﻿walker: 
  on 
  margaritana 
  margaritifera. 
  135 
  

  

  Adams, 
  1905, 
  pp. 
  56-61). 
  But 
  however 
  that 
  may 
  be, 
  after 
  the 
  

   retreat 
  of 
  the 
  ice, 
  the 
  surviving 
  colonies 
  of 
  the 
  species 
  gradually 
  

   spread 
  out 
  and 
  re-peopled 
  the 
  post-Glacial 
  drainage 
  systems, 
  and 
  

   finally 
  attained 
  the 
  distribution 
  of 
  the 
  present 
  time. 
  

  

  The 
  western 
  extent 
  of 
  the 
  present 
  range 
  of 
  the 
  species 
  has 
  been 
  no 
  

   doubt 
  determined 
  by 
  two 
  main 
  factors 
  : 
  first, 
  suitable 
  conditions 
  of 
  

   environment 
  ; 
  and 
  second, 
  opportunity' 
  of 
  access 
  by 
  water 
  toward 
  the 
  

   west 
  from 
  the 
  several 
  points 
  of 
  post-Glacial 
  dispersal. 
  

  

  The 
  first 
  of 
  these 
  has 
  been 
  apparently 
  the 
  preponderating 
  one 
  in 
  

   the 
  case 
  of 
  the 
  species 
  under 
  discussion, 
  although 
  it 
  must 
  be 
  confessed 
  

   that 
  we 
  do 
  not 
  know 
  just 
  what 
  these 
  are. 
  The 
  fact 
  is, 
  however, 
  that 
  

   barring 
  the 
  one 
  citation 
  from 
  the 
  Saskatchewan 
  (to 
  be 
  considered 
  

   later) 
  there 
  is 
  no 
  evidence 
  that 
  since 
  Glacial 
  times 
  it 
  has 
  succeeded 
  in 
  

   extending 
  its 
  range 
  further 
  west 
  through 
  Canada 
  than 
  the 
  neighbour- 
  

   hood 
  of 
  Quebec. 
  

  

  That 
  its 
  range 
  further 
  westward 
  has 
  not 
  been 
  for 
  lack 
  of 
  opportunity 
  

   is 
  shown 
  by 
  the 
  fact 
  that 
  characteristic 
  species 
  of 
  the 
  Atlantic 
  fauna, 
  

   such 
  as 
  Unio 
  com2)lanatus, 
  Dill., 
  and 
  Anodonta 
  marginata, 
  Say, 
  which 
  

   no 
  doubt 
  shared 
  with 
  Margaritana 
  the 
  vicissitudes 
  of 
  the 
  Glacial 
  

   Period, 
  and 
  in 
  Post-Glacial 
  times 
  presumably 
  started 
  from 
  practically 
  

   the 
  same 
  centres 
  of 
  dispersal, 
  have 
  succeeded 
  in 
  obtaining 
  a 
  wide 
  

   range 
  to 
  the 
  west 
  and 
  north. 
  Thus 
  A. 
  marginata 
  is 
  an 
  abundant 
  

   species 
  throughout 
  the 
  whole 
  of 
  the 
  St. 
  Lawrence 
  drainage 
  area 
  

   from 
  Anticosti 
  Island 
  to 
  Lake 
  Superior, 
  and 
  north 
  to 
  Keewatin 
  and 
  

   Hudson 
  Bay. 
  U. 
  complajiatus, 
  though 
  found 
  in 
  but 
  few 
  of 
  the 
  

   southern 
  tributaries 
  of 
  the 
  Great 
  Lakes 
  between 
  Ontario 
  and 
  Superior, 
  

   is 
  the 
  most 
  abundant 
  Unionid 
  in 
  the 
  Lake 
  Superior 
  basin, 
  from 
  which 
  

   it 
  ranges 
  north 
  to 
  the 
  Saskatchewan. 
  

  

  In 
  this 
  connexion 
  the 
  distribution 
  of 
  the 
  well-known 
  A. 
  cataracta, 
  

   Say 
  [fluviatilis, 
  Dill.), 
  is 
  of 
  interest 
  and 
  significance. 
  This 
  species 
  

   is 
  closely 
  related 
  to 
  the 
  European 
  A. 
  cygnea^ 
  and 
  is 
  the 
  east 
  coast 
  

   analogue 
  of 
  the 
  west 
  coast 
  species 
  of 
  the 
  same 
  group. 
  There 
  is 
  reason 
  

   to 
  believe 
  that 
  it 
  was 
  a 
  co- 
  immigrant 
  with 
  M. 
  margaritifera 
  to 
  the 
  

   east 
  coast 
  of 
  I^orth 
  America. 
  It 
  is 
  probable, 
  too, 
  that 
  the 
  common 
  

   European 
  snail. 
  Helix 
  hortensis, 
  Mull., 
  was 
  also 
  a 
  participant 
  in 
  the 
  

   same 
  migration 
  (see 
  Johnson, 
  1906, 
  p. 
  80). 
  It 
  has, 
  however, 
  since 
  

   its 
  arrival 
  in 
  this 
  continent 
  become 
  much 
  more 
  modified 
  from 
  the 
  

   ancestral 
  type 
  than 
  the 
  west 
  coast 
  race 
  and 
  has 
  better 
  claims 
  to 
  

   specific 
  recognition. 
  The 
  range 
  of 
  both 
  races 
  on 
  the 
  two 
  coasts 
  

   is 
  remarkably 
  similar. 
  Both 
  have 
  extended 
  further 
  south 
  than 
  

   Margaritana, 
  but 
  both 
  have 
  substantially 
  the 
  same 
  inland 
  range 
  as 
  

   that 
  species. 
  Thus 
  cataracta 
  is 
  found 
  in 
  the 
  streams 
  of 
  the 
  Atlantic 
  

   drainage 
  from 
  IN^orth 
  Carolina, 
  north 
  to 
  the 
  St. 
  Lawrence. 
  In 
  the 
  

   St. 
  Lawrence 
  di-ainage 
  it 
  does 
  not 
  apparently 
  occur 
  west 
  of 
  Ottawa, 
  

   Ont., 
  and 
  Buffalo, 
  N.Y. 
  It 
  has 
  been 
  cited 
  from 
  some 
  of 
  the 
  

   northern 
  tributaries 
  of 
  Lake 
  Huron 
  and 
  Lake 
  Superior, 
  but 
  Simpson 
  

   questions 
  these, 
  and 
  it 
  is 
  quite 
  probable 
  that 
  the 
  citations 
  are 
  based 
  

   on 
  forms 
  of 
  A. 
  marginata, 
  which 
  does 
  occur 
  in 
  those 
  districts. 
  But 
  even 
  

   if 
  they 
  be 
  correct, 
  it 
  is 
  clear 
  that 
  the 
  western 
  range 
  of 
  the 
  species 
  is 
  

   very 
  much 
  more 
  restricted 
  than 
  either 
  A. 
  marginata 
  or 
  U. 
  complanatus. 
  

  

  