one of these active periods, and their different lengths to indicate the size they 
had attained when that action ceased. 
The quantity of matter accumulated round the base, indicates the degree 
of adhesion and support the animal required ; and, perhaps, bears also pro- 
portion to the quantity of calcareous substance accumulated in the.alimentary 
canal, and from which it became necessary to free it when inconvenient to its 
economy. 
Portions of the column (Pt. 1. fig. 1. and Pu. tr. fig. 2. to 14.) are not 
unfrequently encompassed by a thick spreading ring of caleareous matter, in 
every particular resembling that which covers them at the base, and forms their 
