Geological Society. 135 



April 18th, 1917.— Dr. Alfred ITarker, F.R.S., President, 

 in the Chair, 



The following communication was read : — 



' The Development and Morphology of the Ammonite Septum.' 

 By Prof. Henry Hurd Swinnerton, D.Sc, F.Gr.S., and Artluir 

 Elijah Trueman, M.Sc. 



Two methods of studying the septum (not merely the suture) 

 were used : — 



(1) Cleaning the face of the septum completely. 



(2) Filing away the surface of the whorl in successive layers, and thus 



making a series of sections — called septal sections — of the septum 

 parallel to its periphery. 



An instrument was designed for measuring accurately the varia- 

 tions in level of the face of the septum in relation to a definite 

 datum-plane ; and also the thickness of the layers filed' off from 

 the whorl. 



Dactylioceras commune, Spheeroceras hrongniarfi, and Tracjo- 

 pJiylloceras loscomhi were chosen as types with normally shaped, 

 greath^ depressed, and greatly elevated whorls respectively. 



A contoured plan, of the adult septum of Dactylioceras, shows 

 that half the septum lies approximately in one plane ; and that the 

 posterior folds or lobes occupy a greater area than the anterior 

 folds or saddles. It also confirms the view that the septum is, on 

 the whole, convex forwards. In all three types the axes of the 

 folds remain approximately at right angles to the periphery 

 through all the changes in shape of the whorl. Incompletely formed 

 septa indicate that secretion commences at the umbilical angle and 

 at a definite distance from the preceding septum. 



The examination of adult sutures of various species of Dactij- 

 lioceras shows that the major frillings alone are of systematic 

 importance for that genus. The variations in the minor frillings, 

 and in the sutm*e-line as a whole, throw light on the changes 

 which accompany senile decline. 



The second septum is remarkably like the central portion of the 

 adult septum ; but the flattened portion is relatively less extensive, 

 the folds ai'c sharper, and the whole septum tends to be concave 

 rather than convex. As development advances the successive septa 

 possess a similar resemblance to an increasing area of the adult 

 septum. The outcome of this is, that a series of septal sections of 

 the adult septum closely resembles the developmental series of the 

 suture-line. The same is true also for Sphccroceras and Trayo- 

 phylloceras. 



In no case do the septal sections show a stage comparable with 

 the first suture-line. In TrayopliyUoceras the similarity starts 

 not later than the seventh septum. With these exceptions, septal 

 sections reproduce the main features in the development of the 

 sutures with sufficient accuracy to justify their use for the same 

 purpose, especially when the material for the stiidy of the early 

 stages is inaccessible. 



