806 ‘STRATIGRAPHICAL GEOLOGY.  [Boox VI. — 
Inma, Pecten,’ Perna, Modiola, Lyriodon, Isocardia, Cardium, Venus. 
Inoceramus and Exogyra are specially characteristic, but still more 
so is the family of Hippuritide or Rudistes. These singular forms 
are entirely confined to the Cretaceous system ; their most common 
genera (Fig. 393) being Hippurites, Radiolites, Spherulites, Caprina, 
and Caprotina. Hence, according to present knowledge, the oceur- 
rence of hippurites in a limestone suffices to indicate the Cretaceous 
age of the rock. The most common gasteropods belong to the 
genera Natica, Nerinea, Turritella, Turbo, Solarium, Trochus, Pleuro- 

Fic. 392.—CreTAcEOus LAMELLIBRANOHS. 
a, Exogyra (Ostrea) columba (Lam.)($); b, Ostrea vesicularis (Lam.) (4); ¢, Ostrea 
carinata (Lam.) (4); d, Spendylus (Lima) spinosus (Desh.) (3); e, Inoceramus 
Cuvieri (Sow.) (young spec.) (4). 
tomaria, Cerithium, Rostellaria, Aporrhais, and Fusus. Cephalopods 
must have swarmed in some of the Cretaceous seas (Figs. 394, 395, 
396), Their remains are abundant in the Anglo-Parisian basin and 
thence eastwards, but are comparatively infrequent in the southern 
Cretaceous area. To the geologist they have a value similar to those 
of the Jurassic system, as distinct species are believed to be restricted 
in their range to particular horizons which have by their means been 
identified from district to district. ‘To the student of the history of | 
life they have a special interest, as they include the last of the great 

