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  PROCEEDINGS 
  OP 
  THE 
  GEOLOGICAL 
  SOCIETY. 
  

  

  during 
  the 
  intervals 
  of 
  time 
  that 
  occurred 
  between 
  the 
  different 
  

   horizons. 
  6 
  genera 
  and 
  9 
  species 
  pass 
  to 
  the 
  Kellaways 
  Rock, 
  and 
  

   3 
  genera 
  and 
  8 
  species 
  to 
  the 
  Oxford 
  Clay, 
  viz. 
  Avicula 
  incequivalvis, 
  

   Lima 
  dujdicata, 
  L. 
  pectiniformis, 
  Pecten 
  annulatus, 
  P. 
  crenatus, 
  P. 
  

   demissus, 
  P. 
  lens, 
  and 
  P. 
  vimineus, 
  all 
  moderately 
  deep-sea 
  forms. 
  

   The 
  Corallian 
  beds 
  contain 
  4 
  genera 
  and 
  6 
  species, 
  3 
  of 
  those 
  just 
  

   quoted, 
  with 
  Placwnqpsis 
  incequalis, 
  Ostrea 
  solitaria, 
  and 
  0. 
  flabel- 
  

   loides. 
  The 
  Kimmeridge 
  Clay 
  through 
  the 
  persistent 
  shells 
  Avicula 
  

   incequivalvis, 
  Ostrea 
  solitaria, 
  Pecten 
  crenatus, 
  P. 
  articvlatus, 
  P. 
  de- 
  

   missus, 
  and 
  P. 
  vimineus 
  keeps 
  up 
  the 
  connexion; 
  but 
  no 
  species 
  of 
  

   any 
  Jurassic 
  group 
  passes 
  the 
  Portlands. 
  

  

  Dimyaria. 
  — 
  70 
  genera 
  and 
  024 
  species 
  of 
  this 
  group 
  occur 
  in 
  the 
  

   whole 
  of 
  the 
  Jurassic 
  rocks, 
  and 
  47 
  genera 
  and 
  236 
  species 
  in 
  the 
  

   Inferior 
  Oolite. 
  Of 
  these 
  if 
  pass 
  to 
  the 
  Fuller's 
  Earth, 
  % 
  \ 
  to 
  the 
  

   Great 
  Oolite, 
  % 
  to 
  the 
  Forest 
  Marble, 
  \\ 
  to 
  the 
  Cornbrash, 
  \ 
  to 
  the 
  

   Kellaways 
  Hock, 
  % 
  to 
  the 
  Oxford 
  Clay, 
  and 
  J 
  to 
  the 
  Corallian 
  beds. 
  

   The 
  most 
  important 
  genera, 
  or 
  those 
  most 
  largely 
  represented, 
  

   arc 
  : 
  — 
  

  

  Astarte 
  

  

  with 
  24 
  species. 
  

  

  Area 
  

  

  55 
  

  

  12 
  „ 
  

  

  Cuculla?a 
  

  

  ?? 
  

  

  16 
  „ 
  

  

  Modiola 
  

  

  55 
  

  

  9 
  » 
  

  

  Myacites 
  

  

  ?» 
  

  

  11 
  „ 
  

  

  Mytilus 
  

  

  n 
  

  

  9 
  „ 
  

  

  Pholadomya 
  

  

  >■> 
  

  

  11 
  „ 
  

  

  Tancredia 
  

  

  5* 
  

  

  10 
  „ 
  

  

  Trigonia 
  

  

  J? 
  

  

  34 
  „ 
  

  

  136 
  „ 
  

  

  Thus 
  these 
  9 
  genera 
  out 
  of 
  47, 
  and 
  136 
  species 
  out 
  of 
  236, 
  constitute 
  

   more 
  than 
  one 
  half 
  of 
  the 
  Dimyarian 
  fauna 
  of 
  the 
  Inferior 
  Oolite 
  ; 
  

   1 
  2 
  genera 
  possess 
  only 
  one 
  species 
  each, 
  9 
  genera 
  two 
  species, 
  and 
  

   5 
  genera 
  three 
  species. 
  These 
  smaller 
  genera 
  with 
  few 
  species 
  often 
  

   give 
  way 
  under 
  close 
  examination 
  and 
  study. 
  The 
  genera 
  Cercomya, 
  

   Dreissena, 
  Pullastra, 
  Ptychomya, 
  &c, 
  with 
  only 
  one 
  species, 
  like 
  

   many 
  others, 
  will 
  ultimately 
  be 
  expunged 
  from 
  our 
  lists. 
  Assuming 
  

   that 
  the 
  species 
  in 
  the 
  above 
  nine 
  prolific 
  genera 
  would 
  probably 
  

   have 
  a 
  long 
  range 
  in 
  time, 
  we 
  find 
  that 
  the 
  following 
  numbers 
  

   pass 
  to 
  the 
  Great 
  Oolite 
  — 
  in 
  Astarte 
  10, 
  Area 
  4, 
  Cucullcea 
  only 
  2, 
  

   Modiola 
  5 
  out 
  of 
  9, 
  Myacites 
  only 
  1, 
  Mytilus 
  only 
  1, 
  Pholadomya 
  4, 
  

   Tancredia 
  5, 
  and 
  Trigonia 
  only 
  4 
  out 
  of 
  34 
  ; 
  or 
  36 
  species 
  out 
  of 
  

   the 
  136 
  in 
  the 
  above 
  genera 
  are 
  common 
  to 
  the 
  Great 
  and 
  Inferior 
  

   Oolite. 
  It 
  is 
  easy 
  to 
  account 
  for 
  this 
  on 
  looking 
  into 
  the 
  habits 
  and 
  

   habitats 
  of 
  the 
  genera, 
  their 
  bathymetrical 
  range, 
  the 
  nature 
  of 
  the 
  

   sea-bottom 
  and 
  their 
  probable 
  food. 
  

  

  The 
  20 
  Cornbrash 
  species 
  that 
  are 
  common 
  to 
  the 
  Inferior 
  Oolite 
  

   belong 
  to 
  13 
  genera 
  — 
  Anatina, 
  Astarte, 
  Carclium, 
  Corbicella, 
  Goni- 
  

   omya, 
  Gresslya, 
  Homomya, 
  Isocardia, 
  Lucina, 
  Macrodon, 
  Modiola, 
  

   Quenstedtia, 
  and 
  Trigonia. 
  The 
  Corallian 
  forms 
  yield 
  a 
  single 
  spe- 
  

  

  