.CROMER FOREST-BED. 
188 
types, are mostly recent forms; but as the smaller species are 
almost unknown from the strata beneath tlie Forest-bed, any 
Fig. 45. Fig. 46. 
Cervus verticornis, Dawk. Cervus Sedgwickii, Falc. 
comparison between the deposits must deal exclusively with the 
larger species. The large mammals are fairly well known from 
each horizon. 
The land and freshwater mollusca of the Forest-bed number 
58 species. Five of these are extinct, and five others do not 
now live in Britain.* (See PI. V.) The proportion of extinct 
forms is nearly identical with that found among the marine 
mollusca of the Weybourn Crag. It is commonly stated that 
the rate of change is much slower among the freshwater mollusca 
than among the marine forms. There is, however, no indication 
of this slowness in the Forest-bed, or in any other Tertiary 
deposit that I have examined. What we generally find is that 
among the freshwater mollusca a few large genera have a 
wide range in time, but their specific forms vary as quickly as 
among the marine mollusca. Among the freshwater species two 
or three help to connect the Forest-bed with the Pliocene strata 
of the continent. Tims we find a slug, Limax modioliformisy 
closely allied to a species from the Val d’Arno (Sansania 
Bourgnignati, De Stefani), perhaps identical with it. Nematura 
runtoniana is also more like certain forms from the Yal d’Arno 
than to anything else. The continental species have all a 
southern range. 
In the Weybourn Crag only 10 species of land and freshwater 
mollusca have been found. Of these two are extinct and two 
* If we accept Prof. Sandberger’s determinations there are several more extinct 
forms in the Forest-bed, and the proportion is much greater than among the marine 
mollusca. 
