﻿136 
  PROCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  Now, 
  it 
  would 
  seem 
  to 
  be 
  evident 
  that, 
  if 
  the 
  latter 
  two 
  species 
  

   have 
  been 
  able 
  since 
  the 
  close 
  of 
  the 
  Glacial 
  Period 
  to 
  obtain 
  such 
  

   a 
  wide 
  range 
  to 
  the 
  north-west, 
  M. 
  margaritifera 
  and 
  A. 
  cataracta, 
  whicli 
  

   have 
  apparently 
  always 
  been 
  associated 
  with 
  them 
  since 
  pre-Glacial 
  

   times, 
  could 
  have 
  accompanied 
  them 
  in 
  this 
  western 
  migration 
  had 
  

   the 
  environmental 
  conditions 
  of 
  the 
  more 
  westei'n 
  waters 
  been 
  

   favourable 
  to 
  their 
  existence. 
  

  

  There 
  is 
  apparently 
  no 
  satisfactory 
  explanation 
  to 
  be 
  offered 
  as 
  to 
  

   why 
  the 
  western 
  region 
  does 
  not 
  offer 
  a 
  suitable 
  environment, 
  but 
  

   such 
  would 
  seem 
  to 
  be 
  the 
  fact. 
  Under 
  the 
  assumption, 
  therefore, 
  

   that 
  the 
  east 
  coast 
  Margaritana 
  is 
  an 
  immigrant 
  from 
  Europe, 
  it 
  is 
  

   clear 
  that, 
  if 
  in 
  pre-Glacial 
  times 
  it 
  succeeded 
  in 
  reaching 
  central 
  

   British 
  America, 
  it 
  was 
  wholly 
  exterminated 
  in 
  that 
  region 
  by 
  the 
  ice, 
  

   and 
  that 
  since 
  the 
  close 
  of 
  the 
  Glacial 
  Period 
  it 
  has 
  not 
  succeeded 
  in 
  

   getting 
  into 
  that 
  region, 
  although 
  other 
  associated 
  species 
  have 
  been 
  

   amply 
  able 
  to 
  do 
  so. 
  

  

  Put 
  did 
  it 
  succeed 
  in 
  getting 
  into 
  central 
  Pritish 
  America 
  in 
  

   pre-Glacial 
  times 
  ? 
  There 
  is 
  no 
  direct 
  evidence, 
  so 
  far 
  as 
  I 
  know, 
  

   showing 
  just 
  what 
  the 
  Unione 
  fauna 
  in 
  that 
  region 
  was 
  at 
  that 
  time. 
  

   Put 
  as 
  we 
  know 
  that 
  the 
  Atlantic 
  drainage 
  was 
  prior 
  to 
  the 
  Glacial 
  

   Period 
  peopled 
  by 
  an 
  immigration 
  from 
  the 
  then 
  fauna 
  of 
  the 
  Mississippi 
  

   Yalley 
  through 
  that 
  region, 
  it 
  is 
  to 
  be 
  presumed 
  that 
  the 
  fauna 
  was 
  

   composed 
  of 
  substantially 
  the 
  same 
  species 
  (or 
  their 
  prototypes) 
  that 
  

   now 
  are 
  found 
  iu 
  the 
  Atlantic 
  drainage. 
  The 
  climatic 
  conditions 
  

   of 
  the 
  region 
  at 
  that 
  time 
  were 
  presumptively 
  favourable 
  to 
  the 
  

   existence 
  of 
  such 
  a 
  fauna. 
  They 
  certainlj^ 
  were 
  at 
  the 
  time 
  when 
  the 
  

   eastern 
  migration 
  took 
  place, 
  and 
  there 
  is 
  no 
  reason 
  to 
  predicate 
  any 
  

   substantial 
  change 
  in 
  these 
  conditions 
  until 
  the 
  commencement 
  of 
  the 
  

   Glacial 
  Period. 
  That 
  being 
  so, 
  it 
  is 
  a 
  fair 
  inference 
  that 
  the 
  climate 
  

   of 
  the 
  region 
  at 
  that 
  time 
  was 
  not 
  greatly 
  different 
  from 
  what 
  it 
  is 
  at 
  

   the 
  present 
  time. 
  It 
  is 
  also 
  evident 
  that 
  if 
  at 
  that 
  time- 
  the 
  western 
  

   Tlnionidfe 
  were 
  able 
  to 
  make 
  their 
  way 
  to 
  the 
  Atlantic 
  coast 
  there 
  

   "Was 
  no 
  reason, 
  so 
  far 
  as 
  oppoitunity 
  of 
  access 
  is 
  concerned, 
  why 
  

   Margaritana^ 
  which 
  at 
  that 
  period 
  had 
  arrived 
  on 
  the 
  coast, 
  should 
  

   not 
  have 
  been 
  able 
  to 
  pass 
  westward 
  along 
  the 
  same 
  system 
  of 
  

   waterways, 
  unless 
  prevented 
  by 
  the 
  same 
  unfavourable 
  conditions 
  

   of 
  local 
  environment 
  which 
  have 
  apparently 
  since 
  post-Glacial 
  times 
  

   restricted 
  its 
  range 
  to 
  the 
  westward. 
  It 
  is 
  certainly 
  a 
  fair 
  presumption 
  

   that 
  the 
  species 
  at 
  that 
  time 
  was 
  subject 
  to 
  the 
  same 
  limitations 
  that 
  

   it 
  evidently 
  is 
  at 
  present. 
  There 
  is 
  indeed 
  no 
  evidence 
  that 
  it 
  ever 
  

   did 
  reach 
  that 
  region. 
  And 
  between 
  the 
  two 
  presumptions 
  that 
  

   doubtless 
  is 
  the 
  more 
  reasonable 
  which 
  is 
  in 
  accord 
  with 
  the 
  

   known 
  facts 
  affecting 
  the 
  inland 
  range 
  of 
  the 
  species, 
  at 
  the 
  

   present 
  time. 
  

  

  It 
  might 
  be 
  argued 
  that 
  if 
  Margaritana 
  was 
  a 
  member 
  of 
  the 
  

   pre-Glacial 
  fauna 
  of 
  central 
  Pritish 
  America, 
  it 
  is 
  strange 
  that 
  it 
  did 
  

   not 
  participate 
  in 
  the 
  retreat 
  southward 
  of 
  the 
  fauna 
  upon 
  the 
  advent 
  

   of 
  the 
  Glacial 
  conditions, 
  and 
  from 
  the 
  surviving 
  remnant 
  of 
  that 
  

   migration, 
  after 
  the 
  retreat 
  of 
  the 
  ice, 
  spread 
  out 
  again 
  to 
  the 
  north 
  

   and 
  reoccupy 
  the 
  region 
  from 
  which 
  it 
  had 
  been 
  driven. 
  

  

  