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arcuata), a species of oyster, strictly confined to the Lower Lias, 

 and forming hard scars of limestone at Redcar. Amongst 

 the other bivalve shells of the Lower Lias Cardinia, is most 

 noteworthy and characteristic, though now quite extinct. The 

 beautiful univalve, Pleurotomaria an^Iica, is somewhat numer- 

 ous at Redcar, and belongs to a genus which is abundant 

 in the stratified rocks, but is at the present time almost 

 extinct, a few living examples having been found in the 

 Antilles and Japan. 



The nature of the Lower Lias rocks, shale with thin bands 

 of limestone, indicates the conditions of life at that epoch, 

 viz.. moderately deep water, clear when the limestone bands 

 were formed, and muddier when the shales were deposited. 

 In the Middle Lias still shallower water existed, as is testified 

 by the ripple marked sandstones of the Sandy Series, with 

 their oyster beds and swarms of Cardium truncatum. These 

 littoral conditions were followed by deeper water in which 

 the shaly beds of the Ironstone Series were deposited, a life 

 condition which prevailed up to the close of the Liassic 

 period. 



In the [Middle Lias, which includes the Ironstone Series, 

 we are confronted with a rich and varied fauna quite different 

 from that of the Lower Lias. Not only have nearly all the 

 species died out which are characteristic of the inferior beds 

 but also whole genera. The Arietan Ammonites, so numerous 

 in the Redcar Rocks, are replaced by species belonging to 

 new genera. Of these Mgoceras capricomus, and Amaltheus 

 spinatus, may be taken as typical examples. The Cardinias 

 are reduced to one or two feeble representatives, whilst 

 peculiar genera of bivalve Mollusca become very dominant, 

 viz.: Gresslya, Pleuromya, Goniomya, Arcomya, Ceromya, and 

 Pholadomya. All of them are extinct, except Pholadomya, 

 two or three species of which have been dredged from the 

 bottom of deep parts of the Atlantic. 



A famous bivalve of the Ironstone is Trigonia lingonensic. 

 one of the earlievt members of a genus which attained an 

 extraordinary and beautiful development in later Jurassic 

 times, but is now" only represented by a few species in 



