82 
THE AVONIAN OF THE AVON GORGE 
who is not specially trained to the appreciation of variational 
differences, may confuse the later for the earlier variant if he 
pass immediately from the first locality to the second. 
The case of Zaphrentis aff. Philli'psi which characterizes Z, 
and Zaphrentis aff. Ennishilleni which marks the upper part of 
D, may be cited as an excellent example. 
(2) A particular area may, during a certain period, have been 
subject to peculiar conditions and consequently exhibit a peculiar 
faunal assemblage. If a dominant fossil of such an assemblage 
be selected as a zonal fossil, it will probably be of value only in 
areas which were subject to the same conditions during the same 
period, and the absence of the fossil will not denote the absence 
of deposits of that particular age in areas which do not exhibit 
the special conditions. 
(3) If a species which exhibits very exaggerated characters be 
selected as a zonal fossil, there is a danger of confusing the local 
extinction of a mutation of a gens with the total and general 
extinction of the gens itself. 
Extinction is frequently accompanied by the development of 
excrescences, such as spines and exaggerated ornament in the 
Brachiopods, roots and the excessive development of vesicles in 
the Corals, and these striking moribund characters almost com- 
pletely conceal the less pronounced mutational differences upon 
which we rely for fixing zonal positions. If, then, a gens becomes 
extinct in a particular area, owing to the incoming of unfavour- 
able conditions, the individuals which stayed on in the area 
until it became uninhabitable may exhibit the same moribund 
characters which are to be seen in the last stages of the gens 
itself at a much later time. 
For example, the temporary extinction of the gens of Pro- 
ductus semireticulatus at the top of Si, in the Bristol Area, 
is signalized, in the last representatives, by the extraordinary 
development of spines and marginal accretion A 
On the other hand, the structural complexity or specialization 
which heralds the general extinction of a group affords an ex- 
