28 A. E. Trueman — The Atnmonite Siphuncle. 



Portion of living chamber present. No siphuncular envelope after 43 mm., 

 that is. twelve chambers without. 



5. Oxynoticeras oxynotum. Welsh National Museum. Has no siphuncular 

 tube for twenty-five chambers, the whole of the last whorl. 



6. ? Lioceras sp. Bristol University. Diameter about 150 mm. Body- 

 chamber present. No siphuncular tube in last nine chambers. The body- 

 chamber and the preceding nine chambers have oolite filling, the earlier 

 chambers have calcite. 



It frequently happens that while the chambers of an Ammonite 

 are filled with calcite or other crystals, the body-chamber is filled 

 with material such as clay or oolite, which was able to get in when 

 the animal decayed. But in several of the Ammonites referred to 

 above, not only the body-chamber, but also the chambers without 

 a siphuncular tube, were filled with such material, probably 

 indicating that when the Ammonite died the siphuncle decayed, 

 allowing detrital material to enter those chambers where there 

 was no siphuncular tube. Using this type of evidence it would be 

 jjossible to extend the above list very considerably. 



Othee Indications of a discontinuous Siphuncular Tube. 



While the most obvious way of investigating the continuity of 

 the siphuncular tube is by means of median sections, yet useful 

 information concerning some Ammonites has been obtained from 

 casts and impressions. These cases may be considered in three 

 groups : — ■ 



1. In some casts of Ammonites, especially those which have 

 been rubbed or polished, the siphuncular tube may be seen along 

 the external margin. In such casts it was found that the siphuncular 

 tube did not continue unbroken to the body-chamber. 



For instance, a polished cast of an unidentifiable Ammonite in 

 the teaching collection at University College, CardifE, has a con- 

 tinuous siphuncular tube up to a diameter of 175 mm. Following 

 this are three chambers, the remainder of the shell not being present. 

 The siphuncular envelope in these chambers is discontinuous (see 

 Fig. 2). In the last two chambers short portions of the tube (S 2, 

 S 3) are attached to the front septal neck only ; in the third chamber 

 the portion of the tube is free (SI) and on the side of the cast. It 

 appears, therefore, that the tube was discontinuous in these 

 chambers (and in any others in front that may originally have been 

 present), and that the secretion of the solid tube commenced between 

 the septa, each joortion becoming attached to the septal neck in 

 front. With the death of the animal the siphuncle decayed, and 

 these portions of the tube were unsupported, and the shell having 

 fallen on its side they either hung loosely from their attached ends 

 (S 2, S 3) or slipped along the inside of the shell (SI). 



A similar condition is shown by an example of HojMtes desJiayesi 

 (Leym.) in the Welsh National Museum. In this specimen part of 

 the living chamber is present at a diameter of 70 mm. The siphun- 

 cular tube appears on the outside of the cast as a continuous black 



