12 THE DISPERSAL OF SHELLS. 



(5. lacustre) in an isolated pond dug only twenty years 

 before is certainly worthy of note. 



Pond at Rigsby, LincolnsJiive. — Mr. Mason and Mr. 

 Davy have also favoured me with particulars of another 

 pond, probably much older than the last, and remark- 

 able for possessing a surprisingly rich moUuscan fauna. 

 Situate in the parish of Rigsby (in the " nineteen acres," 

 abutting in part on Rigsby Wood), of small size, sur- 

 rounded by arable land, and apparently perfectly 

 isolated. The nearest ditch, it is true, is within about 

 eleven yards, but the water in the pond is considerably 

 above the level of that in the ditch, and Mr. Mason 

 assures me that no possible flood can have reached the 

 pond ; with the exception of the water carried in 

 through the pipe-tiles of a small under-drain, it gets 

 every drop from rain and surface drainage. It is 

 apparently very old, and was probably dug at a time 

 when the surrounding district was an unenclosed sheep- 

 walk, in which case, of course, it is just possible that 

 sheep or cattle may have been instrumental in intro- 

 ducing shells or ova adhering to their feet or legs.^ 

 There are no plants likely to have been intentionally in- 

 troduced, duckweed {Lemna ini?wj'),di Potamogetoit, some 

 grasses, &c., making up the principal vegetation. Mr. 

 Davy and Mr. E. Woodthorpe, who have searched for 

 shells, have discovered no less than four species of bi- 

 valves and five univalves, representing together five 



' The same may be said of most ponds which appear to be 

 more ancient than the surrounding enclosures, as well as of those 

 situate on large unenclosed pastures. 



