FOSSIL INVERTEBRATED ANIMALS. 



481 



occasionally pyritous, or have been so, and have become iron 

 ore. Some have the surface smooth — others are marked with 

 striae, or ribs — others with tubercles, &c. 



The animals, which produced the fossil shells of the genus 

 Nautilus, were contemporaneous with the ammonites in the 

 earlier ages of the world ; but they have been enabled to resist 

 the causes which proved destructive to the latter, the most 

 recent debris of which are found only in the lower chalk. But 

 the debris of the nautili are to be met with, even up to the era 

 in which the most ancient strata of that substance were depo- 

 sited, in the crag-limestone of later origin, and also, in the 

 living state, in the equatorial regions. The early species were, 

 however, incomparably more numerous than the present, since 

 iscarcely two are known in the living state, while, perhaps, 

 more than thirty have been recognized in the fossil. This 

 genus is of the very small number of those which are found in 

 the ancient as well as the most recent formations ; and if no- 

 thing of it has been found in the strata of the upper chalk, it is 

 probably owing to the soluble nature of those shells which 

 have disappeared in that formation. 



The species of this genus are rather difficult to determine; 

 because, the testa having disappeared in so many instances, 

 there only remains the internal mould, which does not pre- 

 sent all the characters of the shell. Those found in our 

 own country are described and figured by Messrs. Sowerby, 

 with their usual accuracy — one of which, the N. zigzag, is 

 a very singular shell, and has not precisely either the characters 

 of the nautili or those of the ammonites. It might constitute, 

 says M. de France, a genus approximating to the latter, because 

 the tube, or siphunculus, appears to be marginal ; but Mr. 

 Sowerby places it nearest the inside. Denys de Montfort 

 makes a peculiar genus of it, under the name of aganide ; but 

 in his figure he places the siphunculus in the centre. 



M. de France considers that varieties of this same shell have 



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