166 
are casts of stone, and sometimes bear the complete form of the shell, 
although the internal margaritaceous part alone is remaining. Mr. 
Martin found specimens which he at first thought were P. nohilis in 
the neighbourhood of Buxton, but he was afterwards satisfied that it 
was some unknown species. One of these is represented in Plate vi. 
of his interesting 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 Bose, 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 position of calcareous molecules — perhaps by a species of 
crystallization. The fracture of these shells, when viewed by a lens, 
shows exceedingly fine stria;, 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 Coquilles, Tome hi. 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, bmellatus, of Lhwydd, Litli. 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 serection 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 Somersetshire, and in the lime-stone of Gloucestershire. I have 
two of these specimens ; but sufficient of the shell is not visible, to 
