﻿walker: 
  on 
  IlARGARirAXA 
  MABGARITIFEIiA. 
  137 
  

  

  While 
  there 
  was, 
  no 
  doubt, 
  such 
  a 
  recession 
  of 
  the 
  terrestrial 
  

   moUuscan 
  fauna 
  southward 
  at 
  that 
  time, 
  and 
  that 
  the 
  " 
  survivors 
  

   crowded 
  with 
  the 
  boreal 
  forms 
  in 
  a 
  band 
  along 
  the 
  states 
  borderin"- 
  

   the 
  glaciated 
  area" 
  (Pilsbry, 
  1906, 
  p. 
  531), 
  there 
  is 
  no 
  evidence 
  that 
  

   there 
  was 
  awy 
  such 
  survival 
  of 
  the 
  pre-Glacial 
  Unione 
  fauna 
  of 
  that 
  

   region. 
  Indeed, 
  the 
  probability 
  is 
  all 
  the 
  other 
  way, 
  as 
  it 
  was 
  wholly 
  

   impossible 
  for 
  the 
  fliiviatile 
  fauna 
  to 
  escape 
  by 
  migrating 
  to 
  the 
  south 
  

   unless 
  the 
  existing 
  sj'stems 
  of 
  drainage 
  afforded 
  the 
  necessarj^ 
  water 
  

   communications 
  for 
  them 
  to 
  reach 
  the 
  unglaciated 
  area 
  in 
  the 
  south. 
  

   There 
  is 
  no 
  evidence 
  that 
  any 
  such 
  waterways 
  were 
  then 
  in 
  existence. 
  

   And 
  a 
  consideration 
  of 
  the 
  present 
  fauna 
  shows 
  that 
  it 
  is 
  wholly 
  

   unlikely. 
  The 
  eastern 
  migration 
  of 
  the 
  primitive 
  Mississippi 
  fauna 
  

   was 
  undoubtedly 
  a 
  very 
  ancient 
  one, 
  and 
  it 
  is 
  probable 
  that 
  the 
  

   differentiation 
  of 
  the 
  species 
  taking 
  part 
  in 
  it 
  into 
  the 
  distinct 
  races 
  

   now 
  existing 
  on 
  either 
  side 
  of 
  the 
  Appalachian 
  jNIountains 
  took 
  place 
  

   before 
  the 
  advent 
  of 
  the 
  Glacial 
  Period. 
  This 
  is 
  apparently 
  shown 
  by 
  

   the 
  fact 
  that 
  the 
  fossil 
  relics 
  of 
  the 
  Interglacial 
  Unione 
  invasion 
  of 
  

   Canada 
  are 
  specifically 
  identical 
  with 
  the 
  species 
  now 
  living 
  in 
  the 
  

   Ohio 
  Valley 
  (see 
  Simpson, 
  1893, 
  p. 
  592). 
  This 
  would 
  certainly 
  seem 
  

   to 
  show 
  that 
  there 
  has 
  been 
  no 
  radical 
  change 
  in 
  the 
  character 
  of 
  the 
  

   fauna 
  since 
  Glacial 
  times. 
  

  

  The 
  entire 
  area 
  now 
  comprised 
  within 
  the 
  St. 
  Lawrence 
  drainage 
  

   system 
  was 
  within 
  the 
  glaciated 
  area. 
  In 
  order, 
  therefore, 
  for 
  any 
  

   of 
  the 
  pre-Glacial 
  Unione 
  inhabitants 
  of 
  that 
  region 
  to 
  have 
  escaped 
  

   extermination 
  by 
  the 
  ice, 
  they 
  must 
  have 
  succeeded 
  in 
  escaping 
  from 
  

   that 
  region 
  and 
  in 
  obtaining 
  a 
  refuge 
  in 
  the 
  Ohio 
  and 
  Mississippi 
  

   Valleys. 
  If 
  Margaritana 
  and 
  its 
  pre-Glacial 
  associated 
  species, 
  such 
  

   as 
  U. 
  complajiatus 
  , 
  A. 
  marginata, 
  and 
  A. 
  cataraeta, 
  ever 
  succeeded 
  in 
  

   effecting 
  such 
  an 
  escape, 
  it 
  is 
  certainly 
  very 
  remarkable 
  that 
  not 
  

   a 
  single 
  one 
  of 
  them 
  has 
  left 
  any 
  survivors 
  in 
  the 
  Ohio 
  and 
  Mississippi 
  

   Valle3's 
  to 
  bear 
  witness 
  of 
  their 
  ancient 
  place 
  of 
  refuge. 
  And 
  not 
  

   only 
  that, 
  if 
  there 
  were 
  any 
  survivors 
  left 
  south 
  of 
  the 
  ice-cap, 
  why 
  

   did 
  they 
  not 
  accompany 
  the 
  then 
  existing 
  species 
  of 
  those 
  rivers 
  in 
  

   their 
  invasion 
  of 
  the 
  glaciated 
  area 
  upon 
  the 
  retreat 
  of 
  the 
  ice? 
  The 
  

   present 
  fauna 
  of 
  the 
  upper 
  Great 
  Lakes 
  and 
  central 
  British 
  America 
  

   is 
  clearly 
  the 
  result 
  of 
  two 
  distinct 
  immigrations, 
  one 
  from 
  the 
  south 
  

   of 
  the 
  dominant 
  species 
  of 
  the 
  Ohio 
  and 
  Mississippi 
  faunas, 
  and 
  the 
  

   other 
  from 
  the 
  east 
  of 
  the 
  more 
  vigorous 
  species 
  of 
  the 
  Atlantic 
  fauna 
  

   (see 
  Walker, 
  1898). 
  It 
  would 
  seem 
  to 
  be 
  certain, 
  therefore, 
  that 
  

   tlie 
  pre-Glacial 
  Unione 
  fauna 
  of 
  the 
  glaciated 
  area 
  was 
  wholly 
  

   exterminated. 
  

  

  The 
  occurrence 
  of 
  Margaritana 
  monodonta^ 
  Say, 
  west 
  of 
  the 
  Appa- 
  

   lachian 
  Mountains, 
  occupying 
  a 
  territory 
  in 
  which 
  M. 
  margaritifera 
  

   is 
  wholly 
  unknown, 
  is 
  of 
  great 
  interest. 
  It 
  is 
  quite 
  similar 
  apparently 
  

   in 
  general 
  appearance 
  to 
  the 
  Siamese 
  M. 
  Laosensis, 
  and 
  the 
  relation 
  

   of 
  both 
  species 
  of 
  margaritifera 
  would 
  appear 
  to 
  be 
  very 
  much 
  the 
  

   same. 
  It 
  is 
  probable 
  that 
  both 
  are 
  ancient 
  offshoots 
  from 
  the 
  more 
  

   ancient 
  margaritifera 
  stock. 
  

  

  The 
  distribution 
  of 
  31. 
  mo7iodonta 
  is 
  comparatively 
  a 
  limited 
  one. 
  

   In 
  the 
  Ohio 
  its 
  most 
  eastern 
  record 
  is 
  at 
  Cincinnati 
  (Sterki, 
  1907, 
  

  

  