WHITECLIFF BAY. 131 



the fluvio - marine and freshwater group 550 

 feet. 



The assemblage of organic remains in these de- 

 posits is regarded by Professor E. Forbes as indi- 

 cating the following sequence of changes in the 

 condition of the basin in which they were formed. 

 " This fauna, which did not begin until after a 

 considerable thickness of mottled clays, in which 

 no traces of animal organization appear, had 

 been deposited upon the chalk, commences by 

 numerous peculiar mya-form shells, pectunculi, 

 ostrege, and their associates, in a series of sands 

 and clays. The earliest fossiliferous bed is a most 

 remarkable one, consisting of a thin stratum, 

 almost entirely composed of the shells of a ptero- 

 podous mollusk (the ditrupa plana, PL III. fig. 3), 

 which appears to have continued but a short specific 

 time in life, and to have entirely disappeared. In 

 the midst of this group, strata charged with 

 myriads of foraminifera (nummulites), probably in- 

 dicating some change in the depth of the sea, 

 appear, and cease. The sudden conversion of the 

 sea into a freshwater lake, denoted by a stratum 

 of paludina clay, its return into a brackish state, 

 and the consequent reappearance of certain marine 

 animals — its reconversion into a freshwater lake 

 thronged with myriads of fluviatile mollusca (plan- 



