﻿184: DB. A. VATJGHAN ON THE PALJEONTOLOGICAL [May I905, 



different lines and at very different dates in the earth's history, 

 may be included in the same species-group. Two forms which are 

 essentially similar and occur on the same relative horizon, but in 

 distant localities, may with great probability be referred to the 

 same species-group. For example : — Spirifer tornacensis, of the 

 Lower Tournaisian of Belgium, is essentially similar to Sp. aff. 

 clathratus of the Bristol area, which occurs at the same horizon ; 

 consequently, these two species may be referred to the same 

 species-group. 



On the other .hand, a form resembling Productus Martini, which 

 occurs in the uppermost zone of the Carboniferous Limestone in 

 the Bristol area, has almost certainly been immediately evolved 

 from ancestors of the type of Pr. Cora, and is in no way a survivor 

 of the form here denoted by Pr. cf. Martini, which characterizes 

 the lower part of the Carboniferous-Limestone Series in the 

 Bristol area. These two forms are, however, easily distinguished, 

 and should consequently receive distinct specific names ; but, 

 though similar, they cannot be referred to the same species-group, 

 since their lines of evolution are totally distinct. 



The use of ' aff.' before a specific name will imply that the fossil 

 under consideration and that with which it is compared belong to 

 the same species-group. 



The use of ' cf.' before a specific name merely denotes a 

 certain degree of similarity, but does not necessarily imply genetic 

 relation. 



Mutations are strictly the time-variants of a gens, but I 

 include, also, space- variants under the same term, for no sharp 

 distinction can possibly be drawn between the two types of variant. 

 The mutations of a gens usually mark the points of convergence of 

 that gens with cognate gentes. 



In the Bristol area, the Corals are the most valuable genera as 

 zonal indices, for the following reasons : — 



(1) Corals are abundant in all parts of the area, and almost throughout 



the whole of the Carboniferous Limestone. 



(2) The genera are easily distinguished, by the aid of sections, even in 



small fragments. 



(3) They have usually well-defined ranges. 



(4) It seems almost certain that Zaphre?itis, Caninia, Lithostrotion, 



Clisiophyllum, and Lonsclalia are stations on an unbroken line of 

 evolution ; and it may also be possible to demonstrate that Caninia, 

 Cyathophyllum of the type of C. <p, Cyathophyllum of the type of 

 C. Micrchisoni, and Cyathophyllum of the type of C. reyium are 

 corresponding stages in evolution along another radius. 



With regard to (4), it is clear that any system of zonal indices, 

 in which each index is the result (either entire or partial) of 

 evolution from the one which precedes, has a special value, from 

 the fact that the relative order is necessarily the same for all 

 localities. 



