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.  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.]

[Illustration:  Fig. 196.—­Ostrea Couloni.  Lower Greensand.]

[Illustration:  Fig. 197.—­Spondylus spinosus.  White Chalk.]

[Illustration:  Fig. 198.—­Inoceramus sulcatus.  Gault.]