THE JURASSIC PERIOD. 



235 



in the Jurassic deposits, and in many cases highly character- 

 istic. In the marine beds of the Oolites, which constitute by 



Fig. 165. Jurassic Brachiopods. , Leptcena Liassica, enlarged, the small cross below 

 the figure indicating the 'true size of the shell Lias ; b, Spirifera rostrata, Lias ; c, 

 Terebratula^ quadrifida, Lias ; d, d', Rhynchonella varians, Fuller's Earth and Kello- 

 way Rock ; e, Terebratula spha-roidalis, Inferior Oolite ;/, Terebratula digona, Brad- 

 ford Clay, Forest-marble, and Great Oolite. (After Davidson). 



far the greater portion of the whole formation, the Bivalves 

 are of course marine, and belong to such genera as Trigonia, 

 Lima, Pholadomya, Cardinia, Avicula, Hippopodium, &c. j but 

 in the Purbeck beds, at the summit of the series, we find 

 bands of Oysters alternating with strata containing fresh-water 

 or brackish-water Bivalves, such as Cyrence and Corbulce. The 

 predominant Bivalves of the Jurassic, however, are the Oysters, 

 which occur under many forms, and often in vast numbers, 

 particular species being commonly restricted to particular 

 horizons. Thus of the true Oysters, Ostrea distorta is char- 

 acteristic of the Purbeck series, where it forms a bed twelve 

 feet in thickness, known locally as the " Cinder-bed ; " Ostrea 

 expansa abounds in the Portland beds ; Ostrea deltoidea is 

 characteristic of the Kimmeridge clay ; Ostrea gregaria pre- 

 dominates in the Coral-rag ; Ostrea acuminata characterises the 

 small group of the Fuller's Earth ; whilst the plaited Ostrea 

 Marshii (fig. 166) is a common shell in the Lower and Middle 

 Oolites. Besides the more typical Oysters, the Oolitic rocks 

 abound in examples of the singularly unsymmetrical forms 



