Reports and Proceedings—Geological Society of London. 285. 
The stratigraphical relationships are seen most clearly in the highest 
group (C), which is therefore dealt with first. The beds here follow 
each other in perfectly regular succession, with a uniform strike of 
K. 30° N. The basal beds, with a fauna belonging to the zone of 
Monoyraptus cyphus, form a well-marked ridge across country, and 
Upper Birkhill and Gala Beds follow to the north-west. 
The second group (B) occupies a wide tract to the east of the 
Llansawel Group. The coarse basal deposits, and the characteristic 
shales and mudstones, are described from many localities. 
The lowest group (A) has its greatest development on the south of 
Llansawel. 
The structure in the eastern part of the district shows many points 
of interest, and is very much more complicated than in the west. 
The repeated outcrops of the conglomerate in the hilly region around 
Shon Nicholas are described in detail, and these give the clue to the 
structure. 
The paper concludes with a general summary and a brief comparison 
of this district with those of Rhayader and Pont Erwyd. 
Il.—Aprii 27, 1910.—Professor W. W. Watts, Sc.D., M.Sc., F.RB.S., 
President, in the Chair. 
The following communications were read :— 
1. “On the Evolution of Zaphrentis delanouet in Lower Carboni- 
ferous Times.”” By Robert George Carruthers, F.G.S. 
The small simple corals that belong to the gens of Zaphrentis 
delanouet are of common occurrence in the Lower Carboniferous rocks 
of Scotland. Their distribution is remarkably sporadic, but it is 
possible to collect over wide areas of which the stratigraphy is 
definitely known. A large number of specimens have been got 
together (some twelve hundred in all) from horizons scattered 
throughout the sequence. The ontogeny of these specimens has been 
investigated by means of serial transverse sections. 
The evolutionary changes observed aré confined to the disposition 
of the septa, which has influenced the shape of the cardinal fossula 
in a very marked manner. The external characters, and the spacing 
and curvature of both septa and tabule, remain unchanged. 
Zaphrentis delanouei is, typically, a Tournaisian species, and it has 
a wide fossula, expanded inwardly. When the gens first appears in 
the Scottish rocks (in the Cementstone Group of Liddesdale) 
4. delanouet is the predominant form, but is associated with a mutation 
(in Waagen’s sense) in which the fossula is parallel-sided. 
In the higher limestones of Lawston Linn another mutation 
appears, which, for reasons detailed in the paper, is regarded as 
a sport, or offshoot from the direct line of evolution. 
In the succeeding Lower Limestone Group the gens undergoes 
further modification. Adults of the two Cementstone species are 
extremely rare, and the predominant form has a fossula which 
narrows rapidly to the inner end ; in subordinate association a further 
mutation is also developed, in which the septa are short ‘and 
amplexoid. 
In the still higher horizons of the Upper Limestone Group the 
