242 



HISTORICAL PALAEONTOLOGY. 



ticular, are very abundant, and the species are often confined 

 to special horizons in the series. 



Remains of Bivalves (Lamcllibranchiata) are very numerous 

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

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

 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. ; 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 Cyrcncc and Corbula. The 

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



Fig. 165, Jurassic Brachiopods. a, Leptcena Liassica, enlarged, the small cross be- 

 low the figure Indicating the true size of the shell Lias, 6, Spirifera rostrata, Lias ; c, 

 Tere&ralula quadriftda, Lias ; d, d', Rhynchonella varians, Fuller's Earth and Kello- 

 way Rock ; e. Terebratula spficerotdalis, Inferior Oolite ; /, Terebratula digona, Brad- 

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



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 characterizes the 

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



