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power. Plate X. Fig. 23, is another section, similar to the last, but 
having a circular spot in its centre. Plate X. Fig. 24, is another sec- 
tion, differing from the two preceding, in having three darker lines 
pass through it. The section, Fig. 25, appears to be the same fossil 
as Fig. 20, but altered in its form, and, I conjecture, during the life 
of the animal. 
This is rendered more likely, from the appearance of the section 
Fig. 26 ; in which, although the structure differs, the substance being 
marked with spots in the place of lines, it agrees in having a line of 
separation running through the substance, with apparently a central 
cavity, here a similar change of form seems to have taken place. 
In the section Fig. 22, the general structure resembles that of Fig. 20, 
23, 24, and 25, but differs from them in having a line pass along the 
middle ; with two spots of spathose matter nearly at its termination, 
marking the previous existence of a cavity along the circumference of 
the body, and continuous with the central line. 
In the specimen Fig. 26, as well as in several other sections 
with a similar central line, an appearance of alcyonic structure is 
observable. 
How far these different species partook of an alcyonic structure, and 
how much some of them might possess the power of altering their 
general form, cannot perhaps be ascertained from the extent of our pre- 
sent observations ; but, in every species which I have had the satisfac- 
tion of noticing, such a structure evidently exists as must have been well 
adapted to give the animals to which they belonged, the power of regu- 
lating their buoyancy, according to the circumstances under which they 
were placed. Thus, in the ordinary nummulite, if its substance was 
unyielding, the admission of air or of water into its chambers would 
have been sufficient to have produced the effect ; or if of a softer 
substance, allowing of a change of figure, then the diminishing or 
enlarging the capacity of its chambers, by the approximation or the 
separation of its sides, might be sufficient to effect the necessary 
