14 
THE LOWER LIAS OF KEYNSHAM. 
Am. psilonotus zone ( = Zone of Am. planorMs). 
In all the Lower Lias outcrops near Bristol, Am. planorbis 
is confined to a very limited portion of the zone, namely, 
to one or two beds at the very base. Were we to confine 
this zone to the very small thickness of rocks in which Am. 
planorbis actually occurs, the zone would be too narrow to 
afford either a useful or reliable unit of time measurement. 
Hence, by general consent, the zone is made to include an 
upper and larger portion from which Am. planorbis is 
absent, and which is only limited by the first occurrence 
of the next zonal index, ffm. angulatus. A considerable 
vagueness is the necessary result ; for example, if a fossil is 
said to be confined to the Planorbis zone, it is merely implied 
that no specimen of Am. angulatus has been found associated 
with it, but the fossil may or may not occur in association 
witln Am. planorbis. The difficulty can be partially over- 
come by associating a nearly allied ammonite, Am.johnstoni.! 
as joint zonal index. The zone is then best described as 
that of Am. psilonotus., since under this term, Quenstedt 
included both Am. planorbis and Am. johnstoni. The 
usual objection urged against this course is the fact that, in 
certain localities, Am. johnstoni is stated to be a common 
associate of Am. angulatus, and, consequently, there would be 
a marked overlap of zones. We cannot see that any vague- 
ness results, but rather that the circumstances lend them- 
selves to an added exactness of horizoning in the districts in 
which the overlap occurs. The objection seems to be based 
on the impossible striving after conterminous zonal indices. 
It seems to us to be preferable to have a series of overlapping 
indices, rather than one which admits of large gaps between 
its successive terms. Throughout the district from Radstock to 
Sodbury, we have found no overlapping of the Psilonotus and 
Angtilatus zones, but always that there is a considerable gap 
between the beds in which Am. psilonotus occurs and those 
in which typical specimens of Am. angulatus can be found. 
