(After Hall, M'Coy, and Salter.)]
[Illustration: Fig 73.--_Tentaculites ornatus_. Upper Silurian
of Europe and North America.]
The higher groups of the _Mollusca_ are also largely represented
in the Upper Silurian. Apart from some singular types, such as
the huge and thick-shelled _Megalomi_ of the American Wenlock
formation, the Bivalves (_Lamellibranchiata_) present little of
special interest; for though sufficiently numerous, they are rarely
well preserved, and their true affinities are often uncertain.
Amongst the most characteristic genera of this period may be
mentioned _Cardiola_ (fig. 71, A and C) and _Pterinea_ (fig. 71,
B), though the latter survives to a much later date. The Univalves
(_Gasteropoda_) are very numerous, and a few characteristic forms
are here figured (fig. 72). Of these, no genus is perhaps more
characteristic than _Euomphalus_ (fig. 72, b), with its flat
discoidal shell, coiled up into an oblique spiral, and deeply
hollowed out on one side; but examples of this group are both
of older and of more modern date. Another very extensive genus,
especially in America, is Platyceras (fig. 72, a and f),
with its thin fragile shell--often hardly coiled up at all--its
minute spire, and its widely-expanded, often sinuated mouth. The
British _Acroculioe_ should probably be placed here, and the
group has with reason been regarded as allied to the Violet-snails
(_Ianthina_) of the open Atlantic.
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