﻿85 
  

  

  NOTES 
  ON 
  THE 
  BED 
  VARIETY 
  OF 
  PLANORBIS 
  CORNEUS, 
  L., 
  

   AND 
  SOME 
  OTHER 
  FRESHWATER 
  MOLLUSCA. 
  

  

  By 
  W. 
  T. 
  Webster, 
  F.ll.M.S. 
  

  

  {Being 
  the 
  substance 
  of 
  communications 
  made 
  14th 
  June 
  and 
  8th 
  November, 
  

  

  1918.) 
  

  

  TuE 
  specimens 
  of 
  Planorhis 
  conieus, 
  var. 
  rubra 
  (known 
  in 
  Germany 
  

   as 
  Hote 
  posthornschneclcen) 
  here 
  described 
  were 
  collected 
  on 
  Easter 
  

   Monday, 
  1918, 
  from 
  a 
  very 
  small 
  pond 
  near 
  Barnet. 
  The 
  type 
  form 
  

   was 
  exceedingly 
  abundant, 
  while 
  of 
  the 
  red 
  variety 
  about 
  200 
  were 
  

   counted, 
  showing 
  this 
  form 
  to 
  be 
  well 
  established. 
  

  

  Tliis 
  red 
  snail 
  is 
  without 
  doubt 
  a 
  sort 
  of 
  albino, 
  and 
  the 
  deficiency 
  

   of 
  pigment 
  extends 
  somewhat 
  to 
  the 
  shell, 
  which 
  is 
  lighter 
  than 
  the 
  

   normal. 
  The 
  shells 
  of 
  those 
  animals 
  which 
  have 
  wintered 
  in 
  the 
  

   decomposing 
  vegetation 
  have 
  become 
  quite 
  black. 
  This 
  variety 
  

   interbreeds 
  freely 
  with 
  the 
  ordinary 
  form, 
  and 
  the 
  offspring 
  of 
  the 
  

   cross 
  are 
  always 
  dark. 
  The 
  young 
  in 
  some 
  cases 
  are, 
  apparently, 
  

   somewhat 
  lighter 
  in 
  body 
  and 
  shell 
  than 
  the 
  normal, 
  but 
  as 
  they 
  get 
  

   older 
  they 
  cannot 
  be 
  distinguished 
  from 
  the 
  type 
  form. 
  In 
  a 
  few 
  

   instances 
  I 
  have 
  noticed 
  little 
  ones 
  very 
  dark, 
  a 
  suspicion 
  of 
  

   melanism, 
  When 
  found 
  nearly 
  all 
  had 
  been 
  cross-fertilized 
  by 
  

   ordinary 
  type 
  and 
  produced 
  dark 
  offspring, 
  but 
  later 
  on, 
  having 
  

   been 
  kept 
  free 
  from 
  contamination, 
  the 
  same 
  individuals 
  bred 
  true 
  to 
  

   colour, 
  and 
  no 
  dark 
  ones 
  were 
  afterwards 
  produced. 
  

  

  I 
  have 
  in 
  years 
  past 
  had 
  odd 
  specimens 
  of 
  this 
  red 
  variety 
  

   brought 
  to 
  me 
  as 
  coming 
  from 
  Essex 
  ponds. 
  I 
  formerly 
  had 
  some 
  

   doubts, 
  but 
  now 
  think 
  these 
  instances 
  may 
  possibly 
  be 
  accepted, 
  and 
  

   that 
  perhaps 
  this 
  red 
  form 
  is 
  by 
  no 
  means 
  so 
  very 
  rare. 
  

  

  Several 
  specimens 
  have 
  been 
  found 
  with 
  nearly 
  white 
  bodies 
  and 
  

   normal 
  shells, 
  and 
  some 
  have 
  been 
  discovered 
  with 
  red 
  bodies 
  and 
  

   white 
  or 
  nearly 
  wliite 
  shells. 
  Most 
  specimens 
  over 
  one 
  year 
  old 
  are 
  

   completely 
  perforated 
  in 
  the 
  centre. 
  In 
  many 
  cases 
  the 
  hole 
  is 
  

   quite 
  small, 
  whilst 
  in 
  others 
  the 
  perforation 
  is 
  large, 
  and 
  only 
  the 
  

   outer, 
  or 
  body, 
  wliorl 
  remained. 
  In 
  the 
  early 
  summer, 
  in 
  all 
  these 
  

   cases 
  of 
  perforation 
  the 
  remaining 
  extremity 
  of 
  the 
  shell 
  was 
  neatly 
  

   sealed, 
  and 
  there 
  was 
  nothing 
  to 
  indicate 
  that 
  they 
  had 
  ever 
  

   possessed 
  any 
  further 
  apex. 
  Later 
  in 
  the 
  season 
  the 
  sealing 
  became 
  

   ragged 
  as 
  further 
  erosion 
  took 
  place. 
  

  

  On 
  carefully 
  examining 
  the 
  tentacles 
  many 
  peculiarities 
  have 
  been 
  

   observed. 
  Some 
  have 
  both 
  extremely 
  long, 
  slender, 
  and 
  well 
  

   matched. 
  Some 
  have 
  one 
  long 
  and 
  one 
  short; 
  the 
  shorter 
  being 
  

   either 
  left 
  or 
  right 
  indiscriminately. 
  Some 
  have 
  two 
  very 
  short 
  

   tentacles, 
  stunted 
  and 
  conical. 
  A 
  few 
  were 
  found 
  doing 
  well 
  

   entirely 
  without, 
  and 
  the 
  places 
  where 
  tliey 
  should 
  have 
  been 
  were 
  

   indicated 
  by 
  the 
  merest 
  suggestion. 
  Some 
  specimens 
  have 
  the 
  left 
  

   tentacles 
  bifurcated, 
  and 
  these 
  having 
  been 
  isolated 
  and 
  mated 
  all 
  

   the 
  young 
  proved 
  normal. 
  None 
  has 
  been 
  found 
  with 
  a 
  single 
  

   tentacle. 
  

  

  