27 8 



HISTORICAL PALEONTOLOGY. 



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 



Fig. 197. Spondylua spinosui. White Chalk. 



Sponges. Another extraordinary family of Bivalves, which is 

 exclusively confined to the Cretaceous rocks, is that of the 

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



Fig. 198. Inoceramua aulcatus. Gault. 



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 Hippuritidce are known, all of these 

 being Cretaceous, and occurring in Britain (one species only), 

 in Southern Europe, the West Indies, North America, Algeria, 



