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. a, Leptena Liassica, enlarged, the small cross below 
the figure indicating the true size of the shell—Lias; 4, Spirifera rostrata, Lias; ¢, 
Terebratula guadrifida, Lias; d, a’, Rhynchonella varians, Fuller's Earth and Kello- 
way Rock; e¢, Terebratula spheroidalis, Inferior Oolite ; f, 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 Z77govza, 
Lima, Pholadomya, Cardinia, Avicula, Hippopodium, &c. ; 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 Cyrene and Corbule. ‘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 1s 
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 
