﻿838 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM. 
  vol.xx. 
  

  

  end, 
  with 
  a 
  sharp, 
  angular, 
  byssal 
  notch 
  beneath 
  it, 
  separated 
  from 
  the 
  

   body 
  of 
  the 
  sliell 
  by 
  a 
  narrow 
  groove. 
  The 
  dorsal 
  margins 
  of 
  the 
  body 
  

   of 
  the 
  shell 
  are 
  nearly 
  straight 
  and 
  form 
  more 
  thaji 
  a 
  right 
  angle; 
  the 
  

   ventral 
  margin 
  is 
  broadly 
  rounded, 
  nearly 
  semicircular, 
  forming 
  a 
  very 
  

   obtusely 
  rounded 
  angle, 
  where 
  it 
  joins 
  the 
  dorsal 
  margins. 
  Umbos 
  a 
  

   little 
  prominent, 
  with 
  a 
  small, 
  smooth, 
  rather 
  acute, 
  incurved 
  beak, 
  

   which 
  projects 
  a 
  little 
  above 
  the 
  hinge-margin. 
  The 
  surface 
  of 
  the 
  left 
  

   valve 
  is 
  everywhere 
  thickly 
  covered 
  with 
  fine, 
  almost 
  microscopic, 
  radia- 
  

   ting 
  stria', 
  which 
  become 
  a 
  little 
  more 
  distinct 
  on 
  the 
  anterior 
  auricle; 
  

   slightly 
  raised 
  thin 
  lines 
  of 
  growth 
  are 
  often 
  very 
  distinct 
  on 
  some 
  

   parts 
  of 
  the 
  shell, 
  especially 
  on 
  the 
  anterior 
  auricle, 
  where 
  they 
  become 
  

   closer, 
  more 
  regular, 
  and 
  often 
  produce, 
  in 
  crossing 
  the 
  radial 
  striations, 
  

   a 
  quite 
  regular, 
  microscopic 
  decussation; 
  the 
  sculpture 
  on 
  the 
  posterior 
  

   auricle 
  is 
  sometimes 
  similar 
  but 
  finer, 
  although 
  in 
  many 
  specimens 
  the 
  

   surface 
  is 
  nearly 
  smooth 
  or 
  marked 
  only 
  by 
  very 
  fine 
  radial 
  stria'. 
  The 
  

   right 
  valve 
  is 
  less 
  convex 
  than 
  tlie 
  left 
  (its 
  ventral 
  edge 
  does 
  not 
  quite 
  

   reach 
  that 
  of 
  the 
  opposite 
  valve), 
  the 
  umbo 
  is 
  less 
  prominent, 
  the 
  beak 
  

   less 
  acute, 
  and 
  scarcely 
  projects 
  beyond, 
  and 
  often 
  falls 
  short 
  of, 
  the 
  

   hinge-margin; 
  the 
  inequality 
  is 
  less 
  marked 
  than 
  in 
  most 
  of 
  the 
  allied 
  

   sjjecies. 
  The 
  body 
  of 
  the 
  shell 
  in 
  this 
  valve 
  is 
  smooth, 
  except 
  for 
  very 
  

   fine, 
  irregular 
  lines 
  of 
  growth; 
  on 
  the 
  anterior 
  auricle 
  there 
  are 
  from 
  

   three 
  to 
  six, 
  or 
  more, 
  distinct 
  radial 
  ridges, 
  roughened 
  by 
  conspicuous 
  

   lines 
  of 
  growth 
  ; 
  the 
  margin 
  below 
  the 
  byssal 
  notch 
  is 
  entire, 
  without 
  

   pectinidial 
  teeth; 
  the 
  posterior 
  auricle 
  is 
  nearly 
  smooth. 
  The 
  internal 
  

   hinge-plate 
  is 
  thin 
  in 
  the 
  middle, 
  but 
  relatively 
  broad 
  on 
  each 
  auricle, 
  

   and 
  is 
  crossed 
  by 
  numerous 
  fine, 
  well-marked, 
  transverse 
  striations; 
  

   these 
  are 
  much 
  more 
  conspicuous 
  than 
  in 
  any 
  of 
  the 
  related 
  species, 
  

   whether 
  young 
  or 
  old. 
  The 
  resilial 
  pit 
  is 
  small, 
  rounded, 
  situated 
  just 
  

   under 
  the 
  boiik. 
  The 
  inner 
  surface 
  is 
  smooth 
  and 
  glossy, 
  although 
  in 
  

   fresh 
  specimens 
  the 
  external 
  radiating 
  lines 
  show 
  through 
  by 
  transpar- 
  

   ency. 
  There 
  are 
  no 
  internal 
  lira\ 
  

  

  The 
  ground 
  color 
  of 
  the 
  right 
  valve 
  is 
  yellowish 
  or 
  grayish 
  white, 
  

   with 
  more 
  or 
  less 
  numerous 
  light 
  yellowish 
  brown 
  and 
  reddish 
  brown 
  

   spots 
  or 
  blotches, 
  and 
  sometimes 
  with 
  irregular 
  patches 
  of 
  opaque 
  white; 
  

   the 
  right 
  valve 
  is 
  white, 
  sometimes 
  with 
  a 
  few 
  yellowish 
  brown 
  spots. 
  

   Some 
  specimens 
  are 
  nearly 
  destitute 
  of 
  spots. 
  

  

  Length 
  of 
  one 
  of 
  the 
  largest 
  specimens, 
  7 
  mm.; 
  height, 
  mm.; 
  dor- 
  

   sal 
  hinge-margin, 
  4 
  mm. 
  

  

  It 
  has 
  been 
  taken 
  in 
  considerable 
  numbers, 
  live 
  and 
  dead, 
  at 
  three 
  

   stations 
  between 
  Is^. 
  lat. 
  37° 
  1' 
  40", 
  W. 
  long. 
  74^35' 
  40", 
  and 
  N. 
  lat. 
  35^ 
  

   42', 
  W. 
  long. 
  74° 
  54' 
  30", 
  in 
  43 
  to 
  13L> 
  fathoms, 
  1884. 
  

  

  Although 
  very 
  small, 
  this 
  species 
  seems 
  to 
  be 
  adult. 
  It 
  is 
  so 
  distinct 
  

   from 
  all 
  other 
  species 
  of 
  our 
  coast 
  that 
  a 
  detailed 
  comparison 
  is 
  unnec- 
  

   essary. 
  It 
  resembles 
  the 
  young 
  of 
  P. 
  elinton'us 
  more 
  than 
  any 
  other 
  

   native 
  species, 
  but 
  a 
  comparison 
  of 
  specimens 
  of 
  the 
  same 
  size 
  shows 
  

   marked 
  differences. 
  ' 
  

  

  