INTRODUCTION. 



When the ' Crag Mollusca ' was passing through the press, during the years from 

 184G to 1856, our knowledge of the Upper Tertiaries was not only very restricted, but 

 some erroneous ideas existed as to the identity of beds now found to be very different in 

 age. Geologists had begun to consider the Clyde beds as newer than the Crag ; but 

 the Bridlington deposit was regarded as coeval with the Norwich Crag, while the 

 relation of this Crag to the Red was but vaguely ascertained. Moreover, the thick beds 

 of sand which overlie the Red Crag, but which are now found to have no connexion with 

 it, were then regarded as the " unproductive sands of the Crag," while the Glacial beds 

 were known but dimly under the term " Northern Drift," or as the "Boulder Clay/' 

 under which term two very different deposits, the Boulder Till of the Cromer Cliff and 

 the wide-spread Boulder Clay of the East Anglian uplands, were confounded together. 

 Further, the Post-glacial freshwater beds of Clacton, llford, Grays, Stutton, and other places, 

 were thought at this time by many, myself included, to belong to the age of the Crag, 

 which, indeed, so far as any light their freshwater testaceous remains would throw on the 

 question, there was every reason to suppose was the case. 



The advance of geology during the years that have since elapsed has thrown new 

 light on the relations borne by these various newer tertiary beds to each other, and I 

 have been able in consequence to refer the species noticed in the present work to the 

 formations in which they occur, according to what I believe to be their true geological 

 sequence, and accordingly several new formations and localities are referred to throughout 

 the Supplement that are not to be found in the original work. 



Under these circumstances I have thought it desirable to confine this Supplement to 

 fossils from that portion of England in which the succession of the various beds has been 

 systematically worked out, and to avail myself of the labours of my son and his co- 

 adjutor, Mr. E. W. Harmer, of Norwich, who have for several years past occupied 

 themselves in studying these formations in the field, and in regularly mapping them on 

 the one-inch scale Ordnance maps. The area over which they have carried, and now very 

 nearly completed this mapping, is that which extends from the Norfolk coast, as far 

 west as the Wash, to the River Thames, comprising nearly all of the three counties, 

 Essex, Suffolk, and Norfolk ; so that they have had the opportunity of tracing the beds 

 over a wide area, and of thus guarding themselves against the not unlikely error of 

 b 



