﻿HKNDKESON 
  : 
  ON 
  THE 
  GENUS 
  OEEOEELIX. 
  23 
  

  

  geological 
  history 
  of 
  the 
  genus. 
  Altliough 
  land 
  and 
  freshwater 
  

   mollusks 
  must 
  have 
  been 
  abundant 
  during 
  Eocene, 
  Oligocene, 
  and 
  

   Miocene 
  times, 
  and 
  at 
  least 
  as 
  abundant 
  during 
  the 
  Pliocene 
  and 
  

   Pleistocene 
  as 
  to-day, 
  it 
  seems 
  that 
  conditions 
  could 
  not 
  have 
  been 
  

   favourable 
  for 
  the 
  preservation 
  of 
  their 
  shells 
  in 
  the 
  rocks 
  then 
  

   forraintr. 
  In 
  the 
  immense 
  areas 
  covered 
  by 
  Miocene 
  rocks 
  in 
  tlie 
  

   Oreohelix 
  country, 
  it 
  is 
  very 
  seldom 
  that 
  molluscan 
  shells 
  are 
  found 
  

   fossil. 
  However, 
  three 
  species, 
  0. 
  megarche, 
  Ckll. 
  & 
  Hend., 
  

   0. 
  grangeri, 
  Ckll. 
  & 
  Hend., 
  and 
  0. 
  nacimientensis 
  (White), 
  Ckll., 
  

   have 
  been 
  described 
  from 
  the 
  lower 
  Eocene 
  of 
  Wyoming 
  and 
  New 
  

   Mexico, 
  indicating 
  probable 
  pre-Cainozoic 
  origin 
  of 
  the 
  genus. 
  

  

  It 
  is 
  not 
  likely 
  that 
  more 
  than 
  half 
  tlie 
  i-ecognizable 
  living 
  forms 
  

   have 
  yet 
  been 
  found 
  and 
  described, 
  and 
  little 
  is 
  known 
  of 
  the 
  

   distribution 
  of 
  many 
  others. 
  Many 
  of 
  the 
  species 
  are 
  enormously 
  

   vai'iable 
  in 
  characters 
  usually 
  used 
  in 
  differentiating 
  species 
  and 
  sub- 
  

   species, 
  such 
  as 
  sculpture, 
  size, 
  relative 
  width 
  of 
  umbilicus, 
  relative 
  

   lieight 
  of 
  spire, 
  colour, 
  and 
  so 
  on. 
  This 
  makes 
  it 
  necessary 
  to 
  obtain 
  

   large 
  quantities 
  of 
  material 
  from 
  numerous 
  localities 
  in 
  order 
  really 
  

   to 
  understand 
  the 
  various 
  forms 
  and 
  their 
  relationships. 
  

  

  Colonies 
  of 
  Oreohelix 
  often 
  cover 
  only 
  a 
  few 
  square 
  yards 
  or 
  rods 
  

   of 
  ground, 
  and 
  are 
  almost 
  as 
  completely 
  isolated 
  from 
  colonies 
  a 
  mile 
  

   distant 
  as 
  though 
  they 
  were 
  situated 
  on 
  different 
  continents. 
  Some 
  

   well-marked 
  species 
  and 
  subspecies 
  are 
  known 
  only 
  from 
  one 
  small 
  

   colony. 
  Furthermore, 
  in 
  case 
  of 
  species 
  that 
  occupy 
  more 
  than 
  one 
  

   colony, 
  the 
  material 
  from 
  each 
  colony, 
  viewed 
  in 
  mass, 
  may 
  differ 
  

   in 
  certain 
  average 
  characters 
  from 
  that 
  obtained 
  at 
  the 
  nearest 
  

   neighbouring 
  colony, 
  though 
  plenty 
  of 
  individuals 
  may 
  be 
  found 
  in 
  

   each 
  colony 
  to 
  form 
  a 
  complete 
  gradation. 
  Hence 
  it 
  is 
  very 
  

   important 
  to 
  give 
  the 
  exact 
  location 
  of 
  each 
  station. 
  It 
  is 
  often 
  

   desirable 
  to 
  revisit 
  stations 
  for 
  further 
  studies 
  as 
  the 
  investigation 
  

   progresses. 
  By 
  careful 
  attention 
  to 
  topographic 
  notes 
  and 
  other 
  data 
  

   many 
  of 
  the 
  type 
  localities 
  have 
  been 
  re-located, 
  with 
  some 
  very 
  

   interesting 
  results. 
  

  

  The 
  colour 
  bands, 
  normally 
  two 
  in 
  number, 
  are 
  extremely 
  variable. 
  

   In 
  some 
  forms, 
  as 
  for 
  example 
  0. 
  hendersoni 
  dalatni, 
  Hend., 
  the 
  bands 
  

   are 
  confined 
  to 
  the 
  earlier 
  whorls 
  and 
  even 
  there 
  are 
  faint. 
  In 
  some 
  

   colonies 
  of 
  0. 
  strigosa 
  depreasa 
  (Ckll.) 
  there 
  are 
  two 
  very 
  strong, 
  dark 
  

   bands 
  extending 
  to 
  the 
  aperture 
  on 
  nearly 
  all 
  specimens, 
  while 
  in 
  

   other 
  colonies 
  the 
  bands 
  are 
  so 
  light 
  on 
  the 
  later 
  whorls 
  that 
  on 
  most 
  

   specimens 
  they 
  can 
  be 
  detected 
  only 
  with 
  difficulty, 
  and 
  in 
  most 
  

   colonies 
  the 
  prominence 
  and 
  width 
  of 
  the 
  bands 
  vary 
  greatly. 
  

   »In 
  0. 
  cooperi 
  (W. 
  Gr. 
  B.) 
  there 
  are 
  often 
  a 
  number 
  of 
  extra 
  bands, 
  

   especially 
  below 
  the 
  periphery, 
  and 
  it 
  is 
  much 
  more 
  often 
  the 
  case 
  

   witli 
  the 
  immature 
  shells. 
  0. 
  peripherica 
  (Ancey) 
  varies 
  from 
  almost 
  

   pure-white 
  specimens 
  with 
  faint 
  bands 
  showing 
  only 
  on 
  the 
  early 
  

   whorls 
  {Patula 
  strigosa 
  binnei/i, 
  Hemph.), 
  through 
  specimens 
  with 
  

   two 
  narrow 
  bands 
  extending 
  to 
  the 
  aperture 
  (var. 
  gouldi, 
  Hemph.), 
  

   to 
  specimens 
  with 
  two 
  broad 
  bands 
  covering 
  the 
  whole 
  shell 
  except 
  

   for 
  a 
  broad, 
  white 
  peripheral 
  band 
  (var. 
  albofasciata, 
  Hemph.), 
  and 
  

   these 
  may 
  all 
  be 
  found 
  under 
  one 
  bush 
  completely 
  intergrading. 
  

  

  