﻿398 
  

  

  ME. 
  H. 
  WOODS 
  ON 
  THE 
  MOLLTJSCA 
  

  

  [Aug. 
  1897, 
  

  

  ACTINOZOA. 
  

  

  Parasmilia 
  centralis 
  (Mant.). 
  

  

  PORIFERA. 
  

  

  Camerospongia 
  campanulata 
  (Smith). 
  

  

  „ 
  subroiunda 
  (Mant.). 
  

  

  Coscinopora 
  infundibuliformis, 
  Groldf. 
  

   Cystispongia 
  subglobosa, 
  Bonier. 
  

   Guettardia 
  stellata, 
  Mich. 
  

   Leptophragrna 
  Murchisoni 
  (Goldf.). 
  1 
  

  

  Porifera 
  (cont.). 
  

  

  Placotrema 
  cretaceum, 
  Hinde. 
  

   Plinthosella 
  squamosa, 
  Zitt. 
  1 
  

   Plocoscyphia 
  convoluta 
  (Smith). 
  

  

  „ 
  flcxuosa 
  (Mant.). 
  

  

  Ventriculites 
  alcyonoides, 
  Mant. 
  

  

  ,, 
  angustatus, 
  Bom. 
  

  

  ,, 
  decurrens, 
  Smith. 
  

  

  ,, 
  impressus, 
  Smith. 
  

  

  ,, 
  mammillaris, 
  Smith. 
  

  

  Verrucoccelia 
  tubulata 
  ? 
  (Smith). 
  

  

  V. 
  Conditions 
  under 
  which 
  the 
  Chalk 
  Bock 
  was 
  Deposited. 
  

  

  To 
  endeavour 
  to 
  determine 
  the 
  depth 
  at 
  which 
  a 
  fauna, 
  composed 
  

   (with 
  the 
  exception 
  of 
  some 
  of 
  the 
  foraminifera) 
  of 
  extinct 
  species, 
  

   lived, 
  may 
  at 
  first 
  sight 
  appear 
  to 
  be 
  quite 
  hopeless, 
  since 
  we 
  know 
  

   that 
  at 
  the 
  present 
  day 
  different 
  species 
  of 
  a 
  genns 
  have 
  often 
  very 
  

   diverse 
  ranges. 
  I 
  believe, 
  however, 
  that 
  by 
  a 
  study 
  of 
  the 
  predomi- 
  

   nating 
  forms 
  in 
  a 
  fauna, 
  and 
  their 
  association, 
  together 
  with 
  the 
  

   relative 
  numbers 
  of 
  individuals 
  and 
  species, 
  some 
  conclusions 
  of 
  a 
  

   trustworthy 
  nature 
  may 
  be 
  arrived 
  at. 
  

  

  That 
  the 
  fauna 
  of 
  the 
  Reussianum-zone 
  lived 
  in 
  water 
  of 
  less 
  

   depth 
  than 
  the 
  faunas 
  of 
  the 
  other 
  Turonian 
  and 
  Senonian 
  zones 
  

   will, 
  I 
  think, 
  scarcely 
  be 
  disputed. 
  The 
  Chalk 
  Bock 
  is 
  very 
  thin 
  

   generally, 
  yet 
  fossils 
  are 
  more 
  numerous 
  specifically 
  and 
  far 
  more 
  

   abundant 
  individually 
  than 
  in 
  the 
  other 
  zones. 
  It 
  is 
  well 
  known 
  

   that 
  at 
  the 
  present 
  day 
  we 
  find 
  a 
  similar 
  difference 
  in 
  passing 
  from 
  

   comparatively 
  shallow 
  water 
  to 
  greater 
  depths 
  : 
  the 
  number 
  of 
  

   species, 
  as 
  well 
  as 
  the 
  number 
  of 
  individuals, 
  living 
  on 
  the 
  sea- 
  

   bottom, 
  diminishes 
  considerably. 
  In 
  water 
  of 
  no 
  great 
  depth, 
  the 
  

   nature 
  of 
  the 
  materials 
  forming 
  the 
  sea 
  -bottom 
  has 
  a 
  greater 
  

   influence 
  on 
  the 
  character 
  of 
  the 
  fauna 
  than 
  has 
  the 
  actual 
  depth 
  of 
  

   the 
  water.* 
  2 
  The 
  relative 
  richness 
  of 
  the 
  fauna 
  of 
  the 
  Reussianum- 
  

   zone 
  certainly 
  cannot 
  be 
  accounted 
  for 
  by 
  any 
  difference 
  in 
  the 
  

   nature 
  of 
  the 
  sea-bottom, 
  since 
  in 
  its 
  original 
  soft 
  state 
  the 
  

   Chalk-ooze 
  must 
  have 
  been, 
  so 
  far 
  as 
  animal 
  life 
  was 
  concerned, 
  

   uniform 
  in 
  character 
  throughout 
  the 
  Chalk 
  period. 
  We 
  can, 
  there- 
  

   fore, 
  only 
  conclude 
  that 
  the 
  change 
  in 
  the 
  fauna 
  was 
  caused 
  by 
  a 
  

   decrease 
  in 
  depth 
  ; 
  and, 
  since 
  the 
  change 
  is 
  so 
  marked, 
  the 
  decrease 
  

   was 
  probably 
  considerable. 
  

  

  The 
  presence 
  of 
  glauconite-grains, 
  which 
  is 
  one 
  of 
  the 
  distinguish- 
  

   ing 
  features 
  of 
  this 
  zone, 
  also 
  lends 
  support 
  to 
  the 
  view 
  just 
  stated. 
  

   The 
  Challenger 
  Expedition 
  3 
  found 
  that 
  glauconite 
  was 
  'almost 
  

   exclusively 
  limited 
  to 
  terrigenous 
  deposits 
  in 
  more 
  or 
  less 
  close 
  

   proximity 
  to 
  the 
  continental 
  masses 
  of 
  land, 
  while 
  it 
  was 
  relatively 
  

  

  1 
  These 
  species 
  are 
  here 
  recorded 
  from 
  information 
  supplied 
  by 
  Mr. 
  Jukes- 
  

   Browne, 
  whose 
  specimens 
  were 
  determined 
  by 
  Dr. 
  G. 
  J. 
  Hinde, 
  F.B.S. 
  

  

  2 
  See 
  especially 
  remarks 
  by 
  W. 
  A. 
  Herdman, 
  Proc. 
  Liverpool 
  Geol. 
  Soc. 
  

   toI. 
  vii. 
  pt. 
  ii. 
  (1894) 
  p. 
  171 
  ; 
  also 
  Address, 
  Bep. 
  Brit. 
  Assoc. 
  (Ipswich) 
  1895, 
  

   p. 
  698. 
  

  

  3 
  Murray 
  & 
  Benard, 
  Challenger 
  Bep. 
  ' 
  Deep-Sea 
  Deposits' 
  (1891), 
  p. 
  382. 
  

  

  