solvent acid. I repeat, that there is no room whatever for 
doubting that the Foraminiferous ooze both contained 
within and surrounding the Nautiloid shell were originally 
in identical states. Microscopic observation makes this 
sufficiently plain. The differences now observable between 
them have arisen from changes which have taken place 
subsequent to their primary accumulation, and which 
changes have been due to differences of position; the one 
portion has been protected by the thick calcareous Nauti- 
loid shell which would rob the water percolating through it 
of ail its solvent carbonic acid, and thus preserve the con- 
tained Protozoa from destruction, and which protection 
would continue so long as any portion of the Nautiloid 
sliellwall remained undissolved. The other, being unpro- 
tected, would be exposed to the full action of the solvent, 
which would percolate readily amongst the loosely aggre- 
gated, microscopic organisms and speedily act upon their 
fragile shells. 
But there is a yet further feature in this interesting 
specimen requiring notice. The closed chambers of the 
Nautiloid shell are all filled with clear, crystalline, calcareous 
spar. The acidulated water, acting upon the calcareous 
Foraminifera of the ooze, has become converted into a more 
or less saturated solution of bicarbonate of lime. This has 
passed by percolation through the shell of the Nautilus into 
its hollow chambers. Finding there suitable cavities, it has 
gradually filled them up with a crystalline formation of 
calcareous spar, and which of course exhibits no traces of 
the minute organisms from which the calcareous matter was 
primarily derived. A similar crystallisation has filled up 
