THE LOWER LIAS OF KEYNSHAM. 
25 
is the case in any typical specimen of that species. The 
most characteristic fossil is a species of Diadema which is 
fully described below. 
A species of Modiola (identified by us as M. hillanoides) 
is abundant and characteristic, whilst small smooth Pectens 
with faint radial striae and large Limas (especially L. 
hermanni) are very common. 
The thick clay is followed by a series of thin, disjointed 
limestones with clay partings, the whole thickness amount- 
ing to about seven feet. Here ammonites are far from 
common, but typical fragments of Am. angulatus can be 
found in sitit, and can be picked up in greater numbers on 
the spoil heaps, where they are associated with the fossils 
characteristic of this series of beds. 
The commonest and most characteristic fossils which 
occur in these beds are Pleuromya casts, Ostrea irregularis 
and Lima hettangiensis. 
Were we to confine our attention to the Keynsham area it 
is unquestionable that the most useful zonal index of these 
beds would be the casts of Pleuromya (Gresslya) galathea 
which is extremely common, easily recognized and, so far as 
we know, not found at a higher or lower horizon in the 
district ; but, for the purposes of comparison with neighbour- 
ing areas, we have selected the very variable Ostrea irregu- 
laris as the most useful index. 
Large Limas (L. gigantea and L. hermanni) are fairly 
abundant in the lower part of the series, but the smaller 
duplicate Lima {L. hettangiensis) is more characteristic, and , 
consequently, of greater practical value. 
The Calcicosta series, about six feet thick, overlies the 
irregularis beds, from which it is separated by a con- 
spicuous band of clay. 
This series forms one of the most interesting and homoge- 
neous groups in the Lower Lias of the Keynsham district. 
