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the interior of the minute shells secreted by very low forms 

 of animal life, called Foraminifera. These forms seem to live 

 either at the surface or the bottom of the sea, and perhaps at 

 intermediate depths. However that may be, their shells, 

 when the animal died, must have accumulated at the bottom. 

 Here the mineral Glauconite infiltrated into the shell, filling 

 it and permeating it to its minutest pores ; and after this was 

 accomplished and the mineral had become thoroughly solid, 

 the shell, consisting of carbonate of lime, was dissolved away, 

 leaving the cast. But why the Glauconite came to occupy 

 the chamber vacated by the animal is quite beyond our ken 

 at present. It is one of those cases of a mysterious chemico- 

 physical process, the evidences of which we so often meet 

 with in geological research, and which, together with the 

 problems afforded by the iron pyrites-casts of fossils in the 

 Gault, the flints in chalk, and others of a kindred nature, 

 offer so remunerative a field for a patient investigator, pos- 

 sessed of the requisite leisure, and some knowledge of 

 Chemistry and the natural forces. It is noteworthy that in 

 some parts of the Gulf of Mexico this identical process is 

 going on at the present day, and we know that it has hap- 

 pened before in periods far removed, by the occurrence of 

 Glauconite casts of Foram. shells in the Paleozoic rocks of 

 America and Eussia. 



With regard to the animal which secreted the shell, I 

 mentioned that it was of very low organisation. Some of 

 you are no doubt acquainted with the Amaba of our stagnant 

 ponds, and one can hardly conceive of a simpler form of life 

 than that. It has the appearance of a particle of shghtly 

 animated jelly, which consists of protoplasm, the material of 

 primitive living matter. Its only trace of organisation seems 

 to be a difference in the density of the protoplasm of the 

 exterior and interior, the outer layer or extosarc being rather 

 denser than the inner part or endosarc, which is more fluid. 



Some Amaebas possess what is called a contractUe vacuole. 

 This appears as a clear round space filled with liquid in the 

 interior of the animal, and as it appears and disappears at 

 regular intervals it seems to act as a kind of pump, and may 



