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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 a 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 innumerable fine striae, formed by very minute ar- 
ticulated fibres, which proceed 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 dififerences 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- 
pendant on the particular circumstances under which it might have 
been placed. 
A striking peculiarity observable in this fossil, is its complete devi- 
