﻿AND 
  OTHER 
  FAMILIES. 
  27 
  

  

  also 
  differ 
  so 
  completely 
  as 
  those 
  of 
  the 
  rivers 
  on 
  the 
  two 
  sides 
  of 
  the 
  

   great 
  ridge 
  ? 
  In 
  these 
  the 
  distinction 
  does 
  not 
  exist, 
  for 
  we 
  find 
  almost 
  

   every 
  species 
  which 
  is 
  common 
  on 
  the 
  eastern, 
  equally 
  common 
  on 
  

   the 
  western 
  side. 
  There 
  are, 
  however, 
  some 
  species 
  which 
  are 
  not 
  

   uncommon 
  on 
  the 
  western 
  side, 
  hut 
  which 
  do 
  not 
  exist, 
  so 
  far 
  as 
  my 
  

   information 
  extends, 
  on 
  this 
  side. 
  If 
  it 
  be 
  demanded 
  why 
  the 
  line 
  of 
  

   demarcation 
  should 
  not 
  be 
  as 
  perfect 
  for 
  terrestrial 
  as 
  fluviatile 
  shells, 
  

   we 
  might 
  say 
  in 
  answer, 
  that 
  the 
  barrier 
  of 
  a 
  mountain 
  could 
  in 
  time 
  

   be 
  overcome 
  even 
  by 
  the 
  slowly 
  travelling 
  snail. 
  Surely 
  in 
  the 
  lapse 
  

   of 
  time 
  the 
  progeny 
  of 
  those 
  which 
  accidentally 
  began 
  to 
  climb 
  the 
  

   steeps, 
  might 
  descend 
  into 
  the 
  valleys 
  of 
  the 
  opposite 
  side. 
  

  

  In 
  finishing 
  these 
  introductory 
  remarks, 
  I 
  wish 
  to 
  call 
  the 
  attention 
  

   of 
  those 
  naturalists 
  who 
  are 
  conveniently 
  located, 
  to 
  make 
  further 
  ob- 
  

   servations 
  on 
  this 
  branch 
  of 
  the 
  science, 
  which 
  certainly 
  has 
  great 
  in- 
  

   terest. 
  

  

  In 
  describing 
  the 
  Valvata 
  arenifera 
  in 
  my 
  last 
  memoir, 
  Vol. 
  IV. 
  

   page 
  104, 
  1 
  was 
  impressed 
  with 
  the 
  idea, 
  from 
  the 
  circumstance 
  of 
  

   finding 
  a 
  true 
  operculum 
  combined 
  with 
  a 
  spiral 
  tube, 
  that 
  the 
  animal 
  

   must 
  have 
  belonged 
  to 
  the 
  family 
  Peristomiana. 
  I 
  have 
  reason, 
  how- 
  

   ever, 
  since, 
  to 
  doubt 
  the 
  truth 
  of 
  my 
  conclusions. 
  Professor 
  Troost, 
  

   now 
  at 
  Nashville, 
  Tennessee, 
  originally 
  sent 
  the 
  specimens 
  from 
  that 
  

   neighbourhood 
  ; 
  and 
  from 
  his 
  description 
  of 
  the 
  animal, 
  which 
  he 
  has 
  

   recently 
  communicated 
  to 
  me, 
  I 
  am 
  induced 
  to 
  believe 
  it 
  to 
  be 
  a 
  spe- 
  

   cies 
  of 
  Linnean 
  Fhrygania. 
  

  

  