The Morphology and Development of the Ammonite Septum

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Abstract
I. Introduction. Several years ago one of us noted with particular interest the appearance of the sutures upon some badly-weathered ammonites. They were characterized by an absence of frilling and a simplicity of curvature strongly suggestive 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 by variations in the shape of the whorl in different 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-fine. The value of the latter has long been recognized, and a distinguished succession of workers, including Sowerby, Hyatt, Neumayr, and Buckman, have made very 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 comparatively neglected, and much of the information which exists concerning it must be gleaned from figures, frequently undiscussed. There has also been a limited discussion upon the mechanical conditions which have been concerned in modelling the septum, and upon the relations of newly-formed septa to their predecessors. II. Methods of

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