SIPHUNCLE. 265 



Siphuncle. 



It remains to consider the mechanism of the Siphuncle, 

 that important organ of hydrauUc adjustment, by means of 

 ■which the specific gravity of the Ammonites was regulated. 

 Its mode of operation as a pipe, admitting or rejecting a 

 tluid, seems to have been the same as that we have already 

 considered in the case of Nautili.* 



vatus; d, is the dorsal lobe enclosing the siphuncle, and e. f. the auxiliary 

 ventral lobes, which open to receive the inner whorl of the shell. PI. 

 42, Fig. 3. represents a east of three chambers of A. catena, having two 

 transverse plates still retained in their proper place between them. The fo- 

 liated edges of these transverse plates have regulated the foliations of the 

 calcareous casts, which, after the shell has perished, remain locked into one 

 another, like the sutures of a skull. 



The substance of the easts in all these cases is pure crystalline carbonate 

 of lime, introduced by infiltration tiirough the pores of the decaying shell. 

 Each species of Ammonite has its peculiar form of air-chambers, depending 

 on the specific form of its transverse plates. Analogous variations in the 

 form of the air-chambers are co-extensive with the entire range of species 

 in the family of Nautili. 



* In the family of Ammonites, the place of the Siphuncle is always 

 upon the exterior, or dorsal margin of the transverse plates, (See PI, Sti. 

 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 ;^is collar is seen, well pre- 

 served, 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 PI. 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 contracted 

 at its passage through the collar of each transverse plate, with the same me- 

 chanical advantages as in the Nautilus. 



The shell engraved at PI. 42. Fig. 4. from a specimen found by the Mar- 

 quis of Northampton in the Greensand of Earl Stoke, near Devizes, and of 

 which Figs. 5. 6. are fragments, is remarkable for the preservation of its Si- 

 phuncle, distended and empty, and still fixed in its place along the interior 

 VOL. I.— 23 



