Pleistocene Species 133 



extinct within the county or in the British Isles altogether. 

 These forms have been recorded by several authors, the 

 most recent and important papers on the subject being 

 by Mrs Hughes 1 and Mr B. B. Woodward 2 . To these the 

 reader is referred for details and conclusions. The follow- 

 ing notes and the incidence of still living species in the 

 Pleistocene beds already given in this article are extracted 

 from these sources, from Mr F. Cowper Reid's work on the 

 geology of the county 3 , from the article on Mollusca in the 

 "Victoria History" of the county and from malacological 

 papers in various journals. 



The marine shell-bearing gravels of the March district 

 were laid down in the estuaries of the several streams 

 which flowed into the Wash when its southern shores ex- 

 tended through what is now the north of Cambridgeshire. 

 With about 24 species of marine Gasteropoda and about 

 20 species of marine Pelecypoda (all being forms extant 

 at the present day) the March Gravels contain Bithynia 

 tentaculata, Valvata piscinalis, Unio tumidus and Corbicula 

 fluminalis ?, but no Euthyneurous Pulmonata. In the neigh- 

 bourhood of Cambridge, as at Girton and other places, there 

 are gravels which are believed to have been deposited by 

 affluents of the ancient rivers once debouching into the Wash, 

 which then extended further south. In these gravels there 

 are no marine shells but they contain Hygromia hispida, 

 Jaminia muscwum ?, Succinea putris and Pisidium amnicum?. 



After the deposition of these gravels the present river 

 system of the county became established, though there is 

 some dispute as to the stages by which the ancient system 

 passed into it. The gravels of the Ouse Valley at Somersham 

 are among those laid down in the earlier period of the present 

 river system and in them we find Cardium edule along with 

 the estuarine form Corbicula fluminalis. Later than the 

 . Somersham beds are the gravels of Barrington, Barnwell and 



1 loc. cit. 2 Proc. Geol. Assoc., x., 1888, p. 355. 



3 The Geology of Cambridgeshire. 



