ON CEPHALOPODA. sll 
lations with the spirula, with these differences, however, that 
the latter must be still more interior or more concealed by the 
mantle or its lobes, since it is constantly colourless, that the 
cavity occupied by the posterior part of the animal is much 
less large, and particularly that its circumvolutions are much 
less close. ‘The partitions are of the same form, the siphon 
being only almost marginal. Nevertheless, if we should trust 
to the notes taken on the animal of the spirula by MM. La- 
marck and de Roissy, who examined it when in the posses- 
sion of Peron and Lesueur, and even the figure given by the 
latter, it would differ considerably from what we find in 
Rumphius on the animal of the nautilus. 
We find a similar approximation between the partitioned 
or chambered nautilus and a great number of those fossil 
shells which are called ammonites. These shells are likewise 
partitioned, and rolled regularly in the same vertical plane 
from front to rear. The cavity which contained the hinder part 
of the animal appears proportionally less large than in the 
nautilus, and its orifice dilates a little into a trumpet form. 
The siphon, besides, is much more frequently dorsal; but 
more especially the circumvolutions are less close, holding a 
sort of intermediate relation between those of the spirula and 
those of the nautili, so that all the turns of the spire are vi- 
sible, and the aperture is not modified by the last but one. 
Finally, the partitions are more or less sinuous, which doubt- 
less is occasioned by the form of the posterior part of the body 
of the animal. As to the extreme thinness of the shell, that 
appears to have been somewhat exaggerated, and judged of 
only by a few remaining lamine of the nacreous part, which 
is never exterior in any of those shells, nor even in that of the 
nautilus. 
We have much pleasure in referring to a most excellent 
memoir on Naut. Pompilius of Lin., by Mr. Owen, with 
elaborate figures of the animal, its shell, and various parts, 
