A. E. Trueman — The Ammonite Siphuncle. 



29 



band, passing through all but the last two chambers. In the last 

 chamber no trace of any siphuncular tube can be seen, but in the 

 preceding one a short portion of the tube has slipped along the side 

 exactly as in Fig. 2 (S 1). 



These two cases suggest that in some of the sections described 

 the apparent absence of any siphuncular tube may be similarly 

 due to the falling away of the inter-septal portions of the tube. 

 Nevertheless, there is no doubt that the tube was not present in 

 the last few chambers of all the specimens mentioned. 



Fig. 2; — Diagram showing the discontinuous siphuncular tube in a 

 Hildoceratid Ammonite. S represents part of the normal siphuncular tube, 

 with loose portions at Si, Sa, and S3. 



2. Casts of Aetomoceras scijnonianum (d'Orb.) and allied forms 

 found in the phosphatic beds at Radstock, Somerset, afford further 

 confirmation. The siphuncular tube is generally broken away, 

 leaving a groove along the external margin, but the keel is still 

 present on the cast of the body-chamber and on several chambers 

 preceding it. This suggests that there was no siphuncular tube in 

 those chambers, otherwise it would also have been broken away. 



3. Many specimens of Psiloceras cf. planorbe (Sow.), found in 

 the Lias at Lavernock, near Cardiff, have been flattened in preserva- 

 tion, and in many cases the shell was evidently so thin that it is 

 only now represented by the outer horny layer. This is soon worn 

 or even blown away, and then the impression often shows the 

 siphuncular tube as a white band running round the spiral.^ As 

 this does not extend into the outer two whorls it is extremely 

 probable that here again there was no siphuncular tube in the last 

 few chambers. 



The Siphunculae Tube in Young Ammonites. 



There are two possible explanations of this absence or incom- 

 pleteness of the siphuncular envelope in the last few chambers 



^ Mr. F. F. Miskin, F.G.S., kindly examined a number of these Ammonites, 

 and proved that this white material was calcium phosphate, and therefore 

 represents the siphuncular tube. 



