﻿■walker: 
  on 
  mabgaritana 
  masgabitifera. 
  131 
  

  

  separated 
  and 
  liaye 
  developed 
  in 
  different, 
  though 
  in 
  many 
  cases 
  along- 
  

   analogous, 
  lines. 
  

  

  There 
  is 
  reason 
  to 
  believe 
  that 
  the 
  pre-Glacial 
  northern 
  migration 
  

   consisted 
  of 
  comparatively 
  few 
  specific 
  types, 
  and 
  that 
  the 
  peculiar 
  

   genera 
  characteristic 
  of 
  the 
  Mississippi 
  fauna, 
  many 
  of 
  which 
  have 
  the 
  

   head 
  -waters 
  of 
  the 
  Tennessee 
  system 
  as 
  their 
  centre 
  of 
  distribution, 
  

   originated 
  from 
  another 
  migration 
  along 
  an 
  entirely 
  different 
  route. 
  

   But 
  be 
  that 
  as 
  it 
  may, 
  it 
  is 
  significant 
  that 
  nearly 
  every 
  species 
  of 
  the 
  

   comparatively 
  small 
  northern 
  Atlantic 
  fauna 
  has 
  its 
  analogous 
  species 
  

   of 
  general 
  distribution 
  in 
  the 
  present 
  fauna 
  of 
  the 
  northern 
  states 
  west 
  

   of 
  the 
  Appalachian 
  mountains, 
  viz. 
  — 
  

  

  Atlantic. 
  Mississippi. 
  

  

  Lampsilis 
  radiata. 
  Lampsilis 
  luteola. 
  

  

  L. 
  cariosa. 
  L. 
  ventricosa. 
  

  

  L. 
  nasuta. 
  L. 
  subrostrata. 
  

  

  Unio 
  complanatus. 
  Unio 
  gibhosus. 
  

  

  A.lasmodonta 
  heterodon. 
  A.lasmodoHta 
  calceola. 
  

  

  Strophitus 
  tmdulatns. 
  Strophitus 
  edentuhis. 
  

  

  It 
  may 
  perhaps 
  be 
  claimed 
  that 
  M. 
  margaritifera 
  and 
  M. 
  monodonta 
  

   should 
  be 
  included 
  in 
  the 
  above 
  list, 
  but 
  the 
  latter 
  is 
  not 
  a 
  species 
  

   of 
  general 
  distribution 
  in 
  the 
  north 
  central 
  states 
  as 
  the 
  others 
  are, 
  

   and 
  its 
  occurrence 
  in 
  the 
  Ohio 
  and 
  Tennessee 
  systems 
  can 
  apparently 
  

   be 
  better 
  explained 
  in 
  another 
  way. 
  Putting 
  that 
  species 
  aside 
  

   temporarily, 
  it 
  is 
  certainly 
  remarkable 
  that 
  if 
  M. 
  margaritifera 
  

   originated 
  in 
  central 
  North 
  America 
  in 
  pre-Glacial 
  times, 
  and 
  took 
  part 
  

   in 
  the 
  pre-Glacial 
  migration 
  above 
  indicated, 
  it 
  did 
  not 
  leave 
  any 
  traces 
  

   of 
  its 
  former 
  existence 
  in 
  the 
  Mississippi 
  Yalley. 
  As 
  already 
  remarked, 
  

   Avherever 
  it 
  did 
  originate 
  it 
  has 
  extended 
  nearly 
  all 
  round 
  the 
  world, 
  

   and 
  during 
  all 
  the 
  enormous 
  period 
  required 
  for 
  that 
  dispersal 
  and 
  

   under 
  all 
  the 
  varied 
  conditions 
  of 
  environment 
  to 
  which 
  it 
  has 
  been 
  

   subjected, 
  it 
  has 
  always 
  and 
  everywhere 
  maintained 
  its 
  specific 
  

   identity, 
  or 
  varied 
  within 
  comparatively 
  narrow 
  limits. 
  In 
  view 
  of 
  

   these 
  unquestionable 
  facts, 
  it 
  would 
  seem 
  improbable 
  that 
  it 
  could 
  

   have 
  been 
  a 
  member 
  of 
  the 
  pre-Glacial 
  Mississippi 
  fauna, 
  and 
  yet 
  

   nowhere 
  in 
  the 
  Mississippi 
  Yalley 
  have 
  left 
  any 
  remnant 
  to 
  preserve 
  

   a 
  record 
  of 
  its 
  former 
  occurrence 
  in 
  that 
  region. 
  The 
  theory 
  of 
  the 
  

   trans-Mississippian 
  origin 
  and 
  subsequent 
  extermination 
  by 
  the 
  ice-cap 
  

   involves 
  the 
  necessity 
  of 
  its 
  complete 
  emigration 
  from 
  the 
  entire 
  

   Mississippi 
  region 
  south 
  of 
  the 
  glaciated 
  area, 
  before 
  the 
  beginning 
  

   of 
  the 
  Glacial 
  Period, 
  a 
  most 
  violent 
  assumption 
  in 
  view 
  of 
  the 
  fact 
  

   that 
  it 
  is 
  not 
  true 
  of 
  any 
  other 
  species 
  belonging 
  to 
  the 
  pre-Glacial 
  

   fauna. 
  

  

  Taking 
  into 
  consideration, 
  therefore, 
  the 
  apparent 
  impossibility 
  

   that 
  there 
  could 
  have 
  been 
  any 
  extension 
  of 
  the 
  species 
  to 
  the 
  west 
  or 
  

   north-west, 
  and 
  the 
  improbability 
  that 
  a 
  pre-Glacial 
  migration 
  from 
  

   the 
  trans-Mississippi 
  states 
  to 
  the 
  north-east 
  could 
  have 
  taken 
  place 
  

   without 
  leaving 
  some 
  trace 
  somewhere 
  in 
  the 
  vast 
  extent 
  of 
  territory 
  

   traversed, 
  as 
  apparently 
  all 
  the 
  other 
  specific 
  types 
  involved 
  in 
  that 
  

   migration 
  have 
  done, 
  and 
  realizing 
  that 
  such 
  a 
  hypothesis 
  involves 
  

   a 
  migration 
  eastward 
  around 
  nearly 
  the 
  entire 
  globe 
  from 
  central 
  

  

  