Arkell.1956.Jurassic..

LOWER SAXONY 143
names such as Strigoceras truellei (d'Orb.), Strenoceras latidorsatum
Bentz, etc.
In the middle are said to be two subzones characterized by Stephanoceras
humphriesianum (Sow.) (above) and S. umbilicum (Quenst.) (below).
Both contain numerous species of Stephanoceras and Dorsetensia, also
Normannites and its subgenera, Chondroceras, etc., and the lower subzone
also yields Stemmatoceras and Skirroceras.
At the base is a subzone characterized by Stemmatoceras subcoronatum
(Oppel), S. 'coronatum' (Schlotheim non Brug.), but also with various
Normannites and its subgenera, and Sphaeroceras brongniarti, Chondroceras
gervillii, etc.
Zone of Otoites sauzei. This is likewise divided into two subzones
(Kumm, 1952, p. 383) unlikely to have wider significance: the upper is
characterized by O. sauzei (d'Orb.), the lower by Emileia grandis (Quenst.),
but both contain a long list of Otoites, Stephanoceras, Stemmatoceras, etc.,
though Emileia, Frogdenites and Labyrinthoceras are confined to the lower
subzone and Chondroceras to the upper.
Zone of Sonninia sowerbyi. The Sonninia fauna is extraordinarily rich
and has been monographed by Hiltermann (1939). It also yields Hyperlioceras
discites (Waagen) and Fontannesia spp., but few other ammonites.
LOWER BAJOCIAN (up to c. 130 m.)
Clays and shales with layers of mudstone or clay-ironstone nodules
continue the sequence down to the Lias. Thicknesses vary greatly, from
c. 30 m. to c. 130 m. In the Osning district the Opalinum Zone is missing
or represented only in a basal conglomerate; at Bethel near Bielefeld
the thickness is c. 30 m. and the Sinon Subzone (basal Scissum Zone)
rests directly, with basal conglomerate, on the Dispansum Subzone of
the Toarcian. Towards the east the Scissum and Murchisonae Zones
develop sandstones containing oil.
The clays and nodules of Sehnde, near Hanover, contain probably
the fullest and most important sequence of Lower Bajocian Leioceras,
Ludwigia and Graphoceras faunas in the world. The total thickness here
is about 50 m. Understanding of the sequence is unhappily bedevilled
by the state of the nomenclature of these ammonites. Hoffmann (1913)
and Althoff (1940), in their righteous reaction against the chaotic splitting
of Buckman swung too far to the other extreme, and reverted to a trinomial
nomenclature as backward as Quenstedt's, including everything in a single
genus Ludwigia, to which they illegally subordinated even the long-prior
genus Leioceras Hyatt. The succession established by Hoffmann (1913),
and, for the basal beds by Stolley (1909), seems to fall into the standard
zonal sequence as follows:—
Zone of Graphoceras concavum, with G. concavum (Sow.), G. anguliferum
(Buck.) (type from the Discites Subzone), some Sonninids and Hammatoceratids,
including Eudmetoceras cf. amplectens Buck. (Althoff, 1940,
pi. vi, fig. n).
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