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INTRODUCTION. 



A Geological Histoet or details of the different Formations, not comine 

 within the. object of the Palasontographical Society, it is only necessary here to state 

 that this work may be considered as an illustrated description of my Catalogue of 

 Crag Shells, published in the ' Annals and Magazine of Natural History,' 1840-42, 

 including such Species as have since been discovered, in addition to those already 

 enumerated. 



It is intended to include as Crag Shells all those species, hitherto considered as 

 coming under this general and well-understood denomination, from the three different 

 Periods, into which that Formation has been divided by geologists, viz., the Coralline 

 Crag, as representing in this country the remains of the Miocene Period ; the Red 

 Crag, those of the Pliocene Period; and the Mammaliferous Crag, those of the 

 Pleistocene Period. In the latter are included the several species that have been 

 obtained at Bridlington, as that bed is now considered to be a Marine Formation 

 of the Pleistocene Period, and consequently synchronous with the Estuary deposits 

 of Bramerton and Thorpe. The northern beds of the Clyde, &c, may belong to the 

 same Period ; but I am not sufficiently acquainted with those deposits to express 

 an opinion on that point, as I have seen only such shells from them as may be 

 considered identical with existing species. A Catalogue of the Mollusca from the 

 Lacustrine or Fluviatile deposits of Grays,* Clacton,f Stutton,| and Copford,§ 

 will be given as an Appendix to the Second Part of this work, as those beds are 



* Grays, on the River Thames, twenty-one miles from London, in the County of Essex, 

 t Clacton, in Essex, on the coast, seven miles south-west of Walton Naze. 

 X Stutton, in Suffolk, on the banks of the River Stour, six miles south of Ipswich. 

 § Copford, in Essex, about four miles south-west of Colchester. 



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