THE CRETACEOUS PERIOD. 277 



are forms proper to fresh water, belonging to the existing 

 River-mussels (Unio), Cyrena and Cyclas; but most of the 

 Cretaceous Lamellibranchs are marine. Some of the most 



Fig. 195. Crania lyiwtbergensis. The left-hand figure shows the perfect shell, at- 

 tached by Its ventral valve to a foreign body ; the middle figure shows the exterior of 

 the limpet-shaped dorsal valve ; and the right-band figure represents the interior of 

 the attached valve. White Chalk. 



abundant and characteristic of these belong to the great family 

 of the Oysters (Ostreida). Amongst these are the genera 

 Gryph&a and Exogyra, both of which we have seen to occur 



Fig. 196. Ostrea Couloni. Lower Greensand. 



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 Creta- 

 ceous strata. No single genus of Bivalves is, however, so highly 

 characteristic of the Cretaceous period as Inoccromus, a group 

 belonging to the family of the Pearl-mussels (Aviculidce}. The 

 shells of this genus (fig. 198) have the valves unequal in size, 



