A. E. Trueman — Tlie Ammonite Siphunde. 81 



Function of the Siphuncle. 



It is not the writer's intention to enter into any lengthy specula- 

 tion concerning the use of the siphuncle either among Ammonites 

 or nautiloids. Many writers have dealt with the question, and nearly 

 every one has a different explanation to offer. A summary of a 

 number of these suggestions is given by Blake/ and many others have 

 been briefly discussed by Zittel.^ It will, however, be interesting 

 to re-examine several of these theories in the light of the observations 

 recorded. 



The early conception of the Ammonite shell was as an apparatus 

 by which the animal could float on the surface of the sea, or by 

 diminishing the amount of gas in the chambers could sink to the 

 bottom. Various writers suggested ways in which this might have 

 been accomplished, some imagining a siphuncle which was capable 

 of expanding and so expelling the gas, others suggesting that the 

 siphuncle introduced fluid into the chambers and so achieved the 

 same result.^ Both these suggestions were deemed by most writers 

 to be inadmissible when it was found that the siphuncle was sur- 

 rounded by a solid envelope. But if, as we have noticed, in the last 

 few chambers the siphuncle had no envelope, this objection no 

 longer holds, for a very small change in density would suffice to 

 lower or to raise the shell, and this could be made by altering the 

 contents of the last few chambers only.'* Nevertheless, the author 

 would not for a moment suggest that the siphuncle performed any 

 such function. 



In a recent paper Mr. L. F. Spath has put forward another 

 suggestion concerning the use of the siphuncle in Nautilus!' He 

 considers that this strong central siphuncle of Nautilus helps to 

 prevent the animal from falling away from its shell when it moves 

 forward to form a fresh septum. Apparently this suggestion is 

 based on the somewhat amusing but scarcely scientific account 

 of the capture of a specimen of Nautilus pompilius written by a 

 lady and quoted by E. A. Smith.*^ According to this story the animal 

 breaking its siphuncle fell from its shell and showed no desire to re-enter 

 it ! Against this may be given the experience of Dr. Willey, who 

 made saw-cuts in the shells of Nautilus and cut the siphuncles just 

 behind the body, yet the animals lived on, and apparently showed 

 no tendency to fall from their shells.'' It is fairly certain, therefore, 



1 J. F. Blake, British Fossil Cephalopods, vol. i, 1882, p. 39. 



^ K. V. Zittel, Handbuch der Palaeont., Bd. ii, pt. i, p. 348. 



^ T. Wright, Monograph of Lias Ammonites, Palteont. Soc, 1878-86, p. 174. 



* The total weight of the ammonite animal and its gas-filled shell was 

 probably not very different from the weight of water it displaced. See Dr. F. 

 Willey, The Pearly Nautilus, 1902, p. 747. 



® L. F. Spath, "Notes on Ammonites": Geol. Mag., Vol. LVI, 1919, 

 p. 30. 



^ E. A. Smith, "Note on the Pearly Nautilus": Journ. Conch., October 

 1887. 



^ Op. cit., p. 760. 



