Geological Society. 135 
April 18th, 1917.—Dr. Alfred Marker, 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.G.S., and Arthur 
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, Spheroceras brongniarti, and Trago- 
phylloceras loscombi were chosen as types with normally shaped, 
greatly 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 Dacty- 
lioceras shows that the major frillings alone are of systematic 
importance for that genus. ‘The variations in the minor frillings, 
and in the eaturedine 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 are 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 oe septal sections of 
the adult septum closely resembles the developmental series of the 
suture-line. The same is true also for Spheroceras and Trago- 
phylloceras. 
In no case do the septal sections show a stage comparable with 
the first suture-line. In Tragophylloceras 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 study of the early 
stages is inaccessible, 
