In the great river-deposit of the Wealden, the Bivalves are forms
proper to fresh water, belonging to the existing River-mussels
(_Unio_), _Cyrena_ and _Cyclas_; but most of the Cretaceous
Lamellibranchs are marine. Some of the most abundant and
characteristic of these belong to the great family of the Oysters
(_Ostreidoe_). Amongst 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_.
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