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  A 
  MODIFICATION 
  IN 
  THE 
  FOEM 
  OF 
  SHELL 
  (SIPHONARIA 
  

   ALGESIRyE, 
  QUOY) 
  APPARENTLY 
  DUE 
  TO 
  LOCALITY. 
  

  

  By 
  the 
  Rev. 
  A. 
  H. 
  Cooke, 
  M.A, 
  

  

  Read 
  12th 
  May, 
  1911. 
  

  

  When 
  collecting 
  Siphonaria 
  Algesirm, 
  Quoy, 
  in 
  Januarj^ 
  last, 
  on 
  the 
  

   rocks 
  at 
  both 
  sides 
  of 
  the 
  entrance 
  to 
  the 
  Mediterranean, 
  I 
  was 
  struck 
  

   with 
  a 
  variation 
  in 
  the 
  shape 
  of 
  the 
  shell, 
  which 
  appeared 
  to 
  be 
  

   directly 
  due 
  to 
  the 
  nature 
  of 
  the 
  station 
  occupied. 
  

  

  So 
  far 
  as 
  I 
  was 
  able 
  to 
  ascertain, 
  this 
  species 
  occurs 
  on 
  rocks 
  from 
  

   low-water 
  mark 
  up 
  to 
  the 
  hifi;hest 
  point 
  reached 
  by 
  the 
  spray 
  of 
  the 
  

   ordinary 
  tide. 
  At 
  Tangier 
  the 
  normal 
  tide-fall 
  is 
  about 
  6 
  feet, 
  at 
  

   Cadiz 
  probably 
  rather 
  more. 
  Specimens 
  taken 
  from 
  near 
  high-water 
  

   mark 
  at 
  both 
  localities 
  showed 
  an 
  elevated 
  apex, 
  while 
  specimens 
  from 
  

   near 
  half-tide 
  mark 
  were 
  comparatively 
  flattened. 
  It 
  occurred 
  to 
  me 
  

   to 
  have 
  measurements 
  taken, 
  («) 
  of 
  the 
  apical 
  angle 
  of 
  a 
  number 
  of 
  

   specimens, 
  measuring 
  the 
  angle 
  along 
  the 
  long 
  axis 
  of 
  the 
  shell, 
  and 
  

   (h) 
  of 
  the 
  height 
  of 
  the 
  apex, 
  and 
  then, 
  after 
  taking 
  the 
  length 
  of 
  each 
  

   specimen, 
  to 
  obtain 
  (<?) 
  a 
  ratio 
  of 
  height 
  to 
  length. 
  Groups 
  of 
  shells 
  

   taken 
  from 
  what 
  may 
  be 
  called 
  the 
  upper 
  and 
  the 
  lower 
  levels 
  were 
  

   thus 
  contrasted 
  in 
  three 
  points 
  — 
  («) 
  the 
  apical 
  angle, 
  (b) 
  the 
  height 
  of 
  

   apex, 
  {c) 
  the 
  ratio 
  of 
  height 
  to 
  length. 
  

  

  On 
  the 
  coast 
  close 
  to 
  Cadiz 
  a 
  sort 
  of 
  low 
  reef 
  ran 
  out 
  into 
  the 
  sea 
  

   at 
  an 
  angle 
  of 
  about 
  45° 
  to 
  the 
  shore, 
  and 
  shelved 
  very 
  gradually 
  

   shorewards, 
  with 
  a 
  steep 
  fall 
  of 
  about 
  2 
  or 
  3 
  feet 
  on 
  the 
  sea 
  side, 
  so 
  

   that 
  the 
  waves 
  broke 
  at 
  the 
  foot 
  of 
  the 
  steep 
  and 
  washed 
  shorewards 
  

   over 
  the 
  reef. 
  In 
  contrast 
  to 
  this 
  station 
  were 
  isolated 
  blocks 
  of 
  rocks 
  

   measuring 
  10 
  or 
  12 
  feet 
  square, 
  lying 
  on 
  the 
  sand 
  near 
  high-water 
  

   mark, 
  and 
  not 
  habituiillj^ 
  beaten 
  by 
  the 
  waves. 
  Both 
  these 
  stations 
  

   abounded 
  in 
  Siphonaria. 
  The 
  contrast 
  in 
  the 
  shape 
  of 
  specimens 
  was 
  

   marked, 
  those 
  from 
  the 
  reef 
  being 
  often 
  so 
  flattened 
  as 
  to 
  be 
  scarcely 
  

   distinguishable 
  from 
  the 
  rock 
  on 
  which 
  they 
  lived, 
  while 
  on 
  the 
  

   boulders 
  near 
  high-water 
  mark 
  they 
  were 
  elevated 
  and 
  bold. 
  

  

  Series 
  I. 
  Cadiz, 
  from 
  the 
  Lower 
  Level. 
  

  

  ^ 
  I 
  owe 
  all 
  these 
  measurements 
  to 
  the 
  kindness 
  of 
  my 
  friend 
  and 
  pupil 
  

   Mr. 
  D. 
  W. 
  Parkes. 
  

  

  