101 
implantes les uns dans les autres, et grossissant avec les cloisons.” Hist. 
Nat. des Coquit les, Tome V. p. 164. 
But that the tube in the nautilus was partly a membraneous tube, was 
known so long ago as the time of Hooke, who believed it to be a tube 
dilatable or compressible at pleasure ; and that, like the air-bladders of 
fishes, it served, by its expansion or contraction, to render the animal 
buoyant or not. 
In the representation given by Rumphius of the dead animal which 
had inhabited the shell of N. pompilius, a round membraneous process 
is seen in the posterior part of the animal, which exactly agrees with, 
and had evidently been separated from, the siphunculus ; and serves 
to show, that a connection existed between this part and the body of the 
animal. 
In the dried recent shells of the nautilus, the membraneous part of 
the siphunculus is, I believe, seldom found, it being either removed by 
decay, or by the process of slitting the shell, to obtain the display of its 
internal structure : but I am pleased in being able to say, that frequently, 
in fossil specimens, not only is the existence of a continued siphunculus, 
extending through every chamber of the shell, proved, as in Plate VI I. 
Fig. 12 ; but , that it is sometimes to be seen so much larger than the shelly 
part of the tube with which it is joined, as gives reason for supposing it 
to have been capable of a considerable degree of dilatation. This I am 
able to demonstrate in several specimens, as at Plate VII. Fig. 10, in 
which even the anatomy of this part may be ascertained. 
It may be there seen, that the testaceous part of the tube, extending 
through about one fourth of the chamber, is formed by an elegant sinu- 
ous turning of the septum. It also there appears, that the mem- 
braneous tube which has proceeded from the animal is extended over 
the internal surface of the testaceous tube, is reflected a little on 
the exterior surface of the tube, and then returns and passes on to 
the inner surface of the next testaceous tube, and may thus be traced 
