﻿400 
  MR. 
  H. 
  WOODS 
  OK 
  THE 
  UOLLITSCA 
  [Aug. 
  1 
  897, 
  

  

  deep-sea 
  animals 
  are 
  characterized 
  by 
  the 
  possession 
  of 
  thin 
  shells, 
  

   usually 
  of 
  rather 
  small 
  size. 
  It 
  might 
  indeed 
  be 
  urged 
  that 
  the 
  

   occurrence 
  of 
  thick 
  shells 
  in 
  the 
  Chalk 
  Rock 
  can 
  be 
  explained 
  by 
  

   the 
  existence 
  of 
  a 
  higher 
  temperature 
  than 
  is 
  found 
  in 
  the 
  deep 
  sea 
  

   at 
  the 
  present 
  time, 
  since 
  Murray 
  and 
  Irvine 
  ] 
  have 
  shown 
  that 
  the 
  

   secretion 
  of 
  carbonate 
  of 
  lime 
  by 
  organisms 
  is 
  directly 
  related 
  to 
  

   the 
  temperature 
  of 
  the 
  water 
  ; 
  but 
  I 
  know 
  of 
  no 
  evidence 
  in 
  favour 
  

   of 
  the 
  Cretaceous 
  temperature 
  having 
  been 
  high 
  enough 
  to 
  cause 
  

   any 
  great 
  change 
  at 
  abyssal 
  depths. 
  

  

  In 
  discussing 
  the 
  question 
  of 
  depth 
  from 
  the 
  evidence 
  supplied 
  

   by 
  the 
  genera 
  of 
  Mollusca 
  which 
  have 
  living 
  representatives 
  we 
  

   must 
  consider 
  three 
  main 
  points 
  : 
  — 
  (1) 
  the 
  genera 
  which 
  have 
  a 
  

   limited 
  bathymetrical 
  range 
  ; 
  (2) 
  the 
  depths 
  at 
  which 
  the 
  other 
  

   genera 
  are 
  most 
  numerous 
  ; 
  (3) 
  the 
  relative 
  abundance 
  of 
  the 
  genera 
  

   in 
  the 
  Reussianum-zone 
  itself. 
  

  

  So 
  far 
  as 
  their 
  range 
  in 
  depth 
  is 
  concerned, 
  we 
  may 
  conveniently 
  

   divide 
  the 
  genera 
  into 
  five 
  groups 
  : 
  — 
  

  

  1. 
  The 
  first 
  includes 
  Crepidula, 
  Emarginula, 
  Martesia, 
  Ostrea, 
  

  

  Plicatula, 
  and 
  Trapezium, 
  all 
  of 
  which 
  are 
  shallow-water 
  

   forms 
  and, 
  so 
  far 
  as 
  I 
  know, 
  do 
  not 
  live 
  at 
  a 
  greater 
  depth 
  

   than 
  50 
  fathoms. 
  These 
  genera 
  are 
  all 
  rare, 
  and 
  some 
  of 
  

   them 
  very 
  rare, 
  in 
  the 
  Reussianum-zone, 
  and 
  we 
  cannot 
  

   therefore 
  take 
  them 
  as 
  a 
  proof 
  that 
  the 
  deposit 
  was 
  formed 
  

   in 
  such 
  shallow 
  water. 
  2 
  

  

  2. 
  Pleurotomaria, 
  Cardita, 
  and 
  Arctica 
  do 
  not 
  live 
  below 
  a 
  depth 
  

  

  of 
  200 
  fathoms. 
  

  

  Pleurotomaria 
  is 
  one 
  of 
  the 
  commonest 
  gasteropods 
  in 
  the 
  

   Reussianum-zone, 
  and 
  its 
  evidence 
  should 
  therefore 
  be 
  of 
  

   considerable 
  importance. 
  Only 
  four 
  living 
  species 
  are 
  

   known 
  ; 
  they 
  were 
  dredged 
  at 
  various 
  depths 
  between 
  73 
  and 
  

   200 
  fathoms. 
  Two 
  species 
  — 
  P. 
  Adansonia 
  and 
  P. 
  Beyricld, 
  

   both 
  of 
  large 
  size 
  — 
  have 
  been 
  found 
  at 
  the 
  greater 
  depth. 
  

   It 
  is, 
  of 
  course, 
  possible 
  that 
  the 
  range 
  of 
  the 
  genus 
  may 
  have 
  

   been 
  more 
  extended 
  in 
  Mesozoic 
  times 
  when 
  the 
  number 
  of 
  

   species 
  reached 
  its 
  maximum 
  ; 
  we 
  know, 
  at 
  any 
  rate, 
  that 
  

   it 
  is 
  very 
  common 
  in 
  several 
  Jurassic 
  formations 
  which 
  are 
  

   undoubtedly 
  of 
  shallow-water 
  origin. 
  

  

  Cardita 
  and 
  Arctica 
  are 
  not 
  common 
  in 
  the 
  Chalk 
  

   Rock 
  ; 
  the 
  former 
  extends 
  from 
  the 
  Littoral 
  zone 
  to 
  

   150 
  fathoms, 
  and 
  is 
  found 
  mainly 
  in 
  shallow 
  water 
  ; 
  the 
  

   latter 
  ranges 
  from 
  the 
  Laminarian 
  zone 
  down 
  to 
  about 
  

   180 
  fathoms. 
  

  

  1 
  Proc. 
  Roy. 
  Soc. 
  Edin. 
  vol. 
  xvii. 
  (1891) 
  p. 
  79 
  : 
  J. 
  Murray, 
  ' 
  Summary 
  of 
  

   Scientific 
  Results 
  (Challenger 
  Exped.),' 
  pt. 
  ii. 
  (1895) 
  p. 
  1456. 
  

  

  2 
  With 
  regard 
  to 
  genera 
  which 
  appear 
  to 
  be 
  limited 
  to 
  shallow 
  water, 
  we 
  

   shall 
  do 
  well 
  to 
  bear 
  in 
  mind 
  the 
  warning 
  given 
  bv 
  Starkie 
  Gardner: 
  — 
  'When 
  

   we 
  reflect 
  that, 
  for 
  one 
  cast 
  of 
  the 
  dredge 
  in 
  abyssal 
  depths, 
  a 
  thousand 
  have 
  

   perhaps 
  been 
  made 
  in 
  the 
  littoral 
  zone, 
  we 
  must 
  hesitate 
  to 
  pronounce 
  

   definitely 
  that 
  any 
  genus 
  is 
  without 
  deep-sea 
  species' 
  (Geol. 
  Mag. 
  1884, 
  

   P 
  . 
  497). 
  

  

  