HYDRAULIC ACTION OF 8IPHUNCLE. 351 
mode of operation as a pipe, admitting or re- 
jecting a fluid, seems to have been the same as 
that we have already considered in the case of 
Nautili.* 
The universal prevalence of such delicate 
hydraulic contrivances in the Siphuncle, and 
of such undeviating and systematic union of 
buoyancy and strength in the air-chambers, 
throughout the entire family of Ammonites and 
Nautili, are among the most prominent instances 
of order and method, that pervade these remains 
* In the family of Ammonites, the place of the Siphuncle is 
always upon the exterior, or dorsal margin of the transverse 
plates. (See PI. 36. d. e. f. g. h. i., and PI. 42, Fig. 3. a, b.) 
It is conducted through them by a ring, or collar, projecting 
outwards ; this collar is seen, well preserved, at the margin of all 
the transverse plates in PI. 36. In Nautili, the collar projects 
uniformly inwards, and its place is either at the centre, or near 
the inner margin of the transverse plates. (See PI. 31, Fig. 1. 
y. and PI. 42. 1.) 
The Siphuncle represented at PL 36, is preserved in a black 
carbonaceous state, and passes from the bottom of the external 
chamber (d.) to the inner extremity of the shell. At e. f. g. h. its 
interior is exposed by section, and appears filled, like the adjacent 
air-chambers, with a cast of pure calcareous spar. At PI. 42. 
Fig. 3. b. a similar cast fills the tube of the Siphuncle, and also 
the air-chambers. Here again, as in PI. 36, its diameter is con- 
tracted at its passage through the collar of each transverse plate, 
with the same mechanical advantages as in the Nautilus. 
The shell engraved at PI. 42. Fig. 4. from a specimen found 
by the Marquis of Northampton in the Green sand of Earl Stoke, 
near Devizes, and of which Figs. 5. 6. are fragments, is remark- 
able for the preservation of its Siphuncle, distended and empty, 
and still fixed in its place along the interior of the dorsal margin 
of the shell. This Siphuncle, and also the shell and transverse 
