26 PROF. H. H. SWIXNEETO^f A^'D ME. A. E. TRUEMAIS' [vol. Ixxiu, 



3. The Morphology and Developmext of the Ammonite 

 Septum. By Hexry Hurd Swixxerto>, D.Sc, F.Gr.S., 

 Professor of Greology & Geography in University College, 

 Nottingham, and Arthur Elijah Truemax, M.Sc. (Read 

 April 18th, 1917.) 



[Plates II-IV.] 



Contents. 



Page 



I. latrodiiction 26 



II. Methods of Investigation 27 



III. Materials used in the Investigation 31 



IV. The Morphology of the Adult Septum 31 



V. Terminology of Lobes and Saddles 34 



VI. The Formation of the Septum 35 



VII. The Development of the Septum of Dactylioceras commune. 36 



VIII. The Adult Suture of the Dactyloida , 38 



IX. The Adult Suture of Dactylioceras commune 40 



X. Development of the Suture in Dactylioceras 40 



XI. Septal Sections of Dactylioceras 41 



XII. Septal Sections of Tragoi^hylloceras loscomhi 45 



XIII. Septal Sections of Sphseroceras hronyniarti 47 



• XIV. The General Consideration of Septal Sections 49 



XV. Asymmetry in Suture-Lines , .' 51 



I. Introductiox. 



Several years ago one of us noted with particular interest the 

 appearance of the sutures upon some badly-weathered ammonites. 

 They were characterized b}^ an absence of frilling and a simplicity 

 of curvature strongly suggesti\'e of the early stages in development. 

 A preliminary enquiry was immediately made into the development 

 of the sutures of Dactylioceras, and into the changes produced in 

 the adult suture by filing the surface away in imitation of the 

 process of weathering. This made it evident that the similarity 

 already noted was not merely superficial, but had a widely-extended 

 significance. It was with the view of establishing the extent and 

 limitations of this similarity', as determined b}' variations in the 

 shape of the whorl in difterent ammonites, that the present 

 investigation was undertaken. 



As the work advanced, the problem resolved itself into a study 

 of the morphology and development of the septum as opposed to 

 the suture-line. The value of the latter has long been recognized, 

 and a distinguished succession of Avorkers, including Sowerby, 

 Hyatt, Neumayr, and Buckman, have made \^yy effective use of it 

 in solving problems of ammonite affinities. On the other hand, 

 and apart from the suture- line, the septum itself has been com- 

 paratively neglected, and much of the information which exists 

 concerning it must be gleaned from figures, frequently undiscussed. 



