part 1] ox THE MORPTIOLOOT OF THE AMMOXITE SEPTU]Sr. 88 



different directions are here convex there concave. Branco ^ and 

 Hyatt ~ both observe that in median sections of ammonites tlie 

 ])rofile of the septmn, passing as it does along the troughs of the 

 doi'sal and ventral lobes, necessarily appears convex forward. On 

 the other hand, it may be remarked that profile sections along the 

 crests of individual saddles are concave forwards. Thus the form 

 of an anterior fold of the septum is comparable with that of a 

 horse's saddle, or with the heteroccrlous articular surface of a bird's 

 cervical vertebra, since sections along the crest, and at right angles 

 to it, are concave, and convex respectively. 



Note. — J. F. Blake sug-gested that the forward convexity of the septum 

 is evidence of pressure from behind the animal, and that the saddles represent 

 parts of the edg-e which have yielded to the pressure. In more recent work 

 Pfaff •* also concludes that the form of the septum has been determined largely 

 by pressure upon the posterior part of the animal. Probably, as in the Pearly 

 Nautilus (Willey"*), the chambers of the shell were filled with gas secreted by 

 the veins of the mantle. In ammonites the vigour of secretion may have 

 been so great that the gas exerted sufficient pressure upon the soft mantle to 

 make it bulge forward, while the septum was being deposited. The forma- 

 tion of a new septum must always have been preceded by a period of rapid 

 evolution of gas. Perhaps the pressure thus produced helped the forward 

 shifting of the posterior end of the long worm-like body of the ammonite. 



In all three types of ammonites considered, the axes of the folds 

 are always approximately at light angles to the whorl-surface 

 (PI. Ill), and they maintain this relation throughout all the changes 

 in shape of the whorl which take place during development or 

 during the distortions associated with asymmetry (PI. IV, figs. I, 

 4, & 9). _ ^ 



Each pair^of folds behaves independently of the others. Thus, 

 in tlie development of the septum of Dacfi/lioceras commune the 

 axis of the internal saddle is at one stage almost continuous with 

 that of the external saddle (PL III, fig. 3) ; but as, during 

 development, the whorl increases in height, it comes into a similar 

 relation with the first lateral saddle (PL III. figs. I & 2). The 

 same independence is manifested also in the development of the 

 septum in Tragoplnjlloceras.'' 



This independence finds further expression in the variations of 

 arrangement of the folds according to the shape of the whorl. 

 When this is round, as, for example, in Li/foceras, ihej radiate 

 from a central point ; when it is impressed dorsally, as, for instance, 

 in Dacti/lioceras and Spliceroceras (PL III, figs. 1 & 7), the 

 internal folds come into relation with the corresponding external 

 saddles, and the number of dorsal and ventn^l elements is nearly 

 equal; when it is compressed, as in Tragojjliylloceras (PL III, 



^ ' Beitrage zur Entwickelungsgeschichte der Fossilen Cephalopoden ' Palse- 

 ontographica, pt. 2, vol. xxvi (1879-81) p. 50. 



- ' Genesis of the Arietidte ' Smithson. Contrib. to Knowledge, No. 673, 

 1889. 



'^ ' Form & Ban der Ammonitensepten & ihre Beziehungen zur Suturlinie ' 

 Jahresb. des Niedersiichs. Geol. Verein, vol. iv (1911) p. 208. 



"* 'Contribution to the Natural History of the Peai-ly Nautilus' Zool. 

 Results .... New Britain, New Guinea, &c., pt. vi, 1902, 4to. Cambridge. 



^ L. F. Spath, Q. J. G. S. vol. Ixx (1914) pi. xlix. 



Q. J. G. S. No. 289. p 



