﻿Vol. 66.] EVOLUTION OF ZAPHKENTIS DELANOUEI. 531 



Quarry - sections in the group are numerous, the limestones 

 forming the chief source of supply from the Scottish Carboniferous 

 rocks. 



It will be noticed that both the Limestone Groups are of con- 

 siderable thickness, but by far the greater part of the component 

 strata consist of sandstones and shales, the deposition of which 

 probably occupied no great interval of time, 



A consideration of the assemblages from the Lower Limestone 

 localities shows that the three East of Scotland districts (Dunbar, 

 West Fife, and Midlothian) agree fairly well in their mutational 

 percentages. They have about 1 per cent, of Zaphrentis delanouei y 

 4 to 6 per cent, of Z. parallela, about 78 per cent, of Z. constricta, 

 and about 16 per cent, of Z. disjuncta. 



In the middle of the Central Valley, in the Linlithgow district, 

 there is a decided increase in the proportion of Zaphrentis disjuncta, 

 which rises to 23 per cent. Going farther westwards (where all 

 the localities are known to be from the Hurlet horizon) the whole 

 assemblage is seen to become of a more advanced type. In the East 

 Kilbride district Zaphrentis delanouei is unknown, there is only 

 2 per cent, of the parallela mutation, while the proportion of Z. dis- 

 juncta rises to 48 per cent., almost equalling that of Z. constricta. 



Still farther west, in the North Ayrshire district, the parallela 

 mutation entirely disappears, and for the first time the percentage 

 of Zaphrentis disjuncta exceeds that of Z. constricta. 



It is clear, therefore, that on considering assemblages from the 

 Lower Limestone Group as a whole, the percentage of Z. disjuncta 

 rises considerably in crossing the Central Valley from east to 

 west (although the amount is always much smaller than in the 

 Upper Limestone Group). There would seem to be three possible 

 explanations of this fact, namely : — 



(1) The western limestones may be of slightly later date than those in the 

 east. 



(2) The assumption of the disjuncta habit may have arisen, not so much 



from an inherent cause, as from some change in the physical environ- 

 ment (for instance, a shifting ocean-current) advancing slowly from 

 west to east. 



(3) The forms defined as Zaphrentis disjuncta may in reality belong to two 



branches, that prevalent in the west diverging from the original 

 constricta stock more rapidly than the eastern form. 



The question involved is of fundamental importance to thestrati- 

 grapher, as it deals with the relative value, as chronological indices, 

 of lithological and evolutionary faunal lines. 



Now, it is evident that a precise solution of this problem is to be 

 obtained in one way only — by the examination of a large amount 

 of material collected from a wide area (500 or 600 square miles at 

 least) from a single fossiliferous horizon. The Scottish limestones 

 undoubtedly offer a most promising field for such an enquiry, but 

 the data are as yet insufficient. Nevertheless, the facts so far 

 obtained are given, in the hope that they may serve as a basis for 



