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HISTORICAL PALEONTOLOGY. 



Sponges. Another extraordinary family of Bivalves, which is 

 exclusively confined to the Cretaceous rocks, is that of the 



Fig. 197. Spondylus spinosus. White Chalk. 



Hippuritidce. All the members of this group (fig. 199) were 

 attached to foreign objects, and lived associated in beds, like 





Fig. 198. 



sulcatus. Gault 



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 Hippuritidce 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. 



