KENNAED : HOLOCENE NON-MARINE MOLLUSCA OF ENGLAND. 243 



species, wliilst the abnormal conditions often prevailing in these 

 habitats has been reflected in their molluscan inhabitants, and 

 consequently in the list of varietal names. Marshes and fens have 

 been drained, thus reducing the areas suitable for the damp-loving 

 species, whilst the conversion of woodland into pasture and tilth 

 has been of great advantage to some forms and injurious, if not 

 destructive, to others. 



Charlton Wood, the original habitat of Clausilia rolpMi, Gray, 

 was thus destroyed, and as a consequence Dr. Leach noted " no 

 specimens have been taken for several years " {Synopsis Moll. 

 Gt. Britain, 1852, p. 86). The construction of numerous stone walls 

 in many parts of the country has provided welcome oases for such 

 species as Pyramidula rupestris (Drap.) and Lauria cylindracea 

 (Da Cost.). Horticulture, too, has played a considerable part in 

 the dissemination of many of the slugs, as well as such forms as 

 Helicella draparnaldi (Beck) and Opeas pumilum (Pfr.). The 

 growth of motor traffic and the consequent tarring of the roads has 

 had a very adverse efiect on the molluscan fauna of the bordering 

 hedges, ditches, and ponds. The molluscan inhabitants of many 

 ponds in West Kent have thus been quite exterminated. Though 

 hedges are often of modern origin, and thus not a " natural " 

 habitat in the botanical sense, yet in many parts of England, 

 especially in the east and south-east, they are often strips of old 

 woodland left when the land was originally cleared, and have been 

 havens of refuge for the fauna. Building operations, too, have 

 destroyed many recorded localities, and it is useless now to look 

 for Clausilia hiplicata (Mont.) in Hyde Park, Chelsea meadows, or 

 Fulham meadows ; Fruticicola (Zenohiella) subrufescens (Mill.) ^ at 

 Blackheath ; Succinea ohlonga, Drap., and Pisidium supinum, 

 A. Schm., in Battersea meadows ; whilst H. C. Huggins informs me 

 that the original habitat of Jacosta (Xeroclivia) elegans (Gmel.)/'- 

 at Lydden is now buried by the tip from a neighbouring colliery. 



It is thus obvious that it is quite impossible by the most careful 

 collecting to ascertain the true molluscan fauna of any given district, 

 or to say whether a species is a casual, a colonist, a denizen, or a native. 

 To solve these problems one must turn to the Holocene deposits, 

 always remembering the imperfections of the geological record, and 

 the relatively small attention that has been paid to these beds. 

 Some species, too, such as Zonitoides excavatus (Aid.) and 

 Margaritifera margaritifer (Linn.), from their pronounced calcifuge 

 habits, are not likely to be preserved in a fossil state, whilst the 

 constitution of the shell of Fruticicola (Zenohiella) subrufescens (Mill.) 

 will account for its absence from all deposits. 



^ In view of the fact that the species constitutiag these two genera are now 

 frequently referred to under the subgeneric or sectional names direct, the 

 latter have been introduced throughout this address. 



