ration: Fig. 196.--_Ostrea Couloni_. Lower Greensand.]

[Illustration: Fig. 197.--_Spondylus spinosus_. White Chalk.]

[Illustration: Fig. 198.--_Inoceramus sulcatus_. Gault.]

The Univalves (_Gasteropods_) of the Cretaceous period are not
very numerous, nor particularly remarkable. Along with species of
the persistent genus _Pleurotomaria_ and the Mesozoic _Nerinoea_,
we meet with examples of such modern types as _Turritella_ and
_Natica_, the Staircase-shells (_Solarium_), the Wentle-traps
(_Scalaria_), the Carrier-shells (_Phorus_), &c. Towards the close
of the Cretaceous period, and especially in such transitional strata
as the Maestricht beds, the Faxoee Limestone, and the Pisolitic
Limestone of France, we meet with a number of carnivorous
("siphonostomatous") Univalves, in which the mouth of the shell is
notched or produced into a canal. Amongst these it is interesting
to recognise examples of such existing genera as the Volutes
(_Voluta_, fig. 200), the Cowries (_Cyproea_), the Mitre-shells
(_Mitra_), the Wing - shells (_Strombus_), the Scorpion-shells
(_Pteroceras_), &c.

[Illustration: Fig. 199.--_Hippurites Toucasiana_. A large
individual, with two smaller ones attached to it. Upper Cretaceous,
South of Europe.]

[Illustration: Fig. 200.--_Voluta elongata_. White Chalk.]

Upon the whole, the most characteristic of all the Cretaceous
Molluscs are the _Cephalopods_, represented by the remains of
both _Tetrabranchiate_ and _Dibranchiate_ forms. Amongst the
former, the long-lived genus _Nautilus_ (fig. 201) again reappears,
with its involute shell, its capacious body-chamber, its simple
septa between the air-chambers, and its nearly or quite central
siphuncle. The majority of the chambered _Cephalopods_ of the
Cretaceous belong, however, to the complex and beautiful family
of the _Ammonitidoe_, with their elaborately folded and lobed
septa and dorsally-placed siphuncle. This family disappears wholly
at the close of the Cretaceous period; but its approaching
extinction, so far from being signalised by any slow decrease
and diminution in the number of specific or generic types, seems
to have been attended by the development of whole series of new
forms. The genus _Ammonites_ itself, dating from the Carboniferous,
has certainly passed its prime, but it is still represented by
many species, and some of these attained enormous dimensions
(two or three feet in diameter). The genus _Ancyloceras_ (fig.
202), thou