246 PROCEEDINGS OF THE MALACOLOGICAL SOCIETY. 



vanished {Trans. Geol. Soc. Glasgow, 1866, vol. ii, p. 109. and 

 vol. xvi, 1915, p. 108). In Wales, too, an imperfect valve of this 

 species was found in the Perthi Chwaren Cave, near Llandegla, 

 Denbighshire, a sepulchral cave of Neolithic age, and its presence 

 there is clearly due to Neolithic man (J, W. Jackson, Lane. Nat., 

 1913, p. 321). This specimen had originally been recorded as 

 Mya truncata, Linn. {Journ. Ethnol. Soc. Lond., 1871, p. 443). The 

 remaining seven species are all extremely local forms. Whether 

 they are really native species with a formerly wider distribution, 

 or recent introductions, it is impossible to say, for we are dealing 

 with negative evidence, which is too often a broken reed. In this 

 group, I think, Eulota fruticum (Miill.) should also be included, for 

 though it is found in the Pleistocene, yet its restricted modern 

 distribution and its absence from all Holocene beds leads one to 

 infer that it may be only a modern re-introduction. The following 

 species are known from the Holocene, but not from earlier beds : — 



Helicella draparnaldi (Beck). 



Jacosta {Candidula) gigaxii (Pfr.). 



Theba cantiana (Mont.). 

 „ cartusiana (Miill.). 



Helix pomatia, Linn. 



Clausilia hiplicata (Mont.). 

 „ duhia, Drap. 

 „ rolphii, Gray. 



Limncea auricularia (Linn.) (typical form). 



Planorhis stroemii, West. 



Paludestrina jenkinsi (Smith). 



Vivipara vivipara (Linn.). 



Assiminia grayana, Leach. 



Theodoxus fluviatilis (Linn. ) . 



Dreissensia polymorpha (Pall.). 



Psevdanodonta elongata (HolL). 



Pisidium hibernicum, Westld. 

 Helicella rogersi (B. B. Woodw.) probably belongs to this 

 assemblage, for the two Pleistocene records, Ightham Fissure, Kent, 

 and Langwith Cave, Derbyshire, are both cavern deposits, and in 

 each case there had been disturbance. It is, of course, possible that 

 it may yet be found in a sealed bed, but the course I have suggested 

 appears to be the correct one. Thus the vast majority of our 

 living species have been resident here for a much longer period than 

 was formerly considered to be the case, and it may be noted that 

 nearly all the presumed post-Pleistocene immigrants are confined 

 in the British Isles to England. 



Three species have apparently become extinct during the 

 Holocene period. 



Goniodiscus ruderatus (Stud.) is known from the Holocene of 

 Copford, Essex, and Wheatley, Nottinghamshire. It is also known 



