THE CRETACEOUS PERIOD. 269 



abundantly in the Jurassic ; and there are also numerous true 

 Oysters (Ostrea, fig. 196) and Thorny Oysters (Spondylus, fig. 



Fig. 195. Crania Ignabergensis. 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-hand figure represents the interior of the at- 

 tached valve. White Chalk. 



197). The genus Trigonia, so characteristic of the Mesozoic 

 deposits in general, is likewise well represented in the Greta- 



Fig. 196. Ostrea Coitloni. Lower Greensand. 



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

 characteristic of the Cretaceous period as Inoceramus, a group 

 belonging to the family of the Pearl-mussels (Aviculitfct). The 

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

 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 

 19 



