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since, except in the form of the septa, and the disposition of the 
siphunculus, there is no concurrence of character. The fossil shells 
of this genus appear to resemble Spirula fragilis, Lam. Nautilus 
spirulus, Linn, very closely, except in size. In this respect the dif- 
ference is so very considerable, the fossil shells being so much larger, 
as almost to warrant a specific distinction, founded on this circum- 
stance alone. The fossil shells of this genus have hitherto been named 
Lituites, from their resemblance, in form, to a bishop’s pastoral staff; 
but, in these pages, I shall denominate them Spirulites, in concord- 
ance with the name of the genus. 
The siphunculus, in the recent specimens of this shell, differs from 
that of the nautilus, in being formed by one continued shelly tube, or 
by the smaller end of each anterior one terminating in the larger end 
of each posterior one ; but whether this is constantly the case or not, 
in fossil specimens, I am unable to decide ; since, in those specimens, 
in which I have discovered such appearances as would lead to a sus- 
picion of the siphunculus being partly membranous, I have not been 
able to ascertain whether they were fragments of the spirulites or of 
orthoceratites. The very great length of the straight part of the shell, 
in the spirulites, will necessarily occasion this ambiguity, until some 
decidedly distinguishing marks of the orthoceratites and of the straight 
part of the lituites can be discovered. As in the nautilites, so in the 
spirulites, the situation of the siphunculus varies in different speci- 
mens, and perhaps in those which are in every other respect similar. 
We have seen that, in the nautilus, the siphunculus is partly mem- 
branous and probably elastic; whilst, in the shells of this genus, ’we 
find that the whole siphunculus is of shell, and consequently unyielding. 
From this difference of organization, a considerable difference must 
necessarily exist, as to the influence which the introduction of water 
or of air must have on the buoyancy of the several shells : a difference 
depending on the greater quantity admissible in a dilatable than in a 
rigid tube. A more intimate knowledge of the nature of these shells 
