implantes les uns dans les autres, et grossissant avec les cfcisohs/' 'Hist 

 Nat. des Coquilles, 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 VII. 

 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 

 4he 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 



