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curious structure, than that which is discoverable in the very rare fos- 

 sil, Plate X. Fig. 1, 2, 3, 4. It is a tolerably fine lime-stone, nearly 

 of an hemispherical form ; and according to the best information I 

 can obtain, was found in some part of Switzerland. Its superior con- 

 vex surface, represented Fig. 4, has a groove, about a third of an inch 

 in depth, and as much in width, which possesses about three-fourths of 

 its length. The whole of this superior surface derives a smooth downy 

 appearance, from innumerably fine striae, formed by very minute ar- 

 ticulated fibres, which proce'ed from the inferior surface, pass round 

 every part of the circumference, and collect together and terminate in 

 the sides of the central longitudinal cavity. At Fig, 3 is given a mag- 

 nified view of a part of this surface, in which may be seen the appear- 

 ance of articulation observable in the fibres of which it is composed. 



The inferior surface is nearly flat, and is seen, with the naked eye, 

 to be marked by completely concentric ridges and corresponding de- 

 pressions, intersected by numerous lines, radiating from the centre to 

 the circumference. On employing a magnifying glass, it is discovered, 

 as may be seen in the sketch, Fig. 1, that the cylindrical fibres are 

 continued, from the superior convex surface, to the centre of this in- 

 ferior surface, and are connected laterally, by fine transverse linear 

 processes, circularly disposed, as is represented, moderately magnified, 

 Fig. 2. 



Having, fortunately, two of these curious fossils, I was struck with 

 a considerable difference in the appearance yielded by their inferior 

 surfaces. The circular ridges were more prominent, and, of course, the 

 depressions were deeper in one than in the other ; and in that one, a 

 concavity in the centre of the inferior surface was also observable, 

 which did not exist in the other. I was led by these differences to 

 conclude, that they had originated in a power which had been pos- 

 sessed by the animal, of effecting certain changes in this surface, de- 

 pendent on the particular circumstances under which it might have 

 been placed. 



A striking peculiarity observable in this fossil, is its complete 



