270 HISTORICAL PALAZONTOLOGY. 
Sponges. Another extraordinary family of Bivalves, which is 
exclusively confined to the Cretaceous rocks, is that of the 
Fig. 197.—Spondylus spinosus. White Chalk. 
Hippuritide. All the members of this group (fig. 199) were 
attached to foreign objects, and lived associated in beds, like 
Fig. 198. —/zoceranius sulcatis. 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 Hippuritide 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. 
