Vol. 62.] LIMESTONE OF THE MEXDIP AREA. 35^ 



Corresponding to the clatliratus- and mi^-ftrtta-subzones of the 

 Bristol area, we have the clatliratus- and cornucopice-subzoiLes of the 

 Mendip sequence. The reason for adopting Zaphrentis aff. cornucopia* 

 as a subzonal index in the Mendips, in preference to Scliizoplioria 

 aff. resupinata, is as follows : — Scliizoplioria aff. resupinata occurs 

 somewhat sparingly below Horizon y, and attains its maximum 

 immediately above that horizon, that is, in the base of Sgringothgris- 

 Zone : whereas Zaplirentis aff. cornucopice is very common throughout 

 the upper half of the Zaphrentis-Zone, and attains its greatest 

 abundance below Horizon y ; in fact, the extent of the upper 

 subzone is well defined by the range of the latter fossil, in its 

 typical form. 



The differences between the Mendip and Bristol areas, in the range 

 and relative abundance of various fossils throughout this zone, can 

 be studied by comparing my range-diagrams (Pis. XXXIY & XXXV) 

 with those of Dr. A. Vaughan. 1 The important points of difference 

 may be summarized by stating that, in the Mendip area, there 

 is a relative acceleration of the coral-fauna on the 

 brachiopod-fauna, in this part of the sequence. This 

 relative acceleration is exemplified in the following notes. 



In the Bristol area : — 



Mickelinia is rare in Z,. 



The maximum of Syringothyris aff. cuspidata, mut. C lies in Z 2 . that is, 

 below the maximum of Caninia. 



Schizophora aff. resupinata is most abundant at the base of Z 2 , that is, 

 below the maxima of Caninia and Zaphrentis. 



Spirifcr aff. clatliratus and Tar. are uncommon at Horizon y, which 

 level contains the maxima of Caninia and Zaphrentis. 



In the Mendip area : — 



Mickelinia is abundant in the lower part of Z r 



The maximum of Syringothyris aff. cuspidata, mut. C occurs in the 

 Syringotkyris-Zor\Q, that is, above the maximum of Caninia. 



Schizophoria aff. resupinata attains its maximum abundance immediately 

 above Horizon y, that is, abov.e the maxima of Caninia and Zaphrentis. 



Spirifer aff. clatliratus and var. are still abundant at Horizon y ; this 

 level contains the maximum of Caninia, but the maximum of Zaphrentis 

 lies somewhat lower. 



The very strong definition of Horizon y, by the great abundance 

 of Caninia cylindrical is a noticeable feature of the Mendip sequence. 

 (Compare the Clevedon district 2 and the Backwell-Wrington mass 3 

 in the Bristol area.) 



The SYBTKGOTHYEIS-Zoue. 



The most important difference between the Mendip 

 and Bristol areas lies in the development of this part of 

 the sequence. 



1 Quart. Journ. Geol. Soc. vol. lxi (1905) pis. xxviii & xxix (facing pp. 244 & 

 246). 



2 Ibid. p. 228. 3 Ibid. p. 240. 



