Arkell.1956.Jurassic..

32 THE BRITISH ISLES
Besides Garantiana, forms of Spiroceras are characteristic of the zone.
Various Prorsisphinctes, Bigotites, etc. also occur.
Zone of Strenoceras subfurcatum (Schlotheim). This is the Niortensis
Zone of Buckman, but S. niortensis (d'Orb.) is a junior synonym of S.
bajocense (Defr.) and S. subfurcatum has many years' priority as zonal
index. The zone has a limited distribution in Dorset but has yielded
a rich fauna of Strenoceras spp. and many other ammonites, especially
Leptosphinctes, Prorsisphinctes, Cleistosphinctes and Cadomites spp., with
Cadomoceras, Oecotraustes, Strigoceras, Sphaeroceras, etc.
MIDDLE BAJOCIAN (up to 24 m. in S. England, to 150 m. in Scotland)
In Dorset and Somerset the Middle Bajocian is highly condensed,
often only 1 m. thick, but contains representatives of all the zones. In
the Cotswolds it expands into the Ragstones, up to 24 m. thick, but the
highest zone of Stephanoceras humphriesianum is missing. In the Lower
Lincolnshire Limestone ammonites of the lowest subzone of the Sowerbyi
Zone occur 8 ft. above the base (Kent & Baker, 1938; Kent, 1941;
Swinnerton & Kent, 1949; Muir-Wood, 1952). In Yorkshire the
Humphriesianum Zone is developed in marine facies as the Scarborough
Beds, with ammonites; the rest of the substage is presumed to be
represented by the Middle Estuarine Series* (30 m.) and Millepore Bed
and Whitwell Oolite (0-15 m.). In the Inner Hebrides sandstones up
to perhaps 150 m. thick correspond to the Middle Bajocian and yield
large Stephanoceratidae of the Humphriesianum Zone, closely resembling
species found in Dorset. There are also condensed beds with ammonites
of the Sowerbyi Zone. The zones and subzones are as follows, with
the formations as developed in the Cotswolds :
ZONES SUBZONES
Stephanoceras \
humphriesianum (Sow.) 1
Teloceras blagdeni (Sow.) \
S. humphriesianum (Sow.) /
COTSWOLD
FORMATIONS
Absent from Cotswolds
Otoites sauzei (d'Orb.) . i Phillipsiana Beds
Bourguetia Beds
Sonninia sowerbyi (Sow.) -
Witchellia laeviuscula (Sow.) j
Shirbuirnia trigonalis (Buck.)
Hyperlioceras discites
(Waagen) \
Witchellia Grit
Notgrove Freestone
Gryphite Grit
Buckmani Grit
Lower Trigonia Grit
* My suggestion (1933, p. 312) that the 'Estuarine Series' might appropriately be
renamed Deltaic Series seems to be finding favour (Hemingway, 1949 ; Harris, 1953)
but it involves difficulties (see Sylvester-Bradley, 1949a) and a break with the literature
of a century.
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