157 



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 spa those 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 1 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 un- 

 yielding, 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 change of specific gravity. Thus also, 

 in those which have been last described, should they have been of 



