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 Z77gonia, so characteristic of the Mesozoic 
deposits in general, is likewise well represented in the Creta- 
Fig. 196.—Ostvea Coulont. Lower Greensand. 
ceous strata. No single genus of Bivalves is, however, so highly 
characteristic of the Cretaceous period as /voceramus, a group 
belonging to the family of the Pearl-mussels (Avicuiid@). 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 /vocerami attain a length of two or three feet, and 
fragments of the shell are often found perforated by boring 
