166 



are casts of stone, and sometimes bear the complete form of the shell, 

 although the internal margaritaceous part alone is remaining. Mr. Mar- 

 tin found specimens which he at first thought were P. nobilis in the neigh- 

 bourhood of Buxton, but he was afterwards satisfied that it was some 

 unknown species. One of these is represented in Plate vi. of his inte- 

 resting work on Derbyshire petrifactions. 



Plate XI. Fig. 31, is a magnified representation of a minute fossil 

 pinna, resembling Pinna saccata, which I lately discovered in sand from 

 Grignon. 



It is observed by Bosc, that the texture of the Pinna is different from 

 that of any other shell. Instead of possessing any thing of the laminated 

 structure, the shell of the Pinna seems to be formed by the juxta posi- 

 tion of calcareous molecules perhaps by a species of crystallization. 

 The fracture of these shells, when viewed by a lens, shows exceedingly 

 fine striae, perpendicular to the surface of the shell. This structure, he 

 is of opinion, is sufficient to distinguish this shell, even in ever such 

 small fragments, and in a fossil state. Histoire naturelle des Coquittes, 

 Tome in. p. 123. 



This, however, is not strictly the case ; since fragments of other shells, 

 of a similar structure, are frequently found, especially in chalk. Such 

 are, Trichites pactilis undulatus cretaceus, and Trichites pactilis cretaceus, 

 lamellatus, ofUiwydd,Lith.Brit.Ichnogr. Nos. 1751 and 1752, which 

 are the remains of the remarkable shell, of which I have already spoken, 

 under the genus Patella, Letter vi. page 51, of this volume. A similar 

 structure I shall also have to notice, in a fossil oyster. This astonishing 

 secretion by an animal, of calcareous spar, in a crystallized form, appears 

 to be a circumstance highly deserving the attention of the physiologist, as 

 well as of the admirer of crystallography. 



The pinnite is rather a rare fossil. It is however sometimes found, with 

 the Oolites, in the lime-stone of the northern parts of Wiltshire, in Somer- 

 setshire, and in the lime-stone of Gloucestershire. I have two of these 

 specimens; but sufficient of the shell is not visible, to allow of an opinion 





