﻿NEWTON 
  : 
  TERIIARY 
  SHELT.S 
  OF 
  THE 
  ISLAND 
  OF 
  COS. 
  365 
  

  

  " 
  In 
  the 
  freshwater 
  strata, 
  agamst 
  which 
  the 
  marine 
  newer 
  Phocene 
  beds 
  

   abut 
  uneonformably, 
  there 
  are 
  also 
  numerous 
  and 
  weU-preserved 
  fossils 
  : 
  shells 
  

   of 
  Pabidina, 
  Neritina, 
  Melanopsis, 
  Valvata, 
  Unio, 
  Cyclas, 
  and 
  Planorhis 
  

   occur, 
  marking 
  the 
  nature 
  of 
  the 
  deposits. 
  With 
  them 
  teeth 
  of 
  a 
  Cyimnus 
  

   were 
  found, 
  also 
  leaves 
  and 
  stems 
  of 
  plants, 
  and 
  in 
  the 
  uppermost 
  stratum 
  

   shells 
  of 
  the 
  common 
  cockle. 
  

  

  " 
  The 
  great 
  interest 
  of 
  this 
  formation 
  depends 
  on 
  certain 
  appearances 
  

   presented 
  by 
  the 
  moUusca 
  of 
  the 
  genera 
  Paludina, 
  Melanopsis, 
  and 
  Neritina 
  

   found 
  in 
  great 
  quantities 
  in 
  several 
  parts 
  of 
  it. 
  They 
  occur 
  distributed 
  in 
  

   distinct 
  horizons 
  throughout 
  the 
  vertical 
  thickness 
  of 
  the 
  section. 
  These 
  

   horizons 
  form 
  three 
  series, 
  each 
  of 
  which 
  is 
  characterized 
  by 
  a 
  peculiar 
  form 
  of 
  

   Paludina. 
  and 
  of 
  Neritina 
  not 
  present 
  in 
  the 
  other 
  two, 
  and 
  in 
  the 
  two 
  lower 
  

   horizons 
  there 
  are 
  two 
  species 
  of 
  Melanopsis 
  peculiar 
  to 
  each. 
  

  

  " 
  So 
  very 
  different 
  are 
  the 
  several 
  fossils 
  of 
  one 
  zone 
  from 
  those 
  of 
  another 
  

   that 
  at 
  first 
  examination 
  we 
  appear 
  to 
  have 
  before 
  us 
  very 
  distinct 
  and 
  well- 
  

   marked 
  species, 
  and 
  that 
  each 
  series 
  of 
  horizons 
  was 
  characterized 
  hj 
  PaludincB, 
  

   NeritincB, 
  and 
  Afelanojjsides, 
  peculiar 
  to 
  itself, 
  and 
  representative 
  of 
  those 
  

   inhabiting 
  the 
  other 
  divisions. 
  This 
  would 
  be 
  a 
  very 
  startling 
  phenomenon 
  to 
  

   occur 
  within 
  so 
  limited 
  an 
  area. 
  If 
  the 
  successive 
  species 
  be 
  considered 
  distinct, 
  

   we 
  must 
  regard 
  them 
  as 
  mutually 
  representative, 
  and 
  hold 
  that 
  a 
  succession 
  of 
  

   creations 
  and 
  extinctions 
  took 
  place, 
  in 
  this 
  probably 
  limited 
  basin, 
  during 
  

   a 
  brief 
  geological 
  period, 
  or 
  else 
  that 
  a 
  transmutation 
  of 
  species 
  took 
  place. 
  

  

  " 
  A 
  careful 
  study 
  of 
  the 
  forms 
  in 
  question, 
  and 
  an 
  enquiry 
  into 
  the 
  modes 
  

   and 
  capacity 
  of 
  variation 
  of 
  species 
  in 
  the 
  genera 
  to 
  which 
  they 
  belong, 
  and 
  

   among 
  their 
  allies, 
  have 
  convinced 
  us, 
  however, 
  that 
  these 
  curious 
  changes 
  of 
  

   form 
  may 
  be 
  accounted 
  for 
  otherwise, 
  and 
  that 
  in 
  the 
  successive 
  Paludince, 
  

   NeritincB, 
  etc., 
  we 
  have 
  before 
  us 
  only 
  the 
  same 
  species 
  assuming 
  protean 
  

   variations. 
  By 
  reference 
  to 
  the 
  figures 
  it 
  will 
  be 
  seen 
  that 
  the 
  Paludince 
  and 
  

   NeritincB, 
  of 
  the 
  first 
  or 
  lowest 
  zone, 
  have 
  smooth 
  and 
  unwrinkled 
  shells 
  ; 
  that 
  

   those 
  of 
  the 
  second 
  have 
  their 
  shells 
  belted 
  by 
  a 
  strong 
  fold 
  or 
  corrugation, 
  

   whilst 
  those 
  of 
  the 
  uppermost 
  zone 
  are 
  deeply 
  sulcated 
  and 
  surrounded 
  by 
  strong 
  

   spiral 
  ridges. 
  

