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  PROCEEDINGS 
  OF 
  THE 
  CALIFORNIA 
  

  

  sometimes 
  convert 
  the 
  entire 
  interval 
  into 
  a 
  lake, 
  over 
  which 
  the 
  species 
  might 
  

   have 
  been 
  spread, 
  and 
  since 
  modified 
  in 
  its 
  new 
  localities. 
  The 
  chief 
  argument 
  

   to 
  the 
  contrary 
  may 
  be 
  in 
  the 
  fact 
  that 
  no 
  specimens 
  are 
  known 
  to 
  have 
  been 
  

   lately 
  transported 
  in 
  this 
  way. 
  The 
  most 
  limited 
  local 
  varieties 
  are 
  indicated 
  

   by 
  names 
  placed 
  horizontally, 
  and 
  they 
  usually 
  differ 
  more 
  than 
  others, 
  being 
  

   also 
  more 
  distant 
  geographically. 
  

  

  As 
  I 
  have 
  hinted 
  at 
  the 
  probability 
  of 
  J. 
  tudkulata 
  being 
  the 
  progenitor 
  of 
  

   the 
  coast 
  forms, 
  I 
  may 
  remark 
  that 
  the 
  fossil 
  form 
  found 
  at 
  the 
  foot 
  of 
  the 
  

   Sierra 
  Nevada, 
  by 
  Mr. 
  Gabb, 
  has 
  the 
  very 
  high 
  imperforate 
  character 
  of 
  speci- 
  

   mens 
  now 
  living 
  close 
  to 
  the 
  coast, 
  and 
  may 
  have 
  existed 
  in 
  the 
  Miocene 
  epoch 
  

   when 
  the 
  San 
  Joaquin 
  valley 
  was 
  an 
  arm 
  of 
  the 
  sea, 
  in 
  which 
  lived 
  the 
  sharks, 
  

   etc., 
  whose 
  remains 
  now 
  abound 
  not 
  far 
  from 
  the 
  Helix 
  locality. 
  It 
  would 
  then 
  

   have 
  lived 
  close 
  to 
  the 
  edge 
  of 
  this 
  sea, 
  and 
  before 
  the 
  greater 
  part 
  of 
  the 
  coast 
  

   range 
  was 
  elevated 
  above 
  it. 
  The 
  transplanting 
  of 
  the 
  Arionta 
  form 
  to 
  the 
  

   latter 
  may 
  have 
  been 
  after 
  the 
  salt 
  water 
  was 
  replaced 
  by 
  fresh, 
  or 
  in 
  the 
  

   Pliocene 
  epoch. 
  

  

  Omitting 
  for 
  the 
  present 
  a 
  critical 
  comparison 
  of 
  the 
  species 
  and 
  varieties 
  

   which 
  I 
  have 
  made 
  ready 
  for 
  future 
  publication, 
  I 
  intend 
  here 
  to 
  point 
  out 
  only 
  

   the 
  geographical 
  reasons 
  for 
  the 
  conclusions 
  to 
  which 
  I 
  have 
  arrived, 
  as 
  in- 
  

   dicated 
  in 
  the 
  diagram. 
  

  

  Around 
  San 
  Francisco 
  Bay 
  we 
  have 
  three 
  well-marked, 
  and 
  always 
  separ- 
  

   able, 
  species, 
  viz 
  : 
  orrosa, 
  exarata, 
  and 
  Californiensis, 
  all 
  living 
  together 
  on 
  the 
  

   west 
  side, 
  and 
  at 
  nearly 
  equal 
  distances 
  north 
  and 
  south. 
  Rare 
  hybrids 
  only 
  

   exist 
  as 
  connecting 
  links. 
  

  

  The 
  last 
  also 
  extends 
  to 
  the 
  east 
  side 
  of 
  the 
  bay, 
  where 
  it 
  produces 
  three 
  var- 
  

   ieties 
  which 
  mix 
  together, 
  and 
  also 
  with 
  the 
  typical 
  form 
  on 
  the 
  west 
  side. 
  

   Var. 
  nemorivaga 
  is 
  almost 
  equally 
  common 
  on 
  both 
  sides, 
  and 
  extends 
  further 
  

   south 
  than 
  any, 
  there 
  meeting 
  (but 
  not 
  mixing) 
  with 
  var. 
  vinda, 
  at 
  Monterey. 
  

   (I 
  omit 
  "Nickliniana," 
  as 
  it 
  seems 
  founded 
  upon 
  a 
  combination 
  of 
  the 
  charac- 
  

   ters 
  of 
  these 
  with 
  tudiculata.) 
  

  

  ■ 
  Now, 
  all 
  these 
  varieties 
  of 
  Californiensis 
  follow 
  certain 
  definite 
  rules 
  in 
  their 
  

   divergence 
  from 
  the 
  typical 
  form, 
  according 
  to 
  the 
  amount 
  of 
  heat, 
  moisture, 
  

   fog, 
  and 
  wind 
  they 
  are 
  subject 
  to. 
  A. 
  arrosa 
  and 
  exarata, 
  having 
  only 
  a 
  

   narrow 
  range 
  close 
  to 
  the 
  coast, 
  are 
  subjected 
  to 
  little 
  difference 
  in 
  these 
  con- 
  

   ditions, 
  and 
  accordingly 
  vary 
  but 
  little. 
  Californiensii 
  and 
  varieties, 
  on 
  the 
  

   contrary, 
  spreading 
  twice 
  as 
  far 
  inland, 
  gradually 
  change 
  from 
  a 
  high, 
  imperfor- 
  

   ate 
  form 
  to 
  depressed 
  and 
  umbilicate, 
  as 
  they 
  pass 
  into 
  drier 
  and 
  hotter 
  regions, 
  

   with 
  so 
  many 
  intermediate 
  forms 
  that 
  their 
  specific 
  identity 
  and 
  the 
  causes 
  of 
  

   variation 
  can 
  scarcely 
  be 
  doubted. 
  A 
  similar 
  variation, 
  but 
  in 
  less 
  degree, 
  

   occurs 
  in 
  the 
  two 
  others 
  mentioned. 
  

  

  The 
  next 
  centre 
  of 
  distribution 
  for 
  Arionta, 
  is 
  that 
  of 
  ttidiculata 
  and 
  varieties 
  

   on 
  the 
  west 
  slope 
  of 
  the 
  Sierra 
  Nevada, 
  one 
  hundred 
  miles 
  east 
  of 
  the 
  preceding, 
  

   and 
  separated 
  by 
  wide 
  alluvial 
  valleys, 
  in 
  which 
  no 
  specimens 
  of 
  wnj 
  species 
  

   have 
  ever 
  been 
  found. 
  

  

  This 
  species 
  presents 
  a 
  series 
  of 
  varieties 
  analogous 
  to 
  those 
  of 
  Californiensis, 
  

  

  