ngst these are the genera _Gryphtoea_ and
_Exogyra_, both of which we have seen to occur abundantly in the
Jurassic; and there are also numerous true Oysters (_Ostrea_,
fig. 196) and Thorny Oysters (_Spondylus_, fig. 197). The genus
_Trigonia_, so characteristic of the Mesozoic deposits in general,
is likewise well represented in the Cretaceous strata. No single
genus of Bivalves is, however, so highly characteristic of the
Cretaceous period as _Inoceramus_, a group belonging to the family
of the Pearl-mussels (_Aviculidoe_). The shells of this genus
(fig. 198) have the valves unequal in size, the larger valve often
being much twisted, and both valves being marked with radiating
ribs or concentric furrows. The hinge-line is long and straight,
with numerous pits for the attachment of the ligament which serves
to open the shell. Some of the _Inocerami_ attain a length of
two or three feet, and fragments of the shell are often found
perforated by boring Sponges. Another extraordinary family of
Bivalves, which is exclusively confined to the Cretaceous rocks,
is that of the _Hippuritidoe_. All the members of this group
(fig. 199) were attached to foreign objects, and lived associated
in beds, like Oysters. The two valves of the shell are always
altogether unlike in sculpturing, appearance, shape, and size;
and the cast of the interior of the shell is often extremely
unlike the form of the outer surface. The type-genus of the family
is _Hippurites_ itself (fig. 199), in which the shell is in the
shape of a straight or slightly-twisted horn, sometimes a foot
or more in length, constituted by the attached lower valve, and
closed above by a small lid-like free upper valve. About a hundred
species of the family of the _Hippuritidoe_ are known, all of these
being Cretaceous, and occurring in Britain (one species only), in
Southern Europe, the West Indies, North America, Algeria, and
Egypt. Species of this family occur in such numbers in certain
compact marbles in the south of Europe, of the age of the Upper
Cretaceous (Lower Chalk), as to have given origin to the name
of "Hippurite Limestones," applied to these strata.

[Illustration: Fig. 195.--_Crania Ignabergensis_. The left-hand
figure shows the perfect shell, attached by its ventral valve
to a foreign body; the middle figure shows the exterior of the
limpet-shaped dorsal valve; and the right-hand figure represents
the interior of the attached valve. White Chalk.]

[Illust