﻿Vol. 66.] EVOLUTION OF ZAPHBENTIS DELANOTJEI. 529 



of specific delimitation, only too familiar to every palaeontologist, 

 is presented in an acute form. Some way out of the difficulty must 

 necessarily be devised if there is to be any attempt at dealing 

 with questions of stratigraphical distribution. Accordingly, in 

 the preparation of the fossil list (§ VII, p. 535) certain arbitrary 

 distinctions have been drawn between the various mutations. 



Attention has been paid solely to cross-sections cut below the 

 floor of the calyx and in the conical part of the corallum. Taking 

 size to be a test of age (although it can only be a rough one at 

 best), the critical section has been about 7 mm. in diameter, unless 

 the septal characters are so pronounced that reliance can be placed 

 on sections of smaller type (as, for instance, in Z. disjuncta). 



In very many cases sections across the neanic region have also 

 been made ; this has been done wherever the adult characters 

 admitted of a double interpretation (as in the resemblance between 

 Z. parallela and the early, or pseu&o-pci7*allela, forms of Z. dis- 

 juncta). 



The nomenclatorial difficulty is greatest in the case of Z. dis- 

 juncta, owing to the amplexoid septa developed in this species. 

 If the section happens to coincide with a tabula (that is, if it is a 

 'tabular section'), some, or all, of the septa may be long, and 

 joined together in the centre of the section by the rising of the 

 floor of the tabula into the plane of section. These tabular sections, 

 therefore, may present the septal disposition of Z. constricta; in 

 such cases the question as to which species the specimen should be 

 assigned has to be decided by grinding down the section, in order 

 to pass below the floor of the tabula. When this is done, it can be 

 seen at once whether the septa are, or are not, of an amplexoid 

 type, since in the former case the septa are shortest immediately 

 underneath the tabulae. As a matter of fact, Z. disjuncta is 

 frequently identifiable even in tabular sections, the adult septa of 

 this mutation being usually thinner and more irregular than in 

 Z. constricta. 



Stratigraphical Distribution of the Mutations. 



A list is appended to this paper giving the mutational assem- 

 blages met with in various parts of Scotland (§ VII, p. 535). Most 

 of the Lower Carboniferous areas are represented, the fossil localities 

 having been distributed as widely as possible. The latter are 

 grouped together in districts, in each of which percentages are 

 calculated for both Upper and Lower Limestone Groups, wherever 

 the number of specimens is sufficiently great to yield approximately 

 accurate results. The exact horizon is added in each case where the 

 position within the local stratigraphical sequence is known. 



Sufficient material has been collected from the Carboniferous 

 Limestone Series to justify an analysis of the assemblages found 

 therein, and to notice very briefly the relationships of these assem- 

 blages to the stratigraphical lines. Before taking up this matter, 

 the general sequence of strata in the two Limestone Groups may 



