18 
THE LOWEE LIAS OF KEYNSHAM. 
Sub-zone of Am. Msulcatus. 
We use this definition for the beds above the thick top 
block of the Calcicosta series, and below the first perfect 
shale found in the district. This shale is well marked out 
palseontologicall}^ by the great abundance of Anomia and 
Aviculaj and by the occurrence of fish fragments. It is 
probable that most stratigraphists w'ould include also in 
this zone the top block of the Calcicosta series which is 
here grouped as the uppermost bed of the Angulatus zone. 
The argument for the inclusion of this block is often very 
striking, for bisulcate ammonites are in many places very 
abundant in it (we may especially mention exposure 7, 
where the upper surface of the block is, in places, almost 
paved with large specimens of Am. conybeari)., and they are 
never wholly absent. The reasons against its inclusion seem 
to us stronger, for, excepting the ammonites, all the fossils 
it contains are identical with those found in the beds just 
below. Rh. calcicosta^- small gastropods and species of 
Astarte and Cardinia occur in immense numbers throughout 
the Calcicosta beds, but are uncommon above. 
(We attach little importance to the very rare occurrence of 
Am. angulatus in the beds immediately above the top of the 
Calcicosta beds, for these beds are, in every respect, typical 
members of the Bisulcatus zone.) 
It is also possible that the perfect shale with the hard 
bed immediately above it, which we include in the next 
sub-zone, would by many be considered to belong to the 
zone of Am. bisulcatus. The arguments for and against this 
view are given in the discussion of the zone of Am. semi- 
costatus. 
A careful examination of all the large ammonites which 
we have met with from this zone has convinced us that 
Am. bttcklandi^ in its typical form, is a comparative rarity. 
No two specimens exactly agree either in rate of growth ox 
