184 
CKOMER FOREST-BED* 
others no longer inhabit Britain. This is too small a number for 
comparison, but it is noticeable .that with one exception all the 
species occur also in the Forest-bed. The one exception is 
Lithoglyphus fuscus, a river shell now confined to the Danube, 
and unknown recent or fossil in Northern Europe, except at one 
locality in the Weybourn Crag, and one near Berlin. 
The Norwich Crag has yielded 26 species of land and fresh¬ 
water shells. All of these, with the exception of two recent 
forms of Limncea and one of Pupa, are also found in the 
Forest-bed. There are only two extinct species in the list, and 
one other locally extinct. It is thus evident that, in the present 
state of our knowledge, the proportion of extinct land and fresh¬ 
water mollusca is identical with that found in the Forest-bed. 
In the upper Bed Crag the land and freshwater molluscan 
fauna still remains the same as in the higher strata. Of 17 
species known, two are extinct, and three have disappeared 
from Britain. Two of the latter are, according to Wood, arctic 
species of Limnma,^ the other is Corhieula flnminalis. The 
Walton Crag seems to indicate a complete change in the climatic 
conditions, for of the 4 species of Helix known (there are no 
freshwater shells) three are southern and the other is extinct. 
It therefore appears that we have one set of land and freshwater 
mollusca ranging throughout the upper Crag and Forest-bed of 
Norfolk, and that the proportion of extict forms is about the 
same in each division. 
It is instructive to compare the entire fauna of the Forest- 
bed with the recent fauna of Britain, classed according to the 
conditions under Avhich the species live. This mode of analysis 
brings out the fact that while of the Forest-bed aquatic and 
marsh-loving forms we have a fair knowledge, of the others, 
except the land-mammals of the plains, at present very little is 
known. For this purpose the following table, which includes 
only the more important classes, has been drawn up. In the 
first column the approximate number of existing British forms, 
with the addition of a few mammals exterminated within the 
historic period, is shown ; in the second is given the total number 
of species known from the Forest-bed:—■ 
Bats 
British. 
. 14 
Forest-bed. 
0 
Land mammals - 
- 
- 
- 29 
40 
Amphibious mammals (freshwater) 
4 
6 
Marine mammals 
- 
- 
- 28 
11 
Birds - 
- 
- 
- 354 
7 
Beptiles 
- 
5 
Amphibia 
- 
- 
7 
4 
Freshwater fish 
- 
- 
- 52 
10 
Marine fish 
- 
- 210 
6 
Land shells 
- 
- 
- 77 
17 
Freshwater shells 
- 
48 
41 
Marine shells 
- 465 
19 
* Both somewhat doubtful determinations. 
