26 LIAS OF ENGLAND AND WALES: 
A prominent species of Ammonite was usually taken us the 
index to the zone. Ammonites however were not always to be 
found, and recourse was had in some instances to a Crinoid, a 
Brachiopod, or other fossil, The value of these zonal divisions 
has been questioned, because owing to the local absence of Am- 
monites, and the rarity of other fossils, the zones may often be 
indeterminable. Moreover the tendency to assign definite limits to 
them is apt to create a feeling of distrust. There can however be 
no question of the truth of the general sequence of organic remains 
in the Lias, a sequence that can be verified by any one who will 
devote three or four weeks to the Lias cliffs of Dorsetshire. 
Certain groups will be found in sequence, and experience shows 
we may look in vain for Ammonites planorbis or A. Bucklandi in 
the higher beds of the Lower Lias, and we should be equally at a 
loss to find A. capricornus or A. Hcnhyi in the Basement-beds : 
yet no one can fix definitely the limits of the several zones. 
The local lithological divisions do not necessarily coincide with 
the limits within which the index-species of zones may occur 
in abundance. For instance the limestones of the zone of Ammonites 
Bucklandi are locally replaced to a large extent by clay. Never- 
theless the succession of the Liassic strata exhibits some fairly 
persistent types of rock, that accord generally with the fossiliferous 
horizons. This is the case with the basement-beds both of Lower 
and Upper Lias, with the rock-beds of the Middle Lias, and with 
some of the pyritic shales of the Lower Lias. 
While some species of Ammonites appear to have a restricted 
range, there are others that endured for much longer periods, and 
so do not lend themselves to zonal groupings. Thus the value of 
selected index-species varies in different areas, for there is no 
reason to believe that the range of any one species was confined 
over a large area within the same time-limits. Experience 
however tells us that when we find our index-species in fair abun- 
dance, together with forms usually associated with it, AVC may 
confidently assign the strata to the particular zone. To some 
extent we find that the associated species vary as we trace the 
beds across country, for species of Cardinia, Ilippopodium, and 
other forms are prevalent locally at certain stages. Nevertheless 
the most important factor in the faith in zones is that the in- 
coming of the species of Ammonites occurs in a regular sequence, 
and that there is no inversion in the order of the prominent 
groups of fossils. 
We have no evidence to show that any one zonal species was 
exterminated before another came in, in fact we find the earlier 
predominant forms lingering on beside each succeeding species. 
There is thus a blending of the zones, where the succession is 
gradual and complete, but this in no ways interferes with the fact 
of the gradual succession of different forms of life. The great 
difficulty often is to decide how many zones it may be desirable 
to indicate. The more detailed study of particular districts shows 
that we may introduce zones of a more or less local character, 
zones which are established on the evidence of the prevalence of a 
