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 



U 



Fig. 165. Jurassic Brachiopods. a. l.efitre-ia L'ttsnica, 



the figure indicating the true size of the shel Lias ; b, Spir'fe. 



Terebratnla gnadrifida, ] 



way Rock ; e. TerebratnL. _, .... 



ford Clay, Forest-marble, and G 



11 cross below 



true size 01 tne snei i,ias ; o, opirjera rosirata, Lias ; c, 

 Lias ; d, d, Rhyiiclioiielia variant, Fuller's Earth and Kello- 

 la sfihirroida 'is. Inferior Oolite ; f, Teretiratula digoua, 

 ~ it Oolite. (After Davidson). 



far the greater portion of the whole formation, the Bivalves 

 are of course marine, and belong to such genera as Trt'gom'a, 

 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 Cyrenee and Corbula. 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, Ostrca distorta is char- 

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

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

 cxpansa abounds in the Portland beds ; Ostrca ddtoidca is 

 characteristic of the Kimmeridge clay ; Ostrca grfgaria pre- 

 dominates in the Coral-rag ; Ostnca acuminata characterises the 

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

 MarsJiii (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 



