ime upon the scene, but they have not
hitherto been recognised in deposits later than the Lias. The
great majority of the Jurassic _Brachiopods_, however, belong to
the genera _Terebratula_ (fig. 165, c, e, f) and _Rhynchonella_
(fig. 165. d), both of which are represented by living forms
at the present day. The _Terebratuloe_, in particular, are very
abundant, and the species are often confined to special horizons
in the series.

[Illustration: Fig. 165.--Jurassic Brachiopod. a. _Leptoena
Liassica_, enlarged, the small cross below the figure indicating
the true size of the shell--Lias; b, _Spirifera rostrata_, Lias;
c, _Terebratula quadrifida_, Lias; d, d', _Rhynchonella varians_,
Fulter's Earth and Kelloway Rock; e, _Terebratula sphoeroidalis_,
Inferior Oolite; f, _Terebratula digona_, Bradford Clay,
Forest-marble, and Great Oolite. (After Davidson).]

[Illustration: Fig. 166.--_Ostrea Marshii_. Middle and Lower
Oolites.]

[Illustration: Fig. 167.--_Gryphoea incurva_. Lias.]

Remains of _Bivalves_ (_Lamellibranchiata_) are very numerous in
the Jurassic deposits, and in many cases highly characteristic.
In the marine beds of the Oolites, which constitute by far the
greater portion of the whole formation, the Bivalyes are of course
marine, and belong to such genera as _Trigonia, Lima, Pholadomya,
Cardinia, Avicula, Hippopodium_, &c.; but in the Purbeck beds, at
the summit of the series, we find bands of Oysters alternating
with strata containing fresh-water or brackish-water Bivalves,
such as _Cyrenoe_ and _Corbuloe_. The predominant Bivalves of
the Jurassic, however, are the _Oysters_, which occur under many
forms, and often in vast numbers, particular species being commonly
restricted to particular horizons. Thus of the true Oysters,
_Ostrea distorta_ is characteristic of the Purbeck series, where
it forms a bed twelve feet in thickness, known locally as the
"Cinder-bed;" _Ostrea expansa_ abounds in the Portland beds;
_Ostrea deltoidea_ is characteristic of the Kimmeridge clay;
_Ostrea gregaria_ predominates in the Coral-rag; _Ostrea acuminata_
characterises the small group of the Fuller's Earth; whilst the
plaited _Ostrea Marshii_ (fig. 166) is a common shell in the
Lower and Middle Oolites. Besides the more typical Oysters, the
Oolitic rocks abound in examples of the singularly unsymmetrical
forms belonging to the genera _Exogyra_ and _Gryphoea_ (fig.
167). In the former of these are included Oysters with the