A. E. Trueonan — The Amtnonite SipJittncle. 27 



Natural History Museum, Nottingham, shown diagrammatically in 

 Fig. 1, will serve to illustrate the character in question. That 

 the section is cut accurately along the median plane is shown by 

 the position of the keel and the size of the septal necks for the 

 passage of the siphuncle. The siphuncular tube can be clearly seen 

 along the greater part of the spiral, but after a diameter of 38 mm. 

 (S, Fig. 1) no trace of the siphuncular tube can be made out in the 

 succeeding ten chambers. The wide sej)tal necks in the section 

 at that point show that its non-appearance is not due to asymmetry.' 

 Apparently the only conclusion that can be drawn, therefore, is 

 that the siphuncle, which was, of course, continuous throughout 

 the spiral, had an envelope which did not extend through the ten 

 chambers preceding the living chamber. 



Fig. 1. — Diagram of a section of A-steroceras obtusion (Sow.) showing 

 the siphuncular envelope (S). 



Similar facts were also observed in the following Ammonites : — 



1. A large unidentified specimen from the Inferior Oolite in University 

 College, Cardiff. Diameter 330 mm. Part of living chamber jaresent. Six 

 chambers without a siphuncular envelope. These six chambers and the living 

 chamber are filled with oolite, the earlier chambers with calcite. 



2. Parhinsonia sp. University College, Nottingham. A specimen, 

 diameter 240 mm., septate tliroughout (that is, with no part of the body- 

 chamber retained), has no siphuncular tube in the last three-quarters of a 

 whorl. That is, for at least fifteen chambers there was no siphuncular envelope. 



3. Deroceras sjd. Natural History Museum, Nottingliam. Diameter 

 85 mm. Septate tliroughout. No siphuncular tube after diameter 44 mm., 

 that is, at least fifteen chambers without. 



4. Harpoceras exaratum. University College, Cardiff. Diameter 76 mm. 



^ Further, asymmetrjr is of extremely rare occurrence in such keeled 

 ammonites. See Swinnerton & Trueman, Quart. Journ. Geol. Soc, vol. Ixxiii, 

 1918, p. ,54. 



