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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 has 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 East 

 Wear Bay. 



The Ammonite animal was, in all probability like the Nautilus. 



There are two particulars however in which the Ammonitidoe are 

 distinguished from the Nautilidoe. In the first place the Siphuncle, 

 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 diflereuce 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 



