﻿814 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM. 
  

  

  behind 
  the 
  tooth. 
  External 
  to 
  the 
  posterior 
  part 
  of 
  the 
  ligament 
  there 
  

   is 
  a 
  submargiiial 
  thickening 
  or 
  fold, 
  especially 
  in 
  the 
  right 
  valve. 
  In 
  

   the 
  left 
  valve 
  the 
  central 
  tooth 
  is 
  represented 
  by 
  an 
  irregular, 
  bilobed, 
  

   or 
  somewhat 
  V-shaped 
  thickening 
  of 
  the 
  margin, 
  of 
  which 
  the 
  anterior 
  

   part, 
  situated 
  just 
  in 
  front 
  of 
  the 
  beak, 
  is 
  the 
  more 
  prominent; 
  but 
  this 
  

   varies 
  in 
  form 
  in 
  different 
  specimens. 
  The 
  postero-dorsal 
  margin 
  along 
  

   the 
  ligamental 
  region 
  is 
  less 
  thickened 
  but 
  has 
  a 
  distinct 
  rounded 
  

   ridge 
  inside 
  the 
  ligament. 
  

  

  Length 
  of 
  one 
  of 
  the 
  largest 
  specimens, 
  10 
  mm. 
  ,• 
  total 
  height, 
  10 
  mm. 
  ; 
  

   height 
  of 
  cavity, 
  12 
  mm.; 
  breadth, 
  16 
  mm. 
  In 
  a 
  more 
  rounded 
  speci- 
  

   men 
  the 
  length 
  is 
  15 
  mm.; 
  total 
  height, 
  16 
  mm.; 
  height 
  of 
  cavity, 
  

   13.5 
  mm.; 
  breadth, 
  11 
  mm. 
  

  

  A 
  few 
  dead 
  specimens 
  of 
  the 
  typical 
  form 
  (sublevis) 
  have 
  been 
  taken 
  at 
  

   five 
  stations 
  between 
  N. 
  lat. 
  39° 
  15', 
  W. 
  long. 
  68° 
  8', 
  and 
  N. 
  lat. 
  

   37° 
  56' 
  20", 
  W. 
  long. 
  70° 
  57' 
  30", 
  in 
  1,591 
  to 
  1,917 
  fathoms, 
  1883-1880. 
  

  

  Several 
  live 
  and 
  dead 
  specimens 
  of 
  the 
  varietal 
  form 
  (microdonta) 
  have 
  

   been 
  taken 
  at 
  eight 
  stations 
  between 
  N". 
  lat. 
  39° 
  20', 
  W. 
  long. 
  08° 
  3' 
  30", 
  

   and 
  N. 
  lat. 
  36° 
  47', 
  W. 
  long. 
  73° 
  9' 
  30", 
  in 
  1,631 
  to 
  1,859 
  fathoms, 
  

   1885-1886. 
  

  

  Mr. 
  Dall 
  extends 
  the 
  range 
  south 
  to 
  Patagonia, 
  in 
  122 
  to 
  1,635 
  

   fathoms. 
  

  

  Our 
  specimens 
  show 
  considerable 
  variation 
  in 
  form 
  as 
  well 
  as 
  in 
  the 
  

   prominence 
  of 
  the 
  cardinal 
  tooth 
  in 
  the 
  right 
  valve, 
  and 
  thus 
  unite 
  the 
  

   extreme 
  forms 
  P. 
  sublevis 
  Yerrill, 
  and 
  P. 
  microdonta 
  Dall. 
  

  

  CETOCONCHA 
  ATYPHA, 
  new 
  species. 
  

  

  Shell 
  short-ovate, 
  nearly 
  equilateral, 
  and 
  nearly 
  equally 
  rounded 
  at 
  

   both 
  ends, 
  judging 
  from 
  the 
  lines 
  of 
  growth. 
  Umbos 
  rather 
  prominent, 
  

   but 
  less 
  so 
  than 
  in 
  several 
  allied 
  species. 
  Beaks 
  rather 
  prominent 
  and 
  

   curved 
  strongly 
  forward, 
  but 
  not 
  siiiral. 
  Surface 
  somewhat 
  shining 
  

   and 
  slightly 
  iridescent 
  where 
  rubbed, 
  covered 
  with 
  a 
  very 
  thin, 
  yellow- 
  

   ish 
  epidermis 
  with 
  very 
  numerous, 
  minute, 
  granule-like 
  elevations 
  which 
  

   are 
  arranged 
  in 
  regular 
  radiating 
  lines, 
  and 
  are 
  much 
  the 
  most 
  numer- 
  

   ous 
  on 
  the 
  posterior 
  end 
  where 
  the 
  radial 
  rows 
  are 
  closely 
  crowded 
  and 
  

   the 
  granules 
  in 
  each 
  are 
  also 
  near 
  together; 
  on 
  the 
  center 
  the 
  rows 
  and 
  

   granules 
  are 
  more 
  distant, 
  so 
  that 
  the 
  number 
  is 
  only 
  about 
  half 
  as 
  

   great 
  in 
  the 
  same 
  space; 
  on 
  the 
  anterior 
  end 
  they 
  are 
  so 
  scattered 
  that 
  

   the 
  radial 
  rows 
  are 
  indistinct 
  and 
  the 
  granules 
  are 
  a 
  little 
  larger; 
  on 
  

   the 
  lunular 
  area 
  they 
  are 
  nearly 
  obsolete. 
  The 
  anterior 
  end 
  and 
  lunu- 
  

   lar 
  area 
  are 
  marked 
  by 
  rather 
  conspicuous 
  lines 
  of 
  growth 
  which, 
  near 
  

   the 
  dorsal 
  margin, 
  take 
  the 
  form 
  of 
  distinct, 
  raised, 
  concentric 
  ridges. 
  

   The 
  antero-dorsal 
  margin 
  is 
  nearly 
  horizontal 
  and 
  rises 
  up, 
  in 
  a 
  side 
  

   view, 
  in 
  an 
  acute 
  edge, 
  a 
  little 
  higher 
  than 
  the 
  level 
  of 
  the 
  beak, 
  so 
  as 
  

   to 
  produce 
  a 
  broad, 
  comj)ressed, 
  lunular 
  margin. 
  When 
  viewed 
  from 
  

   above, 
  this 
  part 
  of 
  the 
  margin 
  forms 
  a 
  very 
  marked 
  obtuse 
  angle 
  with 
  

   the 
  posterior 
  hinge-margin. 
  The 
  poster© 
  dorsal 
  margin 
  is 
  also 
  nearly 
  

  

  