﻿820 
  rROCEEDlNGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM. 
  

  

  left 
  valve) 
  ; 
  the 
  i)ostero-dorsal 
  margin 
  is 
  usually 
  slightly 
  concave, 
  but 
  

   is 
  soiiietinies 
  nearly 
  straight, 
  and 
  in 
  some 
  cases 
  decidedly 
  concave, 
  and 
  

   slopes 
  gradually 
  to 
  the 
  superior 
  angulation 
  of 
  the 
  rostrum; 
  the 
  liga- 
  

   mental 
  area 
  is 
  narrow, 
  deep, 
  and 
  long, 
  extending 
  for 
  nearly 
  the 
  entire 
  

   length 
  of 
  the 
  dorsiil 
  margin, 
  and 
  is 
  clearly 
  defined 
  by 
  a 
  marginal 
  ridge 
  

   which 
  is 
  sharper 
  on 
  the 
  left 
  valve. 
  In 
  most 
  specimens 
  this 
  valve 
  is 
  

   marked 
  by 
  a 
  slight, 
  ill-defined 
  groove 
  running 
  from 
  the 
  beak 
  to 
  the 
  

   autero- 
  ventral 
  margin, 
  where 
  it 
  often 
  forms 
  a 
  slight 
  emargination, 
  but 
  

   is 
  often 
  scarcely 
  discernible, 
  except 
  by 
  the 
  change 
  in 
  the 
  direction 
  of 
  the 
  

   lines 
  of 
  growth 
  and 
  character 
  of 
  the 
  epidermis. 
  The 
  surface 
  of 
  the 
  

   left 
  valve 
  is 
  covered 
  with 
  irregular 
  concentric 
  ridges 
  and 
  rather 
  uneven 
  

   lines 
  of 
  growth; 
  the 
  right 
  valve 
  usually 
  shows 
  rather 
  regular, 
  concen- 
  

   tric 
  undulations 
  on 
  which 
  are 
  numerous 
  fine, 
  pretty 
  regular, 
  lines 
  of 
  

   growth; 
  this 
  valve 
  is 
  also 
  usually 
  marked 
  by 
  faint, 
  and 
  rather 
  indis- 
  

   tinct, 
  radiating, 
  impressed 
  lines 
  which 
  are 
  more, 
  or 
  less 
  broken 
  and 
  

   often 
  branched 
  or 
  forked; 
  these 
  are 
  scarcely 
  visible 
  without 
  a 
  lens. 
  

   Ei)iderniis 
  thin, 
  brownish 
  yellow, 
  usually 
  mostly 
  peeled 
  off 
  in 
  dry 
  speci- 
  

   mens, 
  but 
  on 
  the 
  anterior 
  end, 
  in 
  front 
  of 
  the 
  radial 
  groove, 
  it 
  is 
  a 
  

   little 
  more 
  persistent. 
  In 
  the 
  left 
  valve 
  the 
  anterior 
  tooth 
  is 
  strong 
  

   and 
  prominent, 
  with 
  the 
  proximal 
  end 
  the 
  thicker, 
  more 
  elevated, 
  

   rounded 
  or 
  clavate; 
  distally 
  it 
  is 
  curved 
  and 
  diverges 
  considerbly 
  

   from 
  the 
  antero-dorsal 
  margin; 
  between 
  this 
  tooth 
  and 
  the 
  resilial 
  pit, 
  

   there 
  is 
  a 
  small 
  central 
  tooth 
  only 
  a 
  little 
  elevated. 
  The 
  resilial 
  pit 
  

   is 
  directed 
  obliquely 
  backward, 
  and 
  its 
  cavity 
  is 
  oblicpiely 
  upturned, 
  

   forming 
  a 
  distinct 
  excavation 
  on 
  the 
  inner 
  surface 
  of 
  the 
  posterior 
  

   tooth 
  with 
  which 
  it 
  is 
  confluent; 
  this 
  posterior 
  tooth 
  is 
  simply 
  a 
  dis- 
  

   tinctly 
  thickened 
  and 
  slightly 
  elevated 
  portion 
  of 
  the 
  postero-dorsal 
  

   margin, 
  which 
  forms 
  the 
  boundary 
  of 
  the 
  ligamental 
  area, 
  it 
  is 
  often, 
  

   but 
  not 
  always, 
  opposite 
  the 
  position 
  of 
  the 
  resilial 
  pit. 
  In 
  the 
  right 
  

   valve 
  the 
  anterior 
  tooth 
  is 
  a 
  slightly 
  raised, 
  somewhat 
  curved 
  ridge 
  on 
  

   the 
  inner 
  surface 
  of 
  the 
  shell, 
  running 
  to 
  tlie 
  middle 
  of 
  the 
  anterior 
  

   muscular 
  scar; 
  the 
  central 
  tooth 
  is 
  shorter 
  and 
  much 
  more 
  elevated, 
  

   most 
  i)rominent 
  at 
  its 
  inner 
  end; 
  between 
  these 
  two 
  teeth 
  there 
  is 
  an 
  

   additional, 
  slightly 
  raised, 
  tooth-like 
  ridge; 
  the 
  posterior 
  tooth 
  is 
  about 
  

   as 
  long 
  as 
  the 
  anterior, 
  and 
  much 
  more 
  elevated 
  and 
  stouter, 
  its 
  distal 
  

   end 
  being 
  the 
  thicker 
  and 
  higher, 
  with 
  a 
  distinct 
  angular 
  summit; 
  the 
  

   oblique 
  resilial 
  pit 
  is 
  excavated 
  out 
  of 
  its 
  anterior 
  surface. 
  The 
  ossicle 
  

   is 
  somewhat 
  elongated, 
  curved 
  or 
  crescent 
  shaped. 
  The 
  interior 
  of 
  the 
  

   shell 
  is 
  only 
  slightly 
  lustrous 
  and 
  shows 
  but 
  little 
  iridescence. 
  

  

  Length 
  of 
  one 
  of 
  the 
  largest 
  specimens, 
  19 
  mm.; 
  height, 
  11 
  mm.; 
  

   thickness, 
  about 
  3 
  mm. 
  

  

  Found 
  in 
  considerable 
  numbers 
  at 
  twenty-three 
  stations, 
  north 
  of 
  

   Cape 
  Cod, 
  off 
  Stellwagens 
  Bank, 
  and 
  oft' 
  Chatham, 
  in 
  10 
  to 
  43 
  fathoms, 
  

   1872-1881. 
  

  

  This 
  species, 
  which 
  is 
  common 
  in 
  the 
  vicinity 
  of 
  Cape 
  Cod, 
  has 
  prob- 
  

   ably 
  been 
  confounded, 
  hitherto, 
  with 
  C. 
  trilineata 
  Say, 
  and 
  C. 
  gonl- 
  

   diana 
  Dall. 
  From 
  the 
  latter, 
  which 
  occurs 
  abundantly 
  in 
  the 
  same 
  

  

  