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  PROCEEDINGS 
  OF 
  THE 
  MALACOLOGICAL 
  SOCIETY. 
  

  

  Among 
  the 
  species 
  which 
  I 
  have 
  examined 
  some 
  individuals 
  of 
  the 
  

   following 
  possess 
  three 
  teeth 
  in 
  the 
  left 
  valve 
  : 
  — 
  

  

  P. 
  lipartita, 
  Desh. 
  P. 
  rohusta, 
  Sowb. 
  

  

  P. 
  typica, 
  Chem. 
  P. 
  Hemprichi, 
  Issel. 
  

  

  P. 
  lapicida, 
  Chem. 
  P. 
  cequistriata, 
  Sowb. 
  

  

  In 
  these 
  species 
  the 
  posterior 
  tooth 
  is 
  long, 
  narrow, 
  and 
  parallel 
  to 
  

   the 
  nymph 
  which 
  supports 
  the 
  sunken 
  ligament, 
  the 
  median 
  is 
  broad 
  

   and 
  bifid, 
  while 
  the 
  anterior 
  is 
  small 
  with 
  a 
  tendency 
  to 
  disappear 
  in 
  

   old 
  shells 
  ; 
  but 
  all 
  agree 
  in 
  having 
  a 
  peculiar 
  character 
  which 
  does 
  not 
  

   seem 
  to 
  have 
  been 
  hitherto 
  noticed, 
  this 
  is 
  that 
  the 
  hinge-plate 
  is 
  

   deeply 
  notched 
  on 
  each 
  side 
  of 
  the 
  median 
  tooth. 
  

  

  In 
  the 
  right 
  valve 
  the 
  usual 
  number 
  of 
  teeth 
  is 
  two, 
  though 
  some- 
  

   times 
  (as 
  in 
  P. 
  lithophaga) 
  one 
  of 
  these 
  is 
  obsolete, 
  and 
  the 
  complete 
  

   dentition 
  is 
  reduced 
  to 
  2-1. 
  The 
  two 
  teeth 
  which 
  are 
  generally 
  

   present 
  are 
  the 
  posterior 
  and 
  the 
  median, 
  and 
  when 
  only 
  one 
  remains 
  

   it 
  is 
  the 
  median 
  which 
  survives. 
  

  

  In 
  genera 
  which 
  have 
  three 
  teeth 
  in 
  both 
  valves 
  as 
  in 
  Chione, 
  Tapes, 
  

   and 
  Venerupis, 
  if 
  attention 
  be 
  paid 
  to 
  the 
  manner 
  in 
  which 
  the 
  teeth 
  

   interlock, 
  it 
  will 
  be 
  seen 
  that 
  the 
  teeth 
  of 
  the 
  right 
  valve 
  pass 
  in 
  front 
  

   of 
  those 
  of 
  the 
  left 
  valve, 
  so 
  that 
  the 
  anterior 
  of 
  the 
  right 
  occupies 
  

   a 
  more 
  forward 
  position 
  than 
  that 
  of 
  the 
  left. 
  , 
  The 
  consequence 
  is 
  

   that 
  if 
  the 
  hinge 
  -area 
  is 
  laterally 
  contracted 
  the 
  right 
  anterior 
  tooth 
  

   is 
  pushed 
  towards 
  the 
  shell-margin, 
  and 
  tends 
  to 
  become 
  obsolete. 
  

   In 
  the 
  same 
  way 
  the 
  left 
  posterior 
  cardinal 
  may 
  become 
  merged 
  in 
  the 
  

   ligaraeutal 
  plate. 
  

  

  This 
  is 
  what 
  has 
  happened 
  in 
  the 
  case 
  of 
  Petricola 
  ; 
  the 
  right 
  

  

  anterior 
  cardinal 
  has 
  become 
  obsolete, 
  and 
  the 
  dental 
  formula 
  of 
  the 
  

  

  1 
  1, 
  n 
  • 
  -en 
  L. 
  I 
  1.2.3 
  I 
  

  

  normal 
  shell 
  is 
  as 
  lollows: 
  =r 
  ' 
  „ 
  . 
  

  

  E. 
  I 
  0.2.3 
  I 
  

  

  Further, 
  in 
  some 
  species 
  the 
  posterior 
  teeth 
  of 
  both 
  valves 
  have 
  

   become 
  obsolete, 
  leaving 
  only 
  a 
  right 
  median 
  to 
  fit 
  between 
  the 
  left 
  

   anterior 
  and 
  median. 
  

  

  As 
  to 
  other 
  internal 
  characters, 
  it 
  need 
  only 
  be 
  said 
  that 
  the 
  pallial 
  

   sinus 
  is 
  deep, 
  broadly 
  rounded, 
  and 
  generally 
  ascending, 
  but 
  in 
  some 
  

   of 
  the 
  Riipellaria 
  section 
  it 
  is 
  nearly 
  horizontal. 
  

  

  Petricolaria. 
  — 
  The 
  shells 
  of 
  this 
  group 
  have 
  a 
  similar 
  hinge, 
  but 
  the 
  

   hinge-plate 
  is 
  so 
  narrow 
  and 
  so 
  deeply 
  notched 
  between 
  the 
  teeth 
  that 
  

   practicallj^ 
  it 
  does 
  not 
  exist, 
  and 
  the 
  teeth 
  seem 
  to 
  spring 
  from 
  the 
  

   shell-margin. 
  In 
  the 
  right 
  valve 
  the 
  plate 
  is 
  rather 
  broader, 
  and 
  is 
  

   generally 
  thickened 
  to 
  support 
  the 
  median 
  tooth. 
  There 
  are 
  three 
  

   narrow 
  teeth 
  in 
  the 
  left 
  valve, 
  the 
  median 
  being 
  only 
  grooved 
  in 
  front, 
  

   not 
  bifid 
  ; 
  in 
  the 
  right 
  valve 
  there 
  are 
  normally 
  two 
  teeth, 
  a 
  broad 
  

   bifid 
  posterior, 
  and 
  a 
  tall 
  projecting 
  median; 
  no 
  anterior 
  tooth. 
  

  

  The 
  pallial 
  sinus 
  is 
  very 
  deep, 
  often 
  reaching 
  to 
  the 
  middle 
  of 
  the 
  

   shell, 
  but 
  is 
  nearly 
  horizontal. 
  

  

  Most 
  of 
  the 
  shells 
  belonging 
  to 
  this 
  group 
  are 
  very 
  inequilateral 
  

   and 
  more 
  or 
  less 
  pholadiform, 
  the 
  umbones 
  being 
  near 
  the 
  anterior 
  

   end 
  and 
  the 
  posterior 
  region 
  being 
  much 
  elongated 
  ; 
  but 
  P. 
  aquistriata 
  

   from 
  Japan 
  is 
  subequilateral 
  and 
  of 
  an 
  elongate-oval 
  shape, 
  and 
  

  

  