In this
formation in the Alps they occur in such abundance as to give
rise to the name of "Calcaire a Dicerates," applied to beds of
the same age as the Coral-rag of Britain. The genus _Diceras_
belongs to the same family as the "Thorny Clams" (Chama) of the
present day--the shell being composed of nearly equally-sized
valves, the beaks of which are extremely prominent and twisted
into a spiral. The shell was attached to some foreign body by
the beak of one of its valves.
[Illustration: Fig. 168.--_Diceras arietina_. Middle Oolite.]
[Illustration: Fig. 169.--_Nerinoea Goodhallii_, one-fourth of the
natural size. The left-hand figure shows the appearance presented
by the shell when vertically divided. Coral-rag, England.]
Amongst the Jurassic Univalves (_Gasteropoda_) there are many
examples of the ancient and long-lived _Pleurotomaria_; but on
the whole the Univalves begin to have a modern aspect. The
round-mouthed ("holostomatous"), vegetable-eating Sea-snails,
such as the Limpets (_Patellidoe_), the Nerites (_Nerita_), the
_Turritelloe, Chemnitzioe_, &c., still hold a predominant place.
The two most noticeable genera of this group are _Cerithium_
and _Nerinoea_--the former of these attaining great importance
in the Tertiary and Recent seas, whilst the latter (fig. 169)
is highly characteristic of the Jurassic series, though not
exclusively confined to it.
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