DaviesieUa llaiigollensis in DerbysJiire. 463 



horizon of these limestones being regarded by the late Dr. Wheelton 

 Hind and Mr. J. T. Stobbs as the upper portion of the Seminula 

 Zone (= 82).^ 



It will be recalled that Dr. Sibly, in his paper on the Faunal 

 Succession of the Midland Area,^ remarks that the North Wales 

 limestones containing D. llangollensis must be regarded as of 

 lower horizon than any beds seen in the typical section of the 

 Midland area. It is probable that when Dr. Sibly was studying 

 this section some 15 or 16 years ago, the little roadside quarry 

 containing D. llangollensis may not have been opened. 



The question of the horizon of the D. llangollensis Eeds calls for 

 some discussion, in which it will be necessary to review the records 

 of the occurrence of the species in different areas. 



The Topley Pike limestones were included by Dr. Sibly in the 

 lower portion of the series correlated with the Dihunophyllum 6 

 sub-zone (= D;^) of the South- Western Province. The presence 

 of D. llangollensis, however, suggests that the limestones con- 

 taining it should, now be transferred to a lower horizon, viz. the 

 Seminula Zone. 



It is important to note that one of Davidson's types of this species 

 (Dav. Monog., pi. xlv, fig. 6) comes from Tidenham Chase, If miles 

 north-east of Chepstow, Gloucestershire. The original specimen 

 is in the Manchester Museum (Eegistered No. L9838). It agrees 

 in every way with the other examples from the Oswestry district 

 figured by Davidson (pi. xlv, figs. 1-4), and was used by him to 

 illustrate the enormous thickening of the ventral valve. The 

 small amount of matrix between the two valves consists of dolomitic 

 limestone. 



This occurrence of the species in the South- Western Province 

 has apparently been overlooked, as I can find no reference to it 

 in any of the numerous papers on the correlation of the Carbon- 

 iferous Limestone. 



With regard to the horizons represented in the Carboniferous 

 Limestone around Chepstow, the late Dr. A. Vaughan gives an 

 outline of the sequence in this area, and a correlation of this sequence 

 with the Avonian of Bristol.'' From this outline it appears that 

 the folloAving zones are represented : Kj, Zj, Zg ; above the latter 

 level there is a lack of exposures and the next higher beds 

 met with belong to the Seminula Zone. For some distance above 

 the Zj level the beds are extensively dolomitized. As in the Bristol 

 area, the Seminula Zone is thick, and contains the typical assemblage 

 of fossils. Dr. Vaughan found no trace of the Dihunophyllum 

 Zone. Unfortunately he gives no list of the Chepstow fossils. 



1 Hind & Stobbs, " The Carboniferous Succession below the Coal -Measures 

 in North Shropshire. Denbighshire, and Flintshire " : Geol. Mag., 1906, pp. 385- 

 400, 445-59, and 496-507. 



2 Sibly, op. cit., 1908, p. 76. 



3 Q.J.G.S., voh Ixi, 1905, p. 251. 



