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. 



GEOGNOSTIC HISTORY OF FOSSIL SHELLS. 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. 



