Reports & Proceedings — Geological Society of London. 231 



The change of lithological and faunal character in the zones 

 which escape overstep is due to : — 



3. Progressive increase in the vertical extent of contemporaneous dolomiti- 

 zation ; supplemented, in the area east of the Taff , by a great development of 

 Modiola--ph9.se deposits in which dolomite mudstones predominate. 



The outcrop now described supplies, therefore, a key to the 

 remarkably attenuated development of the Carboniferous Limestone 

 Series which is known to prevail on the eastern and north-eastern 

 borders of the coalfield. Overstep and actual thinning are both 

 operating in a north-easterly direction to produce great attenuation. 



The unconformity between the Carboniferous Limestone and the 

 Millstone Grit is doubtless due to the earth-movement which caused 

 flagrant unconformity between Lower and Upper Carboniferous in 

 the Forest of Dean. 



A detailed description of the lithological and faunal succession is 

 given. The physical features of the outcrop are described, and 

 attention is drawn to the remarkably perfect adjustment of minor 

 drainage-lines to geological structure. The paper is illustrated by 

 maps on which the zonal divisions are indicated, by horizontal and 

 vertical sections, and by photographs which depict some of the most 

 interesting features of the scenery. 



4. April 18, 1917.— Dr. Alfred Harker, F.R.S., President, in the 



Chair. 



The following communication was read : — 



" The Development and Morphology of the Ammonite Septum." 

 By Professor Henry Hurd Swlnnerton, D.Sc, F.Gr.S., and A. E. 

 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 variations 

 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, Sphcerocerasbrongniarti, and Tragophylloceras 

 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 of whorl shape. Incompletely formed septa indicate that 

 secretion commences at the umbilical angle and at a definite distance 

 from the preceding septum. 



