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.
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