Variation of local Limnaea peregra 117 



throw some light on the way in which discontinuous distribu- 

 tion of species within a small area may arise where a river 

 runs through it with an unrestrained and changeful course. 



The ditches and ponds round Cambridge contain a fair 

 number of species. In many of them Planorbis albus occurs 

 more frequently than in the river and though abundant is 

 rather local. Ancylus fluviatilis occurs in Hobson's Brook. 

 Limnaea peregra is also very common, and the extreme 

 variability of this form is well illustrated by a comparison 

 of specimens from the local waters. Thus in a collection 

 made in the immediate neighbourhood of Cambridge by 

 Mr W. Bateson, F.R.S., the present writer, on a com- 

 paratively superficial survey, obtained the general results 

 shown on pp. 118 and 119. 



