16 
THE LOWEK LIAS OF KEYNSHAM. 
are certainly in very close relation to the top beds of the 
Psilonotus zone. 
In the variable limestones and clays above the Echinid 
Bed, typical specimens of Am. angulatus occur, and the 
maximum development of the species would seem to be in 
these beds, for, in all the quarries where fragments of Am. 
angulatus can be picked up, they are almost invariably 
found in association with the forms peculiar to these beds. 
The most characteristic and abundant fossil, associated 
with Am. angulatus in these beds, is a very puzzling oyster 
which has been assigned to several species and which, in its 
mutations, exhibits the characters of both Grgphea and 
Exogyra. This fossil we have tentatively referred to Ostrea 
irregularis^ and we have selected it as a sub-zonal index of 
at least great local importance. 
Above the sub-zone of 0 . irregularis^ comes a series of 
limestones and thin clays which we term the Calcicosta 
Beds, on account of the extreme abundance of Rhynchonella 
calcicosta in them throughout the district. This is, par ex- 
cellence^ the richest fossiliferous series in the Keynsham 
district, and forms a very homogeneous group both litho- 
logically and palseontologically. Notwithstanding the re- 
markable homogeneity of this series of beds, it would un- 
doubtedly be assigned to two different zones by the rigid 
adherent to zonal division ; for the top thick bed contains 
abundant representatives of the bisulcate group of am- 
monites, whilst the lower beds contain specimens of Am. 
angulatus in considerable number. We have, however, 
found large adult specimens of Am. angulatus in the beds 
immediately above the top of the Calcicosta series, showing 
that there is, in the Keynsham district, a distinct overlap of 
the Angulatus and the so-called Bucklandi zones, though of 
no considerable extent. 
Comparing the sequence in neighbouring districts : — In 
