﻿Vol. 
  53.] 
  OF 
  THE 
  CHALK 
  EOCK. 
  401 
  

  

  3. 
  Turbo, 
  Chlami/s, 
  Spondylus, 
  and 
  Cardium 
  are 
  not 
  found 
  below 
  

  

  the 
  600-fathom 
  line. 
  

  

  Turbo 
  is 
  very 
  common 
  in 
  the 
  Reussianum-zone, 
  and 
  one 
  

   of 
  the 
  species 
  is 
  of 
  considerable 
  size. 
  At 
  the 
  present 
  day 
  

   the 
  genus 
  extends 
  from 
  the 
  Littoral 
  zone 
  down 
  to 
  565 
  

   fathoms, 
  but, 
  being 
  phytophagous, 
  it 
  is 
  of 
  course 
  most 
  

   abundant 
  in 
  shallow 
  water. 
  

  

  Clilamys 
  is 
  very 
  rare, 
  and 
  Cardium 
  not 
  common, 
  in 
  the 
  

   Reussianum-zone 
  ; 
  both 
  are 
  abundant 
  in 
  fairly 
  shallow 
  water. 
  

  

  Spondylus 
  is 
  rather 
  common, 
  particularly 
  Sp. 
  sj)inosus. 
  

   The 
  existing 
  species 
  live 
  in 
  warm 
  seas, 
  mainly 
  in 
  the 
  Littoral 
  

   zone, 
  or 
  at 
  no 
  great 
  depth, 
  but 
  one 
  form, 
  which 
  is, 
  how- 
  

   ever, 
  quite 
  small, 
  has 
  been 
  found 
  at 
  a 
  depth 
  of 
  520 
  fathoms. 
  

  

  4. 
  Natica 
  is 
  found 
  down 
  to 
  about 
  1000 
  fathoms, 
  but 
  is 
  abundant 
  

  

  only 
  at 
  less 
  than 
  100 
  fathoms 
  ; 
  it 
  is 
  moderately 
  common 
  in 
  

   the 
  Reussianum-zone. 
  

  

  Modiola 
  extends 
  down 
  to 
  800 
  fathoms, 
  but 
  is 
  not 
  common 
  

   below 
  100 
  fathoms 
  ; 
  it 
  is 
  moderately 
  rare 
  in 
  the 
  Reussianum- 
  

   zone. 
  

  

  5. 
  Ten 
  genera 
  — 
  namely, 
  Trochus, 
  Aporrhais, 
  Cerithium, 
  Denta- 
  

  

  lium,Limo]psis, 
  Nucida, 
  Area, 
  Cusj)idaria,Wuculana 
  (=Leda), 
  

   and 
  Lima 
  — 
  have 
  species 
  which 
  live 
  at 
  1000 
  fathoms 
  or 
  

   greater 
  depths, 
  but 
  in 
  every 
  case 
  these 
  genera 
  have 
  a 
  great 
  

   bathymetric 
  range 
  ; 
  all 
  occur 
  in 
  shallow 
  water, 
  and 
  nearly 
  

   all 
  are 
  found 
  in 
  depths 
  of 
  only 
  a 
  few 
  (1 
  to 
  5) 
  fathoms. 
  

  

  The 
  living 
  forms 
  of 
  Troclius 
  l 
  are 
  abundant 
  in 
  the 
  Littoral 
  

   and 
  Laminarian 
  zones, 
  but 
  the 
  genus 
  is 
  not 
  uncommon 
  in 
  

   fairly 
  deep 
  water 
  : 
  indeed, 
  Agassiz 
  2 
  speaks 
  of 
  the 
  Trochidse 
  

   as 
  being 
  well 
  represented 
  in 
  deep 
  water. 
  Trochus 
  is, 
  I 
  

   believe, 
  the 
  commonest 
  gasteropod 
  in 
  the 
  Reussianum-zone, 
  

   nevertheless 
  it 
  is 
  represented 
  by 
  only 
  two 
  species 
  ; 
  in 
  water 
  

   of 
  less 
  than 
  100 
  fathoms 
  there 
  would 
  almost 
  certainly 
  have 
  

   been 
  a 
  larger 
  number 
  of 
  species. 
  

  

  Aporrliais 
  is, 
  also 
  very 
  common. 
  Existing 
  species 
  are 
  

   most 
  numerous 
  within 
  the 
  100-fathom 
  line, 
  and 
  are 
  not 
  

   common 
  beyond 
  it. 
  

  

  Cerithium 
  : 
  there 
  are 
  two 
  species 
  in 
  the 
  Chalk 
  Rock, 
  one 
  

   being 
  very 
  common. 
  The 
  genus 
  is 
  abundant 
  in 
  the 
  Littoral 
  

   zone 
  or 
  at 
  no 
  great 
  depth, 
  and 
  rare 
  beyond 
  100 
  fathoms. 
  

  

  Dentalium 
  : 
  the 
  Dentaliidae 
  are 
  characteristic 
  of 
  deep 
  

   water, 
  but 
  also 
  occur 
  in 
  shallow 
  regions. 
  The 
  genus 
  is 
  

   eomewhat 
  rare 
  in 
  the 
  Reussianum-zone. 
  

  

  Nucida 
  and 
  Area 
  are 
  common 
  in 
  the 
  Chalk 
  Rock 
  ; 
  they 
  

  

  1 
  Dr. 
  W. 
  F. 
  Hume,' 
  Nat. 
  Sci.' 
  vol. 
  vii. 
  (1895) 
  p. 
  391, 
  has 
  stated 
  that 
  the 
  ' 
  pre- 
  

   sence 
  of 
  Trochus, 
  Turbo, 
  and 
  Solarium 
  in 
  the 
  Chalk 
  Rock 
  is 
  paralleled 
  at 
  the 
  

   present 
  day 
  by 
  the 
  same 
  association 
  found 
  at 
  Culebra 
  Island 
  at 
  a 
  depth 
  of 
  

   390 
  fathoms.' 
  Solarium 
  does 
  not 
  occur 
  in 
  the 
  Chalk 
  Rock, 
  and 
  the 
  association 
  

   of 
  Trochus 
  and 
  Turbo 
  proves 
  nothing, 
  since 
  they 
  both 
  occur 
  in 
  the 
  Littoral 
  

   zone 
  as 
  well 
  as 
  at 
  various 
  greater 
  depths. 
  

  

  2 
  'Three 
  Cruises 
  of 
  the 
  Blake,' 
  vol. 
  ii. 
  (1888) 
  p. 
  67. 
  

   Q.J.G.S. 
  No. 
  211. 
  2e 
  

  

  