Vol. 62.] LIMESTONE OF THE MENDIP AREA. 357 



(3) Athyris cf. expansa, which is an important fossil in this zone in the 



Mendips, is not known in the Bristol area. 



(4) Caninia cylindrica attains its second maximum in the upper part of 

 this zone in the Mendips, but in the Lower Semimda-Zone in the 

 Bristol area. This is another example (see above, p. 355) 

 of the relative acceleration of certain coral-groups on the 

 brachiopod-progression in the Mendips as compared with 

 the Bristol area. 



The SuMiNULA-Zone. 



The differentiation of this zone into a lower subzone (S x ) in 

 which the Seminula-fa,\m& contains certain survivors of the 

 Syringothyris-i&ima,, and an upper subzone (S 2 ) possessing a limited 

 but essentially-Kidwellian fauna, is a well-marked feature in the 

 Mendips, as in the Bristol area,, The only noticeable dif- 

 ferences between the two areas are to be found in the 

 characters of S x ; and part of the difference between the 

 Mendip and Bristol development of S x is undoubtedly 

 due to the fact, that the lowest portion of this subzone 

 consists of fossiliferous limestones in the Mendips, and 

 of relatively-unproductive shales and dolomites in the 

 Bristolarea. The most important points of difference are the 

 following : — 



(1) In the Bristol area, Seminula becomes abundant before giganteid 



Proditcti are common, and is very prolific in S r In the Mendips, on the 

 other hand, giganteid Producti are abundant throughout S 15 whereas 

 Seminula only becomes abundant in the upper part of that subzone. 



(2) Athyris cf. expansa and A. cf. glabristria, mut. S 1} two fossils which are 



important in S x of the Mendip sequence, are not met with in the Bristol 



(3) The base of S x in the Mendips contains, locally, a rich faunal assemblage, 1 



which is especially characterized by the abundance of Carcinophyllum 

 mendipense, Camarophoria isorhyncha, Spirifer cf. furcatus, and Athyris 

 cf. glabristria, mut. S x . No similar development is known in the 

 Bristol area. 



(4) Caninia cylindrica attains its second maximum in S t in the Bristol area. 



In the Mendip area, Caninia cylindrica attains its second maximum in 

 the Upper Syrinyothyris-Zone, and is not common in S r 



The DiBUNOPHYLLUM-TiOne. 



The development of shales in this part of the Mendip sequence 

 is small, when compared with that in the Dibmiojphyllum-Zone of 

 the Bristol area. 



The 6(j> and Lonsdaliasubzones. 



The faunal characters of these subzones in the Mendip area are 

 more nearly identical with those of the same subzones in the Bristol 



1 It would seem that this fauna had a very local distribution. For example, 

 the base of the Seminula-Zone is exposed in both the Cheddar and Burrington 

 sections ; but, whereas the former section 6hows a fine development of this 

 fauna, I have failed to discover any trace of a similar horizon at Burrington. 

 The Milton-Koad Quarries, in the Weston-super-Mare district, show a develop- 

 ment precisely similar to that which is noted at Cheddar. 



