beaks
"reversed"--that is to say, turned towards the hinder part of
the shell; whilst in the latter are Oysters in which the lower
valve of the shell is much the largest, and has a large incurved
beak, whilst the upper valve is small and concave. One of the
most characteristic _Exogyroe_ is the _E. Virgula_ of the Oxford
Clay, and of the same horizon on the Continent; and the _Gryphoea
incurva_ (fig. 167) is equally abundant in, and characteristic
of, the formation of the Lias. Lastly, we may notice the
extraordinary shells belonging to the genus _Diceras_ (fig. 168),
which are exclusively confined to the Middle Oolites. 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. One of the limestones of the Jura,
believed to be of the age of the Coral-rag (Middle Oolite) of
Britain, abounds to such an extent in the turreted shells of
_Nerinoea_ as to have gained the name of "Calcaire a Nerinees."
In addition to forms such as the preceding, we now for the first
time meet, in any force, with the Carnivorous Univalves, in which
the mouth of the shell is notched or produced into a canal, giving
rise to the technical na