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manner the method of reasoning by which the geologist extracts 
from a fossil its own history. It is Nautilus striatus from the lias 
rock at Whitby, and the interior of the cells is filled exclusively 
with calcareous spar, while the siphuncle contains nothing but lias 
mud. This proves that no communication existed between the 
siphuncle and the air chambers, otherwise the lias mud passing 
from the bed of the sea through the tube must have found its way 
into them. Note also that this fossil siphuncle is fractured in two 
places. These gaps could not have existed when the mud early 
found its way in, but must have been formed after petrifaction. 
The spar was introduced subsequently to these fractures by slow 
infiltration through the walls of the cell, and now it fills the whole 
cavity. But neither has the spar found its way into the tube, nor 
has the mud of the tube escaped into the chambers. 
The animal itself, as I said, occupies only the outer enlarged 
chamber (in this respect the fossil represented is imperfect). The 
substance of the body resembles that of the Cuttle-fish or the 
Octopus. Around the mouth a number of arms are arranged, but 
these are not provided with suckers like these of the shelless octopus. 
The nautilus is seldom seen, being in all probability a deep sea 
animal ; only a few specimens have ever been dissected. It must 
not be confounded with the Paper Nautilus, which has a ribbed 
shell much more delicate in its structure, and unprovided with 
compartments. 
We are now, I hope, in a position to understand better the 
structure and nature of the ancient relations of the Nautilus, whose 
habitations are distributed so abundantly in the Gault of Hast 
Wear Bay. 
The Ammonite animal was, in all probability like the Nautilus. 
There are two particulars however in which the Ammonitide are 
distinguished from the Nautilide. In the first place the Siphunele, 
instead of piercing the centre of each septum, runs round the outside 
of the coil of the shell, occupying what we call the dorsal position. 
In every other particular it is like that of the Nautilus, i.e. it was 
membranous, and protected by a sheath or collar at each septum. 
The second point of differeuce is more important—the character of 
the septa by which one compartment is separated from another. 
In the Nautilus they are simple and symmetrically curved, the 
concave side being always towards the mouth of the shell. It is 
evident that a transverse section of these septa would give a simple 
curved line. But it is not so in the Ammonite family. While the 
dividing walls are flat or nearly so in the centre, their edges are 
angular, folded, or sinuated, so than when they make their im- 
pressions on the outer walls of the shell, as they frequently do, they 
confer upon it a considerable amount of ornamentation. Notice 
