THE LOWER LIAS OF KEYNSHAM. 
18 
Rhetic serieSj rather than with the beds above, is a species 
of Isodonta. This view, that the White Lias is best classed 
with the Lias, has been already maintained by the late 
Edward Wilson, and for the same reasons, namely, the 
identity of the molluscan fannad 
We consequently include all the rocks above the Gotham 
marble, and below those containing ammonites, in the lowest 
zone of the Lias as he suggests. 
In accordance with custom, we accept 0. liassica as the 
zonal index for these beds, since its maximum is undoubtedly 
in the sub-ammonite beds, where it occurs in thousands. 
There are, however, two objections to its adoption as a zonal 
index, which apply equally to all oysters, viz : — Firstly, 
the difficulty of definition and separation from allied oysters, 
and secondly, their extensive range. 0. liassica is, in certain 
forms, impossible to separate from mutations of 0 . irregularis^ 
and it ranges almost through the Psilonotus zone. The further 
objection to 0. liassica, that it is far from common in the 
Sun Bed, applies only to the system of zoning advocated in 
this paper. We are inclined to think that Pleuromya 
croivcombeia would be a much more satisfactory index. It 
is easily recognized, both in the young Pleuromya-like form, 
and, when full grown, is unmistakable on account of its 
Panopea-like aspect. It is common in all the beds of the 
zone, including the Sun Bed, and in certain bands is ex- 
tremely abundant, whilst we have never found it either in 
the Rhetic shale below or in the Psilonotus Beds above.^ 
The fact, however, that the most important character of 
the beds, as distinguishing them from those above, is the 
absence of ammonites, renders the choice of a zonal index a 
matter of less importance than usual. 
^ Ehetic Bocks at Pylle Hill, Bristol, vol. xlvii. p. 546. 
2 Moore notes the occurrence of this fossil with Avicula contorta, 
Q.J.G.tS., voL xvii. 
