34 THE CARBONIFEROUS LIMESTONE OP BURRINGTON COMBE 
other hand, marks the top of the Syringothyris Zone very 
clearly, but does not form a good horizon of faunal overlap, 
the separation of the Syringothyris and Seminula faunas 
being apparently sharp. In the Bristol Area, Dr. Vaughan 
has adopted Syringothyris aff. laminosa as a sub-zonal 
index for that part of the zone lying between Horizon j 
and Horizon At Burrington, however, since Syringo- 
thyris aff. laminosa is never common in this zone, though 
it occurs throughout, and since it attains a maximum 
in the Zaphrentis Zone, I do not propose to use it as an 
index. All that part of the zone lying between Horizon y 
and Horizon ^ is, therefore, referred to as the Main Syringo- 
thyris Zone. 
The characters of this zone throughout the Bristol Area 
are so widely different from those of the Burrington series 
that a detailed comparison would necessarily be of great 
length ; more especially so because, in the Bristol Area 
itself, the lithological and palaeontological characters of 
the zone vary from point to point. I shall, therefore, 
confine myself to a few broad comparisons, taking the 
Avon Section as a type, and only referring to other parts 
of the Bristol Area when it is necessary to bring out special 
features. 
The lithological characters of the zone are widely 
different from those of the same zone at Burrington. In 
the Avon Section we have, in ascending order from Hori- 
zon y — 
(1) Limestones appreciably dolomitic. 
(2) A thick band of oolite — the Caninia or Gully Oolite. 
(3) Shales with thick beds of pure dolomite and subsidiary beds 
of oolite. 
(4) Massive limestones with thiri shale partings 
(I), (2) and part of (3) form the laminosa sub-zone. 
The remainder of (3) with (4) forms the lower part of the 
Seminula Zone, i.e. the semireticulatus sub-zone. The 
