﻿22 
  PllOCEEDINGS 
  OF 
  THE 
  MALA.COLOGICAL 
  SOCIliTi'. 
  

  

  the 
  list 
  will 
  be 
  greatly 
  augmented 
  by 
  future 
  work. 
  In 
  the 
  larger, 
  

   more 
  continuous 
  ranges 
  of 
  the 
  Rocky 
  Mountain 
  system, 
  where 
  

   favourable 
  localities 
  are 
  not 
  so 
  strictly 
  isolated 
  by 
  intervening 
  

   expanses 
  of 
  wholly 
  unfavourable 
  territory, 
  but 
  few 
  species 
  and 
  

   subspecies 
  are 
  found. 
  Thus 
  only 
  three 
  forms 
  [0. 
  cooper 
  i, 
  0. 
  strigosa 
  

   depressa, 
  and 
  0. 
  hendersoni) 
  liave 
  been 
  recognized 
  along 
  the 
  eastern 
  

   slope 
  and 
  foot-hills 
  of 
  the 
  Rockies 
  from 
  Northern 
  New 
  Mexico 
  to 
  

   Montana 
  and 
  South 
  Dakota, 
  a 
  distance 
  of 
  more 
  than 
  800 
  miles, 
  and 
  

   one 
  of 
  those 
  forms 
  is 
  known 
  from 
  only 
  one 
  small 
  colony. 
  On 
  the 
  

   other 
  hand, 
  Western 
  Utah, 
  Southern 
  Idaho, 
  Southern 
  New 
  Mexico, 
  

   Arizona, 
  and 
  Nevada, 
  comprise 
  an 
  immense 
  arid 
  region, 
  occupied 
  by 
  

   small 
  mountain 
  ranges 
  separated 
  by 
  dry, 
  treeless, 
  and 
  shrubless 
  

   plains 
  and 
  valleys. 
  Even 
  different 
  slopes 
  and 
  gulches 
  within 
  any 
  

   given 
  range 
  are 
  well 
  insulated, 
  from 
  the 
  standpoint 
  of 
  Oreohelix. 
  

   Many 
  of 
  the 
  mountains 
  are 
  composed 
  of 
  limestone, 
  thus 
  favouring 
  

   the 
  snails, 
  while 
  shrubbery 
  affords 
  shelter 
  from 
  the 
  hot 
  sun's 
  rays. 
  

   This 
  arid 
  condition, 
  with 
  many 
  mountain 
  ranges 
  providing 
  numerous 
  

   localities 
  favourable 
  to 
  snails, 
  separated 
  by 
  wide 
  expanses 
  of 
  

   unfavourable 
  plains, 
  has 
  prevailed 
  from 
  at 
  least 
  far 
  back 
  in 
  Pliocene 
  

   times, 
  though 
  probably 
  there 
  was 
  some 
  slight 
  fluctuation 
  in 
  climatic 
  

   conditions 
  there, 
  as 
  elsewhere, 
  during 
  Pleistocene 
  times, 
  which 
  may 
  

   have 
  allowed 
  temporary 
  communication 
  between 
  some 
  colonies 
  now 
  

   Avell 
  isolated, 
  thus 
  making 
  tlie 
  problems 
  more 
  complex. 
  Hence 
  one 
  

   is 
  not 
  surprised 
  to 
  find 
  in 
  that 
  region 
  a 
  large 
  number 
  of 
  distinct 
  

   forms, 
  each, 
  with 
  a 
  few 
  exceptions, 
  occupying 
  a 
  very 
  small 
  area, 
  

   though 
  two 
  forms 
  [cooperi 
  and 
  depressa) 
  are 
  found 
  here 
  and 
  there 
  

   almost 
  throughout 
  the 
  range 
  of 
  the 
  genus. 
  

  

  A 
  favourite 
  habitat 
  is 
  loose 
  talus, 
  locally 
  called 
  "rock 
  slide", 
  at 
  

   the 
  base 
  of 
  a 
  limestone 
  ledge 
  or 
  cliff, 
  though 
  good 
  colonies 
  are 
  also 
  

   sometimes 
  found 
  in 
  talus 
  composed 
  of 
  sandstone, 
  quartzite, 
  granite, 
  

   or 
  gneiss. 
  The 
  snails 
  occur 
  near 
  the 
  base 
  and 
  lateral 
  edges 
  of 
  the 
  

   talus, 
  where 
  shade 
  from 
  overhanging 
  shrubs 
  and 
  dwarf 
  trees 
  minimizes 
  

   evaporation. 
  During 
  droughts 
  they 
  retire 
  to 
  the 
  crevices 
  between 
  

   the 
  rocks 
  and' 
  cling 
  to 
  the 
  sheltered 
  surfaces, 
  ready 
  to 
  emerge 
  

   promptly 
  after 
  each 
  rainfall. 
  Some 
  forms 
  are 
  also 
  abundant 
  under 
  

   brush 
  on 
  calcareous 
  slopes 
  where 
  there 
  is 
  no 
  loose 
  talus, 
  or, 
  rarely, 
  

   in 
  flat 
  stream 
  valleys 
  far 
  from 
  rocks. 
  Occasionally, 
  flourishing 
  

   colonies 
  are 
  found 
  where 
  there 
  is 
  no 
  loose 
  talus 
  and 
  very 
  scant 
  

   vegetative 
  cover. 
  In 
  September, 
  1917, 
  I 
  discovered 
  one 
  form 
  in 
  ' 
  

   abundance 
  during 
  very 
  dry 
  weather, 
  in 
  Wyoming, 
  clinging 
  to 
  the 
  

   barren 
  face 
  of 
  a 
  limestone 
  ledge 
  where 
  there 
  was 
  no 
  vegetative 
  cover 
  

   whatever 
  for 
  snails, 
  either 
  on 
  or 
  near 
  the 
  ledge. 
  Since 
  they 
  are 
  

   vegetarians, 
  the 
  only 
  source 
  of 
  food 
  appeared 
  to 
  be 
  the 
  lichens 
  

   closely 
  clinging 
  to 
  the 
  rocks. 
  

  

  The 
  climatic 
  factor 
  looms 
  very 
  large 
  in 
  determining 
  the 
  distribution 
  

   of 
  plants 
  and 
  animals 
  in 
  the 
  South-Western 
  United 
  States. 
  Since 
  

   snails 
  are 
  more 
  nearly 
  immobile 
  than 
  most 
  land 
  animals, 
  a 
  thorough 
  

   knowledge 
  of 
  the 
  geological 
  history 
  and 
  present 
  distribution 
  of 
  

   Oreohelices 
  may 
  yet 
  furnish 
  the 
  key 
  to 
  the 
  Tertiary 
  and 
  Pleistocene 
  

   climatic 
  history 
  of 
  the 
  region. 
  But 
  little 
  can 
  now 
  be 
  said 
  of 
  the 
  

  

  