26 A. E. Truema7i — TJie Amnionite Siphuncle. 



The Ammonite Siphuncle. 



By A. E. Trueman, D.Sc, F.G.S., University College, Cardiff. 



TT is well known that the chambers of Cephalopod shells are con- 

 -*- nected by an organ called the siphuncle, which passes through 

 the sejDta by the septal necks. Many investigators who have studied 

 these shells have made careful observations on the siphuncle, hoping 

 thereby to throw light on its functions. Such workers as Branco, 

 Blake, Zittel, and Owen, among many others, have written on this 

 problem, while more recently an elaborate study of the structure 

 and constitution of the siphuncle has been made by Grandjean,^ who 

 gives a detailed account of the organs associated with it. Among 

 the many points established by Grandj can's work, not the least 

 interesting is the fact that the siphuncular tube of Ammonites and 

 Belemnites is composed of calcium j)hos23hate, and not, as had 

 previously been stated, of calcium carbonate.^ But in spite of 

 Grandjean's careful examination of the less obvious characters of 

 the siphuncle, it appears that certain facts, much more easily 

 observed, have hitherto been overlooked. 



Terms. 

 The word " siphuncle " in Ammonite literature is used somewhat 

 loosely, just as the word " siphon " is used in the same connexion 

 by French authors. As Grandjean pointed out, however, the 

 '■ siphon " (or siphuncle) proper is a membranous organ; the tube 

 of calcium phosphate found in fossils, and commonly referred to 

 as the siphuncle, is simjjly the solid sheath, which it will be more 

 convenient for the present jDurpose to call the siphuncular tube 

 or envelope. 



Continuity of the Siphuncular Tube. 



When examining some large sections of Ammonites the writer 

 noticed that the siphuncular tube was not continuous through 

 all the chambers to the body-chamber, even when the sections 

 were perfectly median. It was at first thought that this was an 

 unusual character, but when other sections were examined similar 

 facts were noticed, and a more systematic inquiry was therefore 

 Undertaken. Ammonites were examined by kind permission in the 

 College and Museum collections at Bristol, Cardiff, and Nottingham, 

 and while only a small proportion of the ammonites seen were 

 sufficiently complete or accurately cut to give the desired in- 

 formation, in every specimen which showed enough detail the same 

 rule was found to hold good. 



A section of Asieroceras obtusum (Sow.), of diameter 70mm. in the 



^ F. Grandjean " Le Siphon des Ammonites et des Belemnites ": Bull. Soc. 

 Geol., France, ser. iv, vol. x, 1910, p. 496. 



^ This further point of resemblance between Ammonites and Belemnites is 

 interesting in view of Dr. W. D. Lang's recent suggestion concerning their 

 classification (Proc. Geol. Assoc, vol. xxx, p. 59). 



