238 MOLLUSCA. 
operation of the corpuscular forces. In some cases the cal- 
careous matter of the shell has become impregnated with 
foreign ingredients, or has totally disappeared, leaving in 
its place ferruginous or siliceous depositions. But the most 
curious circumstance in the chemical history of these fos- 
sils, is the preservation of the animal matter of the shell in 
its original form and order of arrangement, even when the 
calcareous matter of the shell has been changed into com- 
pact or granular limestone. This very important fact we 
owe to the ingenuity of Mr. Parkinson, who, by treating 
the shell for a length of time with greatly diluted acid, ab- 
stracted the calcareous matter, and obtained a distinct view 
of the cartilaginous membranes of the shell. The student 
will in general observe, that the cavities of those shells, 
which present an external opening, are filled with the same 
sort of matter as the rock in which they are enclosed, while 
the cavities, of the multilocular testacea, for example, which 
have no external communication, are filled with matter in- 
variably of a crystalline structure, even when not different 
from the substance of the rock. 
Grocnostic History or Fossit SHEeiis.—It appears 
evident that the advancement of this branch of conchology 
must, in a great measure, depend on the accurate discrimina- 
tion of the fossil species, and the relations of the rocks in which 
they are contained. It is only within the last twenty years, 
therefore, that our knowledge of this branch of the subject 
has been acquired. ‘The members of the Wernerian and 
geological societies have contributed largely to our stock of 
knowledge: but much yet remains to be brought to light. 
The following notices may be regarded as embracing the prin- 
cipal facts which have been ascertained. 
