Arkell.1956.Jurassic..

428 JAPAN AND KOREA
comprises the genera Hecticoceras and Horioceras, and two Perisphinctids.
One is a 'nucleus' (or young) correctly likened by Kobayashi (1935, pi.
xiii, figs. 4, 5) to Properisphinctes. The other is a larger fragment incorrectly
assigned by him (1935, pi. xiii, figs. 7, 8) to Sigaloceras, for from the
photographs it is apparent that the primary ribs are too long and the
venter too wide for Sigaloceras, and the fragment seems to be some form
of Binatisphinctes or Poculisphinctes ('Trinisphinctes').
The Lower (in part perhaps early-Middle) Kimeridgian faunule
comprises Lithacoceras tarodaense Kob., Aulacosphinctoides cf. steigeri
(Shimizu) and (found separately) Ataxioceras kurisakaense Kob. &
Fukada. These Perisphinctids are all known only from poorly-preserved
fragments, but their general age cannot be doubted. (Figures in Shimizu,
1927; 1927a; 1930; Kobayashi, 1935; Kobayashi & Fukada, 1947).
Other fragments, too poor to be worth discussing, have been figured, also
an indeterminable Oppeliid (Yehara, 1927; Shimizu, 1931). The age
indicated by the ammonites is confirmed by the occurrence of spines of
Balanocidaris glandaria ('Cidaris glandifera') which characterizes Lower
Kimeridgian deposits in East Africa and the eastern Mediterranean.
The sole indication of a later date is a pelagic crinoid, Pseudosaccocoma
japonica, belonging to a genus of the European Tithonian.
NORTH-EASTERN HONSHU
(a) Southern Kitakami massif (mainly Rikuzen Province)
About 350 miles north-east of the Hida plateau and Tetori River area,
there is an isolated group of Jurassic outcrops near the north-east coast
of Honshu, in the Kitakami Mountains and near Shizukawa Bay, in
the old province of Rikuzen (now Miyagi Prefecture). Hettangian and
Sinemurian genera of ammonites were long ago figured from here
(Yokoyama, 1904a).
General descriptions have been published by Mabuti (1933) and Mori
(1949), and a revision based on more ammonites has been undertaken by
Matsumoto & Ono, who in 1951 most kindly sent me a draft of their
unpublished paper, with figures of some of the ammonites. For this
friendly collaboration I am most grateful.
The Kitakami Jurassics belong tectonically to the outer or Pacific
suite, but they combine stages represented on both sides in SW. Japan,
with some additions.
The thickness of the Jurassic in the Kitakami massif was estimated by
Mabuti (1933) and Mori (1949) at about 1000 m. The revised measurements
by Matsumoto & Ono (1951 MS.) make it 700 m.; but in a
neighbouring area it has been estimated at 2000 m. (Shiida, 1940). The
stratigraphical nomenclature is confusing because there are several
physically separate and tectonically distinct groups of outcrops, for which
successive authors use new or emended formation names, to suit local
developments. In the following table the names used by Mabuti (1933)
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