  

  " 
  Such 
  changes 
  of 
  form 
  take 
  place 
  sucaong 
  LittorincB 
  and 
  NeritincB, 
  even 
  now, 
  

   in 
  places 
  where 
  alternations 
  of 
  fresh 
  and 
  salt 
  water 
  affect 
  the 
  mollusca, 
  and 
  in 
  

   brackish- 
  water 
  localities. 
  There 
  are 
  phenomena 
  in 
  the 
  Cos 
  beds 
  which 
  warrant 
  

   us 
  in 
  referring 
  the 
  remarkable 
  peculiarities 
  of 
  the 
  fossils 
  therein 
  found 
  to 
  

   a 
  similar 
  cause. 
  That 
  an 
  influx 
  of 
  salt 
  water 
  changed 
  the 
  character 
  of 
  the 
  

   basin 
  in 
  which 
  they 
  lived 
  towards 
  the 
  close 
  of 
  its 
  existence 
  is 
  evident 
  from 
  the 
  

   presence 
  of 
  the 
  Cardium 
  edule 
  in 
  its 
  ujDpermost 
  part. 
  That 
  some 
  such 
  cause 
  

   had 
  previously 
  been 
  in 
  action 
  is 
  probable 
  from 
  the 
  fact 
  that 
  the 
  pulmoniferous 
  

   testacea 
  found 
  in 
  this 
  formation 
  are 
  confined 
  to 
  the 
  lowest 
  series 
  of 
  horizons. 
  

   One 
  of 
  the 
  authors 
  has 
  elsewhere 
  shown 
  that 
  no 
  species 
  of 
  moUusk 
  can 
  live 
  for 
  

   any 
  length 
  of 
  time 
  on 
  the 
  same 
  ground. 
  

  

  " 
  A 
  change 
  of 
  ground 
  is 
  necessary 
  for 
  its 
  prosperity; 
  otherwise 
  it 
  dies 
  off. 
  

   But, 
  as 
  the 
  fry 
  of 
  even 
  the 
  most 
  sedentary 
  testacea 
  are 
  active 
  creatures 
  of 
  

   a 
  different 
  form, 
  organised 
  for 
  swimming, 
  when 
  all 
  the 
  adult 
  animals 
  upon 
  

   a 
  ground 
  are 
  destroyed, 
  their 
  descendants 
  may 
  survive 
  their 
  destruction 
  and 
  

   replace 
  them, 
  providing 
  the 
  ground 
  be 
  sufficiently 
  changed 
  during 
  the 
  interval. 
  

  

  " 
  Now 
  these 
  two 
  facts, 
  first 
  of 
  the 
  nature 
  and 
  causes 
  of 
  the 
  variations 
  among 
  

   such 
  testacea 
  as 
  present 
  such 
  curious 
  changes 
  of 
  form 
  in 
  the 
  Cos 
  fresh-water 
  

   beds, 
  and 
  second, 
  of 
  the 
  necessity 
  of 
  a 
  change 
  of 
  ground 
  for 
  the 
  well-being 
  of 
  

   a 
  species, 
  and 
  the 
  manner 
  in 
  which, 
  owing 
  to 
  the 
  nature 
  of 
  the 
  larva, 
  such 
  

   change 
  may 
  be 
  effected 
  on 
  the 
  same 
  spot, 
  have 
  led 
  us 
  to 
  propose 
  the 
  following 
  

   solution 
  of 
  the 
  Cos 
  problem. 
  

  

  "The 
  lowest 
  series 
  of 
  horizons 
  were 
  deposited 
  in 
  the 
  basin 
  when 
  it 
  was 
  

   purely 
  a 
  fresh-water 
  one, 
  and 
  in 
  it 
  we 
  found 
  Paludince, 
  etc., 
  in 
  their 
  normal 
  

   condition, 
  associated 
  with 
  ordinary 
  fresh-water 
  mollusca. 
  These 
  latter 
  are 
  

   killed 
  off' 
  by 
  an 
  influx 
  of 
  salt 
  water, 
  sufficient 
  to 
  render 
  the 
  basin 
  slightly 
  

   brackish. 
  This 
  influx 
  takes 
  place 
  at 
  a 
  time 
  when 
  the 
  mollusks 
  of 
  the 
  uppermost 
  

  

  