172 R. G. Carruthers—Coral Zones in Carboniferous Limestone. 
ie. in the Cyathaxonia or Dg sub-zone. It must be remembered, 
however, that C. rushiana shows a close approximation to the well- 
known Tournaisian coral C. cornu, Mich., and that the Dibunophylla 
are not widely differentiated from many of the Clisiophyllids found in 
Lower Visean, Upper Tournaisian, and Upper Devonian beds. 
The discovery of these two fossils at a Lower Visean horizon adds 
another link to a chain of evidence bearing on the nature of the zones 
adopted for the Carboniferous Limestone. “Facts have now accumulated 
in sufficient number to warrant a brief discussion of the value of these 
zones as time indices. In this respect, without wishing to criticize 
unduly a zonal system whose application to stratigraphical problems 
has met with much success, it is nevertheless desirable that the 
limitations of the system should be pointed out. 
So far as the corals are concerned, it must be said that the zones are 
not based upon the presence of forms which are in true genetic 
sequence so much as upon a succession of unrelated faunal phases. 
They are, in fact, dependent upon a series of physical conditions (not 
of necessity expressed lithologically), each of which is favourable to 
certain gentes only. Almost invariably a gens flourishing at one 
particular level is not found in the beds immediately above or below, 
but reappears after a considerable interval in a more or less modified 
form. Abundant illustrations of such a fact are supplied by the 
distribution of the corals in the South-Western and Midland Provinces. 
Thus it is found that the gens of Cleistopora geometrica disappears 
entirely above the A zone, unless it recurs at the top of the Visean 
under the guise of Paleacis cyclostoma. Again, the gentes of Zaphrentis 
delanouet in Z,, and of Z. omaliusa, Caninia cornucopia, and Cyathaxonia 
cornu in Z, y, and C, are not again found above those zones until the 
highest Visean beds are reached. They then reappear in the D, or 
C yathasonia sub-zone in a series of mutational forms (in Waasens 
sense), whose difference from the parent stock is not great, considering 
the interval of time that has elapsed. 
The sudden and abundant appearance of Caninia gigantea in x, of 
Lithostrotion junceum, Lonsdaleia floriformis, and many other members 
of the rich Upper Visean fauna, also cannot be accounted for by any 
observed genetic connexion with immediate predecessors, but must 
probably be ascribed to immigration. It cannot be said that ancestors 
of these forms have been found in the preceding zones, although in all 
probability they are present in the Clisiophyllids and the simple and 
compound Cyathophylla of the Upper Devonian. Many other instances 
of a similar nature might be given, but their enumeration is scarcely 
necessary for our present purpose. 
The explanation of these phenomena seems to lie in an application 
of the old theory of ‘colonies’, the various gentes having lived in 
outlying areas until conditions favourable to their existence recurred, 
It is not for a moment to be denied that over a very large area, 
embracing the South-West of England, North and South Wales, the 
Midlands, and even the West of Ireland and Belgium, the sequence of 
physical conditions, and consequently of fossils, is usually in the same 
order. Sooner or later, however, local areas are met with where 
different conditions have prevailed. Consequently there is an 
