WEYBOURN CRAG. 
145 
portions of the same shallow sea. One cannot understand how, 
under these circumstances, a prolific and accommodating species 
such as this could flourish for a series of years in one area, and 
yet not spread over an equally suitable district barely four miles 
away. The incoming of Tellina balthica, coupled with the 
marked increase of Arctic forms in the Weybourn Crag, is, I 
think, good evidence that we are dealing with a horizon slightly 
newer than the Norwich Crag. 
As the Wey bourn Crag is the highest division of the Pliocene 
series in which is found a marine molluscan fauna of any extent, 
this will be a convenient point at which to compare the different 
horizons, and to show what climatic and other changes are in¬ 
dicated by their respective faunas. The land and freshwater 
species will be more conveniently treated of in the next chapter, 
when we come to deal with the prolific assemblage occurring in 
the Cromer Forest-bed. A great discrepancy between the tem¬ 
perature of the sea and of the air is indicated by the fossils, and 
we cannot well deal with the comparatively few land animals of 
the Norwich Crag before the whole of the evidence has been 
brought forward. 
Marine Mollusca of tne Pliocene Beds* 
i 
Total. 
Arctic. 
Mediterranean. 
Extinct. 
Weybourn Crag - 
53 
9 
0 
5 
Chillesford Crag - 
90 
7 
2 
14 
Flnvio-mariiie Crag 
112 
9 
7 
18 
Red Crag of Boyton, &c. 
199 
13 
23 
55 
Red Crag of Walton 
148 
2 
22 
‘ 50 
Coralline Crag - 
420 
1(?) 
75 
169 
Cast in a tabular form, the figures show at a glance the gradual 
refrigeration of the climate all through the Newer Pliocene period. 
This refrigeration may be taken as the dominant characteristic of 
the period, for there is a steady decrease of southern and extinct 
forms, and a gradual replacement by northern species, till the 
marine Mollusca seem to indicate conditions almost Arctic in their 
severity. On the other hand there is so great a resemblance of 
the faunas throughout, so gradual a change, and so many charac¬ 
teristic species range upwards into the Weybourn Crag, that we 
cannot without violence separate this zone from the other Pliocene 
deposits. 
* The numbers given are only approximate. Differences of determination will 
probably affect each zone equally, and will not alter the comparative results. 
E 60798. ^ 
K 
