SUPPLEMENTARY NOTES. 457 



but as we know that the Nautilus Pompilius possesses in its peri- 

 cardial fluid and siphuncle a sufficient apparatus to effect this 

 purpose, and thereby to cause the rising and sinking of this 

 animal; and as we find in the Ammonites and many extinct 

 families of fossil chambered shells, a siphuncle and air-cham- 

 bers, very similar to those of the Nautilus; we may infer from 

 analogy, that mechanisms so similar, as to those parts which 

 have escaped destruction, were connected with soft and perishable 

 parts, corresponding with the pericardial apparatus in the living 

 Nautilus. 



It is of little importance, however, to the statical theory of 

 siphuncular action here proposed, whether the fluid alternately 

 admitted to and rejected from the siphuncle be derived from the 

 Pericardium, or from any other source within the body, or even 

 from the sea; in the former case, we have ascertained the existence 

 of a mechanism whereby the movements of the pericardial fluid may 

 be effected, as in the Nautilus Pompilius; in the latter cases the 

 mechanism for adjusting the passage of the fluid to and from the 

 siphuncle remains yet to be discovered. 



In the case of siphons which are surrounded by unyielding 

 rigid shell throughout their whole extent, (as in the Nautilus 

 Sypho,) the elasticity of the air within the chambers cannot aid 

 the muscular power of the siphuncle, in regulating the action of 

 any fluid within that tube; and if the hypothesis suggested (P. 

 271, Note,) respecting this species should be inapplicable to 

 it, and to other animals which have an inflexible shell around the 

 siphuncle, their method of moving the fluid to and from this organ 

 is yet unknown. 



In the case of jointed sheaths like those at PI, 32, Fig. 3, d, c, 

 f, and PI. 33, each calcareous joint (e,) if composed of rigid shell, 

 may have articulated with the collars of the adjacent transverse 

 plates (/», i,) so as to form a moveable collar valve, of which the 

 superior margin being raised a little on the outside of the upper 

 collar [h,) would leave an opening between the lower margin of the 

 valve and the inside of the subjacent collar (z;) through this open- 

 ing air might pass from the contiguous air-chamber into the space 

 between the calcareous sheath and membranous siphon, as often 

 as it was emptied of its pericardial fluid, and when this fluid filled 



VOL. I.— 39 



