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presented Plate X. Fig. 1, 2, 3, 4, and described in the twelfth letter. 
The propriety of this opinion we will proceed to ascertain. 
On examining the concavity of the Maestricht fossil, d, it will be 
found to correspond very closely with the convex surface of the above 
fossil, Fig. 4. The structure of the fibres disposed round the concavity 
of the former, shewn in their representation, as magnified with a lens 
of but moderate powers, Plate XII. Fig. 11, y; agreeing, as nearly as 
could be expected, with the magnified appearance of the fibres of 
the former, as represented Plate X. Fig. 3. Considering that the fibres 
of that fossil are shewn on its external, whereas, in the Maestricht 
fossil, they are displayed on the internal surface. Sufficient corre- 
spondence seems to exist between the two fossils, to manifest the 
probability, at least, of their having both derived their existence from 
a similar source. In both are observable fibres passing along the sides, 
uniting in a line at the superior part of the body, and connected in a 
similar manner by lateral processes. In a word, in both are seen the 
same appropriate arrangements, for enabling the animal to adapt its 
form to the varying circumstances, under which its existence must have 
been supported. • 
Should, however, any doubt remain respecting their agreement, it 
must be removed by viewing the circular impression e, on the same 
mass, which agrees in every respect, especially in its alternate risings 
and depressions, with the impression which might be expected to be 
yielded by the inferior part of a small specimen of the fossil depicted 
Plate X. the inferior surface of which is sketched Fig. 1, of the same 
plate. Faujas St. Fond gives also a representation of this surface, 
exactly corresponding in its size with the deep concave impressions. 
From his figures, the circumstance of these two different appearances 
proceeding from two different surfaces of the same body, is rendered 
evident, and is, so far, good evidence of their being the impression 
of the superior and inferior surfaces of the body to which I have 
attributed their origin. 
