ON LOWER CARBONIFEROUS ZONAL NOMENCLATURE. iY 
development of Orionastrea phillipsi. This coral has been noted from various widely 
distributed Lower Carboniferous areas, it has been used as a local index, and is one 
whose contemporaneity of development can be readily tested, the more so as its 
occurrence is not dependent on any peculiarity of lithological conditions. It is 
therefore proposed that this zonal fauna which occurs immediately above the zone D 
shall be denoted by the zonal index O. It should be noted that it is not intended 
that the zone O should be co-extensive with the development of Orionas(rea phillipsi. 
For instance, it is probable that the rare occurrence of O. phillipsi in the upper beds of 
the Lower Carboniferous of the Avon Gorge is an early manifestation of this form, 
and occurs in D, along with the maximum development of O. ensifer. The applica- 
tion of this suggested zonal scheme to areas the faunal succession of which has been 
already described is shown in Table I. 
The Subdivision of the Zone D. 
The zone D was originally divided into D, and D,, and these sub-zones possess 
characteristic faunal assemblages that have been identified in most areas where a 
normal coral-brachiopod fauna is developed. This subdivision has been departed 
from in later papers by various authors who have either recognised higher horizons, 
which they have referred to by such zonal indices as Dy or else expressed the phasal 
character of the fauna by such symbols as P or D, and D,.3, which latter have been 
variously used as denoting higher horizons or phasal faunas. Dy has not received 
general acceptance, and the limitation of the zone D as proposed above excludes 
the horizons previously denoted by Dy from the zone D. The Committee therefore 
recommends that it should be abandoned as a zonal index. The question of phasal 
faunas and their indices is discussed later. There remains the advisability of the use 
of D,. D,; was originally proposed by Sibly* for a faunal assemblage which occurs in 
Derbyshire above a normal D, fauna and which he divided off as the zone of 
Cyathaxonia rushiana. This fauna differs from the normal coral-brachiopod fauna 
in that it contains numerous Zaphrentids and allied genera and a specialised brachiopod 
fauna, and generally approaches to a shale fauna. Such a faunal assemblage has 
been termed a Zaphrentid-phase by Vaughan and Dixon. D, and Ds.3 were later 
used by other authors as Zaphrentid-phasal indices of high horizon, a use which the 
Committee recommend should be discontinued. On the other hand, the use of D, 
for the upper subdivision of the zone D has much to recommend it. In most cases 
where the fauna of the D zone above D, has been investigated and where it is of the 
same phase throughout, it has been divided into faunally distinct upper and lower 
series. This distinction of the two faunal assemblages was noted by Vaughan 
and I. Thomas. Vaughan divided the upper D fauna of Anglesey into D2, and Dys, while 
I. Thomas preferred to indicate the same faunas as D, and D,.5 A similar division 
of the upper D beds is found in the Settle district and has been separated as the - 
Lower Lonsdaleia beds and the Lamellibranch bed (P. pugilis band).6 The same 
faunal sequence occurs in the Dale Country’ and in the Whitehaven district.8 It 
thus seems that where the coral-brachiopod fauna is continued throughout D the 
upper part contains two distinct faunal assemblages which may be referred to as 
upper and lower D,, or—and this seems to the Committee more acceptable—D, 
and D;. It seems probable, too, that D, would then connote a faunal assemblage 
of the same horizon as that for which Sibly coined the term D,, for it is probable 
that the zone of Cyathaxonia rushiana is the Zaphrentid-phasal equivalent of D, as 
defined above and which would thus retain its original horizon. D, is therefore 
defined as the upper division of the D zone with a faunal assemblage as described by 
the various authors mentioned above. If there is need of an index form, Lonsdaleia 
duplicata duplicata is the most suitable, as this form, although occasionally found 
much earlier, attains its maximum at this horizon. 
4 Sibly, T. F., ‘ Faunal Succession in the Carboniferous Limestone of the Midland 
Area,’ Q.J.G.S., vol. lxiv., 1908, pp. 34-82. 
5 Greenly, E., ‘The Geology of Anglesey,’ vol. ii., Mem. Geol. Surv., London, 
1919, pp. 608-612. F 
6 Garwood, E. J., and Goodyear, E., op. supra cit., pp. 201-295. 
7 Hudson, R. G. S., op. supra cit., pp. 182-183. 
8 Edmunds, C., op. supra cit., p. 83. 
1925 Ss 
