129 
The total number of species in the Norwich crag, rejecting those 
varieties formerly considered to be distinct species, is 111, of which 
19 belong to land or freshwater genera. This comparatively small 
number of species, whether compared with the testacea of the Bri- 
tish seas or the Fauna of the Suffolk crag, and not due to want of 
activity on the part of collectors, or a paucity of specimens, Mr. 
Lyell explained by showing, that in seas, the water of which is 
only brackish, as that of the Baltic, or any great estuary, species are 
far less numerous than in the salt sea, latitude, climate, and other 
conditions being the same. A similar scarcity of species exists also 
im the fluvio-marine deposits alone the Rhine, between Basle and 
Mayence. 
Of the 92 marine shells of the Norwich crag, Mr. Wood has 
recognised 73 species found in the red crag, and therefore it might 
be inferred that the two formations are nearly of the same age; but 
on applying the test of the proportions of recent species, Mr. Lyell 
ascertained that the Norwich crag, both with respect to the marine 
and the freshwater shells, contains between 50 and 60 per cent., 
whereas in the red crag there are only 30 per cent., and in the 
coralline but 19. 
Mr. Charlesworth had previously implied that the Norwich beds 
were the most recent, by stating his belief that shells had been 
washed out of the red crag into the Norwich; and both he and 
Mr. 8S. Wood had recognised in the Norwich beds a nearer approach 
to the existing British Fauna. 
The only known freshwater testacea of the red crag of Suffolk 
were collected by Mr. S. Wood at Sutton, and consist of three 
specimens of Auricula myosetis and one of the variety of Planorbis 
marginatus, with a slightly prominent keel: both of these shells occur 
in the Norwich crag. Among the other freshwater species of the 
Norwich crag is the Cyrena trigonalis, found also at Southwold 
and Crostwick. ‘The land shells consist of Helix hispida, H. ple- 
bium, and a species found at Southwold by Capt. Alexander, bearing 
a strong resemblance to Helix Touronensis, so common in the faluns 
of Touraine. All the 92 marine species, except two or three, are 
found either in the red crag or living, so that a very small number 
were peculiar to this period. It is important to notice, that a large 
proportion of the recent shells in the coralline crag have not been 
met with in red or Norwich; but this absence Mr. Lyell attributes 
to the fragile nature of many of these shells, and in some cases to 
their having been peculiar to deep or tranquil seas. 
In determining the above results, the utmost care was taken to 
exclude all those shells which might have been washed out of the 
red crag into the Norfolk, or did not live in the waters by which the 
latter was deposited. - 
Should these numerical conclusions hereafter require some mo- 
dification, still the Norwich crag will be referable to the older 
Pliocene period, and the red and coralline to different parts of the 
Miocene. 
From an equally careful examination by the author, Mr. Wood, 
