30 A. E. Truer)ian — The Ammonite SipJnvncle. 



of Ammonites. It may have been an old-age or adult character, 

 indicating feebleness of powers of phosphate secretion, or it may 

 have been a normal feature of growth, the envelope being formed 

 by a siphuncle but always leaving several chambers in which it was 

 incomplete. 



To test the truth of this latter suggestion sections were cut 

 through a number of young Ammonites, those selected being young 

 individuals with at least a portion of the body-chamber preserved, 

 and not simply the inner whorls of adults. In each example 

 investigated it was found that in the chambers preceding the living 

 chamber there was no siphuncular envelope. 



The following specimens were cut : — 



Hildoceras cf. hifrons (Brug.), from Grantham. Diameter 24 mm. 

 Scarcely any sign of a siphuncular envelope in the last whorl. 



Dactylioceras sp. from Grantham. Diameter 3' 5 mm. No 

 siphuncular tube in last two chambers. 



Amhlycoceras sp. from Leckhampton. Diameter 4'2 mm. No 

 siphuncular tube in last three chambers. 



It therefore appears that in many, if not all, ammonites, young 

 and old, the siphuncle is without an envelope in the chambers 

 preceding the body-chamber. The number of chambers in which 

 there is no envelope apparently increases with the age of the 

 individual, but probably depends on other conditions also. 



Previous Observations on the Extent of the 

 Siphuncular Tube. 



So far as the writer is aware, this incompleteness of the siphuncular 

 envelope has not previously been recorded, although, curiously 

 enough, Zittel has described an exactly opposite instance.^ In 

 some casts of A. elimatus from Williamswitz the siphuncular tube 

 is not preserved, and the groove which presumably marks its 

 position appears to persist a short way into the body-chamber. 

 Beyond this Zittel gives no information, and apparently did not 

 refer to it further in his later works. While this observation may 

 indicate that in some species or individuals the secretion of an 

 envelope proceeded more quickly than in those described above, it 

 is doubtful whether the secretion actually occurred within the 

 body-chambers, and it must be pointed out that Zittel was relying 

 for his evidence not on the actual siphuncular envelope but on the 

 extent of a groove which may have contained it. 



Grand] ean suggested that Zittel's observation affords a possible 

 explanation of the formation of the siphonal membranes, the 

 association of which with the covered siphuncle he was unable to 

 understand.^ It may be briefly noted that the facts recorded 

 above make it possible to give an equally simple explanation. 



• In Oppel, Palseont. Mitt. Mus. d. k. bayer. Staates, Bd. ii, 1868, pp. 79-80. 

 I have to thank Mr. F. T. Ingham, B.Sc, for supjDlymg particulars of this 

 paper. 



2 Op. cit., p. 502. 



