Arkell.1956.Jurassic..

34 THE BRITISH ISLES
much more satisfactorily than in the past, for in the Jura the ammonite
faunas of the Saxon and English basins meet and are combined in one
succession. A gap in the English sequence exists between the Scissum
and Murchisonae Zones, but it has turned out to be not so great as was
supposed from the Hanover succession, where six subzones were intercalated
between opalinum and murchisonae by Hoffmann (1913) (see below,
p. 144), and it is probably bridged to some extent by the Ancolioceras
beds of Buckman, which represent some of the lower parts of the
Murchisonae Zone. There are visible signs of pre-Murchisonae Zone
erosion in Yorkshire.
ZONES SUBZONES
COTSWOLD
FORMATIONS
Graphoceras concavum Graphoceras concavum (Sow.) -| Tilestone
Snowshill Clay
Harford Sands
Ludwigia murchisonae •
Brasilia bradfordensis Buckman -J
Ludwigia murchisonae (Sow.) j
'Ancolioceras spp.'
Upper Freestone
Oolite Marl
Lower Freestone
Pea Grit
? Lower Limestones
Tmetoceras scissum Tmetoceras scissum (Benecke) Scissum Beds
Leioceras opalinum Leioceras opalinum (Reinecke) (absent)
The Scissum Zone is upheld as a separate zone because it is distinct
in the Jura (where it is found to be contemporaneous with the German
subzone of Costileioceras sinon Bayle sp.) and has been recognized in
Canada and Argentina. It therefore has wide stratigraphical significance,
despite the isolated rare occurrence of a minute Tmetoceras in the condensed
Bradfordensis and Concavum Bed in Dorset (Bomford, 1948).
Buckman and Richardson traced the Scissum Zone as a distinct formation
over most of the outcrops of southern England. Its discrimination as a
separate zone between those of Opalinum and Murchisonae is due to
Buckman (1898), but Tmetoceras scissum had already been proposed as
index species for a Mediterranean zone, supposed to be equivalent to
these two northern zones, by Neumayr (1871, Der Penninische Klippenzug,
p. 509).
The Scissum and Opalinum Zones acquire great economic importance
in the English Midlands, where they are developed as the Northampton
Sand, with its valuable low-grade ironstone. The ammonites of this
include Leioceras opalinum and Tmetoceras scissum, with Bredyia and
Lytoceratids (Hollingworth & Taylor, 1946, 1951).
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