Arkell.1956.Jurassic..

THE BRITISH ISLES 21
at the base of the Portlandian at Swindon and about Oxford, at the base
of the Neocomian in Lincolnshire, and at the base of the Aptian at
Faringdon and in Cambridgeshire.
Zone of Pectinatites pectinatus (Phillips). In Dorset 36 m. of clays and
shales down to a conspicuous white paper-shale stone band (lowest of
the Three White Stone Bands); inland Shotover Grit Sands with doggers
at Swindon and about Oxford and Thame. Abundant large ammonites
of the genera Pectinatites, Wheatleyites (with coarse-ribbed body-chamber),
Keratinites (with rostrate aperture), Paravirgatites. (Many figures in
Buckman, Type Ammonites).
MIDDLE KIMERIDGIAN
Zone of Subplanites wheatleyensis (Neaverson). In Dorset, the Dicey
Clays, 34 m., including cuboidal cementstone. Inland 3-4-5 m. of clay
with cementstone septaria containing S. wheatleyensis.
Zone of Subplanites grandis (Neaverson). In Dorset, the Bituminous
Shales, 27 m. Various Subplanites common in the lower part. Absent
inland?
Zone of Subplanites spp. (vimineus Schneid?). In Dorset, the Cattle
Ledge Shales 24 m. The numerous crushed Subplanites still remain to
be worked out. Absent inland?
Zone of Gravesia gigas (Zieten). Zone of Gravesia gravesiana (d'Orb.).
In Dorset, the Hen Cliff Shales, 21 m., with crushed Lithacoceras and
Gravesia. Salfeld recognized species of the group of G. irius (d'Orb.)
in the upper part, and of the group of G. gravesiana (d'Orb.) in the lower
part. Present in Scotland.
LOWER KIMERIDGIAN
Zone of Aulacostephanus pseudomutabilis (de Loriol). In Dorset, the
Kimeridge Bay Shales, 70 m.+. These are the lowest beds exposed in
Purbeck, but a boring at Broad Bench, Kimeridge, proved a further
174 m. of Kimeridge Clay below sea-level. Species of Aulacostephanus
are common throughout the zone. A. pseudomutabilis and A. eudoxus
occur together and no subdivisions have been established. In the lower
part Aspidoceras longispinum (Sow.) and Aptychus latus Parkinson abound,
and some bedding-planes are whitened with crushed Amoeboceras krausei
(Salfeld) and A. anglicum (Salfeld). At Ringstead Bay and Black Head
these occur perfect in a cementstone band (Arkell, 1947, pi. iv). Wellpreserved
Aulacostephanus spp. also occur at Speeton in Yorkshire (Pavlow
and Lamplugh, 1892, pp. 88-9, pi. iv, figs. 7, 8).
Zone of Rasenia mutabilis (Sowerby). Shales with crushed, iridescent,
fine-ribbed Raseniae of the group of R. mutabilis (Sow.) extend from
Dorset to Yorkshire and Scotland (see Arkell, 1933, pi. xxxix, 5; 1947,
pi. iv, 4).
Zone of Rasenia cymodoce (d'Orbigny). The wonderfully-preserved
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