﻿402 
  MR. 
  H. 
  WOODS 
  ON 
  THE 
  MOLLUSCA 
  [Aug. 
  1 
  897 
  

  

  are 
  widely 
  distributed 
  in 
  rather 
  shallow 
  water, 
  and 
  have 
  also 
  

   some 
  abyssal 
  species. 
  

  

  Caspidaria 
  is 
  found 
  in 
  all 
  seas 
  and 
  at 
  all 
  depths, 
  but 
  the 
  

   species 
  are 
  most 
  abundant 
  in 
  deep 
  water 
  and 
  abyssal 
  regions 
  ; 
  

   since, 
  however, 
  only 
  one 
  specimen 
  has 
  been 
  seen 
  in 
  the 
  Chalk 
  

   Rock, 
  we 
  canuot 
  take 
  it 
  as 
  evidence 
  of 
  very 
  deep 
  water. 
  

  

  Nuculana 
  ( 
  = 
  Leda) 
  also 
  occurs 
  at 
  all 
  depths, 
  but 
  is 
  rather 
  

   a 
  characteristic 
  deep-water 
  form. 
  1 
  Only 
  two 
  specimens 
  

   have 
  been 
  found 
  in 
  the 
  Chalk 
  Rock. 
  

  

  To 
  sum 
  up 
  the 
  preceding 
  evidence 
  furnished 
  by 
  the 
  Mollusca, 
  we 
  

   see 
  that 
  we 
  have 
  in 
  the 
  Reussianum-zone 
  some 
  genera 
  (Cuspidaria, 
  

   Nuculana, 
  Dentalium) 
  which 
  are 
  characteristic 
  of 
  deep 
  water, 
  but 
  

   are 
  not 
  confined 
  to 
  it 
  ; 
  others 
  which 
  are 
  limited 
  to 
  shallow 
  water 
  

   (0 
  to 
  50 
  fathoms). 
  But 
  neither 
  group 
  forms 
  the 
  predominating 
  

   element 
  in 
  the 
  Reussianum-iauna 
  ; 
  we 
  may 
  therefore 
  consider 
  that 
  

   this 
  zone 
  was 
  not 
  laid 
  down 
  either 
  in 
  shallow 
  or 
  in 
  very 
  deep 
  

   water. 
  The 
  commonest 
  genera, 
  although 
  at 
  the 
  present 
  day 
  most 
  

   abundant 
  in 
  shallow 
  or 
  rather 
  shallow 
  water, 
  do 
  occur 
  fairly 
  often 
  

   in 
  water 
  of 
  moderate 
  depth 
  (say 
  from 
  100 
  to 
  500 
  fathoms), 
  but 
  

   these 
  genera, 
  numerous 
  in 
  the 
  Reussianum-zone 
  as 
  the 
  individuals 
  

   may 
  be, 
  are 
  there 
  represented 
  by 
  only 
  one 
  or 
  two 
  species 
  in 
  

   each 
  case, 
  whereas 
  in 
  shallow 
  water 
  they 
  would 
  almost 
  certainly 
  

   have 
  been 
  represented 
  by 
  a 
  larger 
  number. 
  I 
  think, 
  therefore, 
  that 
  

   it 
  is 
  unlikely 
  that 
  the 
  Reussianum-zone 
  was 
  formed 
  at 
  a 
  depth 
  

   greater 
  than 
  about 
  500 
  fathoms 
  ; 
  this 
  is 
  further 
  supported 
  by 
  the 
  

   fact 
  that 
  in 
  dredging 
  beyond 
  this 
  depth 
  a 
  marked 
  decrease 
  is 
  

   noticed 
  in 
  the 
  number 
  of 
  individuals 
  as 
  well 
  as 
  species 
  of 
  both 
  

   lamellibranchia 
  2 
  and 
  brachiopoda. 
  3 
  

  

  I 
  consider, 
  then, 
  that 
  the 
  Chalk 
  Rock 
  was 
  laid 
  down 
  between 
  the 
  

   depths 
  of 
  about 
  100 
  and 
  500 
  fathoms. 
  From 
  the 
  number 
  of 
  

   species 
  found 
  in 
  it, 
  several 
  of 
  which 
  belong 
  to 
  genera 
  common 
  in 
  

   fairly 
  shallow 
  water, 
  I 
  think 
  it 
  was 
  probably 
  deposited 
  somewhat- 
  

   nearer 
  the 
  former 
  than 
  the 
  latter 
  limit, 
  although 
  the 
  depth 
  must 
  

   necessarily 
  have 
  varied 
  in 
  different 
  places. 
  The 
  absence 
  of 
  the 
  

   Chalk 
  Rock 
  in 
  Lincolnshire 
  is 
  most 
  likely 
  due 
  to 
  the 
  greater 
  depth 
  

   of 
  water 
  which 
  existed 
  in 
  that 
  region. 
  

  

  I 
  have 
  not 
  studied 
  in 
  detail 
  the 
  other 
  groups 
  of 
  fossils 
  found 
  in 
  

   the 
  Reussianum-zone. 
  I 
  may, 
  however, 
  mention 
  that 
  the 
  Hexacti- 
  

   nellid 
  sponges 
  are 
  well 
  represented, 
  and 
  therefore 
  strongly 
  favour 
  

   the 
  view 
  that 
  the 
  lower 
  limit 
  was 
  not 
  less 
  than 
  about 
  100 
  fathoms, 
  

  

  1 
  The 
  occurrence 
  of 
  the 
  genera 
  Caspidaria 
  (=Necera), 
  Nuculana 
  (=Leda), 
  

   and 
  Dentalium 
  is 
  noteworthy, 
  since 
  Dr. 
  Gwyn 
  Jeffreys 
  and 
  many 
  later 
  writers 
  

   have 
  regarded 
  the 
  absence 
  of 
  these 
  forms 
  in 
  the 
  Upper 
  Chalk 
  as 
  evidence 
  in 
  

   favour 
  of 
  its 
  shallow-water 
  origin. 
  On 
  this 
  subject 
  see 
  J. 
  G. 
  Jeffreys, 
  Eep. 
  

   Brit. 
  Assoc, 
  for 
  1877 
  (1878), 
  p. 
  86 
  ; 
  J. 
  S. 
  Gardner, 
  Geol. 
  Mag. 
  1884, 
  pp. 
  496- 
  

   506 
  ; 
  A. 
  R. 
  Wallace, 
  « 
  Island 
  Life,' 
  2nd 
  ed. 
  (1892) 
  p. 
  89. 
  

  

  2 
  E. 
  A. 
  Smith, 
  ' 
  Report 
  on 
  the 
  Lamellibranchiata,' 
  Challenger 
  Exped. 
  (1885) 
  

   p. 
  6. 
  

  

  3 
  T. 
  Davidson, 
  ' 
  Report 
  on 
  the 
  Brachiopoda,' 
  Challenger 
  Exped. 
  (1880) 
  

   pp. 
  3,4. 
  

  

  