flood (if Jcumuvij, 1918 159 



away from the river. The specimens can hardly have come from 

 there, and it is more likely that the colony lived out of doors and 

 nearer to the river. Nevertheless its progenitors may have been 

 'escapes'. The greenhouses below the railway-bridge have now 

 been out of use for some time, and the snails that were in them 

 must have been forced to seek new quarters. 



Most of the shells, both land and fresh-water, were perfect or 

 nearly so, and all of them were empty. Neither Mr Gray nor myself 

 found a single specimen with any remains of its former inhabitant. 

 The greater number were very fresh in appearance, but some of 

 the land-shells had evidently been exposed to the weather for some 

 time, and some of the fresh-water shells had lain in the mud long- 

 enough to become discoloured or incrusted as if the process of 

 fossilization had begun. The specimens of Vitrea draparnaldi, it 

 may be noted, were all fresh-looking. 



Apart from the extent of the shelly deposit, its freedom from 

 silt below the railway-bridge was perhaps its most important feature, 

 for it shows that even a muddy river like the Cam may produce a 

 purely calcareous deposit. 



The fact that the shells were all empty indicates that those 

 belonging to the river must have lain in its bed for some time; and 

 in this connection an observation made by Mr Gray is of interest. 

 Some years ago at Bottisham, when dredging operations were going 

 on, he noticed that the mud brought up by the dredger was full of 

 fresh-water shells. 



During floods the river digs up its bed and, as on the occasion 

 here described, it may deposit the shells in one place and the silt 

 in another. In the case of an artificially controlled stream like the 

 Cam, floods are comparatively rare ; but in an unrestrained river 

 we may reasonably expect them to be both more numerous and 

 more extensive. It seems quite possible therefore that neither the 

 clayey fresh-water limestones of the Wealden nor the purer fresh- 

 water limestones of the Purbeck series required lagunary conditions 

 for their formation. 



