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  PATELLA 
  VULGATA, 
  LINN.EUS, 
  AND 
  ITS 
  SO-CALLED 
  VARIETY, 
  

   PATELLA 
  DEPRESSA, 
  PENNANT. 
  

  

  By 
  tlie 
  Ptev. 
  A. 
  H. 
  Cookk, 
  Sc.D., 
  F.Z.S. 
  

  

  Read 
  12th 
  January, 
  1917. 
  

  

  Pennant, 
  British 
  Zoology, 
  vol. 
  iv, 
  8vo 
  ed., 
  1777, 
  p. 
  142, 
  described 
  his- 
  

   Patella 
  depressa 
  us 
  a 
  " 
  shell 
  much 
  depressed, 
  the 
  vertex 
  approximating 
  

   nearly 
  to 
  one 
  edge. 
  More 
  oblong 
  tlian 
  the 
  former" 
  [yidgata]. 
  

  

  Forbes 
  & 
  Hauley, 
  Hist. 
  British 
  MoUusca, 
  vol. 
  ii, 
  p. 
  428, 
  under 
  the 
  

   name 
  athletica. 
  Bean, 
  regard 
  depressa 
  on 
  conchological 
  grounds 
  as 
  

   a 
  true 
  species, 
  but, 
  after 
  mentioning 
  that 
  tlie 
  two 
  forms 
  "inhabit 
  

   different 
  levels, 
  the 
  vulgata 
  being 
  always 
  in 
  the 
  higher 
  zone 
  ", 
  add 
  

   the 
  singular 
  statement, 
  "We 
  have 
  sought 
  in 
  vain 
  for 
  differences 
  

   between 
  the 
  structure 
  of 
  the 
  tongue 
  in 
  this 
  and 
  the 
  common 
  species." 
  

  

  Jeffreys, 
  Brit. 
  Conch., 
  vol. 
  iii, 
  p. 
  237, 
  regards 
  depressa, 
  Penn., 
  as 
  

   a 
  var. 
  of 
  vulgata, 
  and 
  " 
  cannot 
  find 
  a 
  single 
  permanent 
  character 
  which 
  

   will 
  serve 
  to 
  distinguish 
  " 
  them 
  or 
  the 
  var. 
  intermedia, 
  Knapp. 
  

  

  The 
  authors 
  of 
  the 
  List 
  of 
  British 
  Marine 
  MoUusca 
  ])ublished 
  by 
  

   the 
  Conchological 
  Society 
  {Journ. 
  Conch., 
  vol. 
  x, 
  1901, 
  ]). 
  16) 
  appear 
  

   to 
  have 
  been 
  of 
  the 
  same 
  opinion, 
  since 
  they 
  kept 
  depressa 
  as 
  one 
  of 
  

   the 
  vars. 
  oivulgata, 
  but 
  in 
  their 
  revised 
  edition 
  (1902) 
  they 
  treat 
  them 
  

   as 
  separate 
  species. 
  

  

  If 
  the 
  radula 
  of 
  a 
  specimen 
  of 
  vulgata, 
  the 
  shell 
  of 
  which 
  measures 
  

   2 
  inches 
  in 
  length, 
  be 
  laid 
  out 
  alongside 
  the 
  radula 
  of 
  a 
  specimen 
  of 
  

   depressa, 
  whose 
  shell 
  is 
  also 
  2 
  inches 
  long, 
  it 
  will 
  at 
  once 
  be 
  noticed 
  

   that 
  the 
  radula 
  of 
  depressa 
  is 
  markedly 
  shorter 
  than 
  that 
  of 
  vulgata, 
  

   probably 
  by 
  as 
  much 
  as 
  i 
  to 
  f 
  inch. 
  Two 
  other 
  points 
  will 
  strike 
  

   the 
  observer 
  : 
  the 
  radula 
  of 
  depressa 
  is 
  much 
  broader 
  than 
  that 
  of 
  

   vtdgata, 
  and 
  the 
  nascent 
  portion 
  is 
  relatively 
  much 
  shorter 
  and 
  

   terminates 
  more 
  abruptly. 
  Thus 
  a 
  radula 
  of 
  vulgata 
  measuring 
  

   84'2 
  mm. 
  long 
  is 
  "8 
  mm. 
  broad, 
  with 
  perhaps 
  25 
  nascent 
  and 
  190 
  adult 
  

   rows, 
  while 
  a 
  radula 
  of 
  depressa 
  55 
  '6 
  mm. 
  long 
  is 
  1*6 
  mm. 
  broad, 
  ajid 
  

   has 
  6-7 
  nascent 
  and 
  112 
  adult 
  rows. 
  

  

  If 
  a 
  long 
  series 
  of 
  the 
  two 
  forms 
  be 
  taken, 
  and 
  the 
  radulae 
  of 
  shells 
  

   of 
  equal 
  length, 
  from 
  the 
  same 
  locality, 
  examined, 
  this 
  difference 
  

   becomes 
  more 
  striking. 
  Beginning 
  with 
  shells 
  measuring 
  58'8 
  mm. 
  

   (the 
  largest 
  procurable 
  of 
  both 
  species) 
  and 
  forming 
  a 
  series 
  with 
  

   shells 
  decreasing 
  each 
  time 
  by 
  an 
  equal 
  amount 
  (r6 
  mm.), 
  we 
  find 
  

   that 
  in 
  every 
  case 
  the 
  radula 
  of 
  depressa 
  is 
  markedly 
  shorter 
  than 
  

   that 
  of 
  vulgata. 
  Sometimes 
  the 
  amount 
  of 
  difference 
  is 
  so 
  large 
  as 
  to 
  

   be 
  startling; 
  thus 
  in 
  the 
  case 
  of 
  shells 
  measuring 
  55-6 
  mm. 
  the 
  

   radula 
  of 
  vulgata 
  is 
  89 
  ram. 
  long, 
  that 
  of 
  depressa 
  47'8 
  mm., 
  

   a 
  difference 
  of 
  41 
  '2 
  mm., 
  or 
  more 
  than 
  \h 
  inches. 
  In 
  the 
  case 
  of 
  

   shells 
  measuring 
  44'6 
  mm. 
  the 
  radula 
  of 
  vulgata 
  (100*2 
  mm.) 
  is 
  more 
  

   than 
  double 
  the 
  length 
  of 
  that 
  of 
  depressa 
  (44'6 
  mm.). 
  In. 
  the 
  

   younger 
  specimens 
  the 
  difference 
  is 
  just 
  as 
  marked. 
  And 
  if 
  the 
  

   length 
  of 
  the 
  radulae 
  of 
  the 
  whole 
  series 
  is 
  added 
  up, 
  the 
  average 
  

   length 
  of 
  a 
  vulgata 
  radula 
  is 
  found 
  to 
  be 
  63-6 
  mm., 
  while 
  that 
  of 
  

  

  