THE CARBONIFEROUS LIMESTONE OF BURRINOTON COMBE 33 
Orthotetes loses its predominance, and is rare above the 
top of the horizon. Very few forms characteristic of the 
Seminula Zone occur below the top of this horizon, and 
these are distinctly rare, e.g., Seminula aff. flcoidea only 
occurs rarely, and no Lithostrotion has been recorded. 
Seminula aff. flcoidea and Lithostrotion first occur in any 
abundance at a considerable vertical distance above the 
top of Horizon Chonetes papilionacea is abundant 
at repeated levels throughout the Syringothyris and 
Seminula Zones, and its occurrence both at and above 
Horizon S is, therefore, no evidence of a faunal overlap.^ 
In the division of the Carboniferous Limestone into 
two great stages, a lower or Tournaisian and an upper or 
Visean, the line of separation must be drawn at the top of 
Horizon and is, as the above remarks show, clearly 
defined in this section. This horizon is, therefore, an 
extremely important one ; further work in the Mendip 
Area will doubtless add greatly to our knowledge of its 
characters. 
Comparison with the Bristol Area. 
The development of this part of the sequence at Burring- 
ton differs greatly from that in the Bristol Area. 
At Burrington we have an unbroken series of massive 
limestonesextending upwards from Horizon y to the base 
of the Seminula Zone, and containing abundant corals 
and brachiopods throughout. This series may be very 
satisfactorily designated the Syringothyris Zone. Syringo- 
thyris cuspidata, in the typical form, occurs commonly 
throughout and attains a maximum in this zone. More- 
over, this form is not common below Horizon y, and dies 
out at Horizon S. At the base of the zone. Horizon y 
forms a well-marked horizon of overlap between the 
Zaphrentis and Syringothyris faunas. Horizon S, on the 
1 Vide footnote, p. 35. 
D 
