40 THE CARBONIFEROUS LIMESTONE OP BURRINGTON COMBE 
zon <y is reached. The most striking features of the resu- 
pinata sub-zone, by comparison with the Bristol develop- 
ment, are the entrance of Caninia, Michelinia and Am- 
plexus in abundance and the persistence of Spirifer aff. 
clathratus and var. as abundant forms. 
Horizon y is finely developed. The beds are extremely 
fossiliferous and exhibit the following striking features : — 
(1) An extraordinary abundance of Caninia cylindrica. 
(2) The by no means uncommon occurrence of several brachiopods, 
which in the Bristol Area are characteristic of the lower 
resupinata beds and absent or very rare at this horizon. 
(3) A well-marked overlap of the Zaphrentis and Syringothyris 
faunas. 
C. Syringothyris Zone. 
Between the top of the Zaphrentis Zone and the base 
of the Seminula Zone there is a thick series of massive 
limestones, mainly oolitic. This series is fossiliferous 
throughout and is well defined as the Syringothyris Zone. 
At the base. Horizon y exhibits a clear faunal overlap. 
Throughout the main zone we see the gradual decline 
of the Tournaisian fauna, though Syringothyris cuspidata, 
which attains a maximum, and Productus semireticulatus 
(muts.) are very characteristic throughout. 
At the top of the zone. Horizon S, which forms a well- 
marked faunal level and marks the end of the Tournaisian 
facies, is well developed. 
A few Visean forms enter near the top of this zone, but 
on the whole the separation of the Tournaisian and Visean 
facies is sharp : the line of separation must be drawn at 
the top of Horizon S. 
D. Seminula Zone. 
This zone consists of limestones which are sometimes 
thin-bedded, but chiefiy massive. In the middle of the 
zone the limestones are oolitic and near the top they are 
concretionary. The top beds are very compact in texture. 
