149 



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 conca- 

 vity of the former, shewn in their representation, as magnified with a 

 lens of but moderate powers, Plate XII. Fig. 11, /, 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. Suffi- 

 cient correspondence seems to exist between the two fossils, to mani- 

 fest the probability, at least, of their having both derived their exist- 

 ence 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, ex- 

 actly 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 at-< 

 tributed their origin. 



