OF NORFOLK AND SUFFOLK. 319 



George Sowerby. The number of shells of the Norfolk crag 

 known in 1833, when Woodward published his list, amount- 

 ed, according to that author, to 85 species ; but so many of 

 these consisted of mere varieties and monstrosities of a few of 

 the commonest species, especially Littorina, Fusus, and Pur- 

 pura, that we have found it necessary to reduce the 85 species, 

 as named in that list, to about 58, and several even of this num- 

 ber must again be excluded from a genuine list of Norfolk crag 

 shells, on the ground of their consisting of fragments, probably 

 washed in from pre-existing beds of the red or Suffolk crag. 

 The total number known in 1833 being thus brought down to 

 less than 60, has been again nearly doubled by the additions 

 recently made, especially by Mr. Wigham, 19 out of 111 con- 

 sisting of land and freshwater shells. 



It will naturally be thought that the total number is very 

 small, whether as compared to the shells of the British seas, 

 or to the Fauna of the Suffolk crag, especially when it is con- 

 sidered that the scantiness of this number is not owing to any 

 want of industry on the part of collectors, nor to any paucity 

 of individual shells. But I have already stated that the de- 

 posit has a fluvio-marine character, and it is well known that 

 in brackish water, like the Baltic, or in any great estuary, the 

 variety of species is far less considerable than in the salt sea, 

 latitude, climate, and other conditions being the same. A 

 similar scantiness in the list of species has been remarked in 

 those tertiary formations which extend along the valley of the 

 Rhine, from Basle to Mayence, and in which great numbers 

 of land and freshwater shells are intermingled with marine 

 species, the same strata including also the bones of Mamma- 

 lia, and among others, at Eppelsheim, of the Dinotherium 

 and Mastodon lougirostris. 



Of the 92 marine shells of the Norwich crag, Mr. Wood has 

 recognized 73 as common to the red crag. This enormous 

 proportion of species common to both (about 78 per cent.), 

 struck me so forcibly when collecting at South wold and Nor- 

 wich, that 1 at first began to suspect, that by increasing our 

 knowledge of the fossils of the Norwich beds we should even- 

 tually prove them and the red crag to be nearly, if not wholly 

 of the same age. But the application of another test, name- 

 ly, the per-centage of recent species, soon led to a very dif- 

 ferent result, for both in the marine and freshwater shells of 

 the Norwich crag, we have found between 50 and 60 per cent, 

 of recent species, and those almost exclusively northern, and 

 nearly all British shells ; whereas in the red crag, as I shall 

 afterwards more fully explain, there are only 30 per cent., 



