100 SECTIONAL ADDRESSES. 
his most admirable account of the evolution of Zaphrentis delanouei 
Carruthers has shown the importance of cutting serial sections, for 
the stages seen in the adult of early forms are often characteristic of 
adolescence in forms at higher horizons. Thus in Z. delanouei evolu- 
tionary stages are confined to the shape of the cardinal fossula and 
the length of the major septa, and different time-variants (mutations, 
Waagen) show striking differences between these. 
In Z. delanouei s.s., which occurs in the Cementstone Group 
300-400 ft. below the base of the Fells Sandstone, the transverse 
sections show septa meeting in the centre of the corallum and a large 
cardinal fossula expanded towards the inner end; together with this 
form there occur others which agree with Z. delanouei in their adolescent 
stage, but in the adult a stage is reached in which the walls of the 
fossula become parallel and finally show a tendency to constriction at 
the inner end. Since this mutation marks an important evolutional 
change as regards the fossula, it is termed Z. parallela. 
At a considerably higher horizon, in the Lower Limestone Group, 
the cutting of sections of a fresh mutation foreshadowed in the Cement- 
stones reveals no trace remaining of what may be termed the delanouei 
stage; but the parallela stage is distinct, and with growth the inner end 
of the fossula narrows, whilst in sections of the adult stage the con- 
striction becomes very pronounced, the septa being, however, still joined 
together in the centre of the corallum. Again, on account of a further 
change in the character of the fossula this mutation may be distin- 
guished as Z. constricta. Within the Lower Limestone Group are also 
found forms representing a further change; these do not pass through 
the parallela stage, but start at the constricta stage, and on further 
growth the septa shorten until they separate at the centre of the 
corallum. This again is an important and easily recognised stage 
(Z. disjuncta), and this mutation is said by Carruthers to show amplexoid 
characters (=amplexoid trend, Lang). The geological value of these 
changes lies mainly in the fact that they are continuous in time and 
characteristic of different stratigraphical horizons, apart from whether 
they are progressive or retrogressive, but it is clear that careful dis- 
crimination may at times have to be made between these. 
At the mere thought of coping with the many evolutional problems 
connected with the Lower Paleozoic Brachiopods the heart of the most 
vigorous paleontologist amongst us might well fail him. I suppose 
that there is no single worker on the Lower Paleozoic rocks who has 
not at one time or another realised the stupendous nature of the problem 
that awaits us here. We Have, I feel sure, all been conscious of the 
fact that many of the so-called long-ranged species are not really quite 
the same, but show certain differences at different horizons with which 
in the course of our field-work we have become familiar and can recog- 
nise, so that for the sake of our own convenience we have often given 
them the field-names; but when we try to analyse these differences 
paleontologically each character seems so slight as to be trivial and 
unimportant ; nevertheless, in bulk they may be important and the two 
extremes quite distinct. This may well be illustrated by the case of 
the Dalmanellas as represented by the species D. elegantula, a name 
