﻿DEEP-WATER 
  MOLLUSCA—VEBBILL 
  AND 
  BUSH. 
  831 
  

  

  Altbougli 
  this 
  gTouj) 
  agrees 
  with 
  Amusium 
  in 
  having 
  internal 
  ribs 
  

   without 
  corresponding 
  external 
  grooveSj 
  it 
  seems 
  to 
  be 
  allied 
  rather 
  to 
  

   Chlanij/s. 
  It 
  may 
  be 
  regarded 
  as 
  a 
  division 
  of 
  the 
  latter 
  in 
  which 
  the 
  

   external 
  radial 
  ribs 
  have 
  degenerated. 
  

  

  LEPTOPECTEN 
  Verrill, 
  1897. 
  

  

  LiplopiiU 
  II 
  (siihiicims 
  o( 
  ClilamyK) 
  Vkkuill, 
  Trams. 
  Comi. 
  Ac;i(l.. 
  X, 
  pp. 
  ti!(, 
  S!), 
  1(1 
  

  

  ISHT. 
  

  

  7///«'. 
  — 
  IjiptoiHcien 
  monothneriH 
  (Conrad 
  ). 
  

  

  Shell 
  thin, 
  translucent, 
  oblique, 
  broadly 
  rounded, 
  with 
  strong, 
  

   rounded 
  radial 
  ridges 
  or 
  folds, 
  like 
  corrugations, 
  which 
  appear 
  in 
  

   reverse 
  on 
  the 
  interior 
  surface. 
  The 
  internal 
  ribs 
  are 
  not 
  angulated 
  by 
  

   a 
  deposit 
  of 
  shell, 
  nor 
  distinctly 
  thickened. 
  Margin 
  with 
  broad 
  scal- 
  

   loi)s. 
  The 
  exterior 
  surface 
  is 
  covered 
  with 
  fine 
  divergent 
  camptonectes 
  

   sculpture, 
  botli 
  on 
  the 
  ribs 
  and 
  intervals. 
  The 
  ribs 
  do 
  not 
  increase 
  in 
  

   number 
  with 
  age 
  but 
  become 
  broader 
  and 
  more 
  flattened. 
  Auricles 
  

   large 
  and 
  broad, 
  thin, 
  corrugated. 
  Byssal 
  notch 
  large 
  and 
  deep. 
  Pec- 
  

   tinidial 
  teeth 
  prominent. 
  Hinge-plate 
  thin 
  and 
  but 
  little 
  differentiated. 
  

   Cardinal 
  ridge 
  thin 
  and 
  small, 
  close 
  to 
  the 
  ligament, 
  crossed 
  by 
  fine 
  

   incisions. 
  

  

  PLACOPECTEN 
  VerriU, 
  1897. 
  

  

  I'lacopcelin 
  (subgenus 
  of 
  Chlamys) 
  Verkill, 
  Trans. 
  Conn. 
  Acad., 
  X, 
  pp. 
  (>!>, 
  8!», 
  91, 
  

   pi. 
  XVIII. 
  ligs. 
  1-7: 
  pi. 
  XX, 
  ligs. 
  7, 
  8, 
  8a; 
  pi. 
  xxi. 
  figs. 
  l-2a, 
  1897. 
  

  

  Type. 
  — 
  Flacopecten 
  cJintonlus 
  (Say). 
  

  

  Shell 
  large, 
  compressed, 
  broadly 
  rounded, 
  rather 
  thin, 
  with 
  sim]»le 
  

   sharp 
  edges, 
  meeting 
  evenly 
  ventrally, 
  but 
  gaping 
  considerably 
  at 
  

   both 
  ends, 
  especially 
  when 
  adult. 
  Valves 
  only 
  slightly 
  unequal 
  in 
  

   form, 
  the 
  right 
  one 
  being 
  a 
  little 
  flatter, 
  but 
  they 
  differ 
  in 
  color 
  and 
  

   somewhat 
  in 
  sculpture, 
  the 
  right 
  one 
  being 
  smoother 
  and 
  paler. 
  Both 
  

   have 
  fine 
  radial 
  lines 
  or 
  riblets, 
  and 
  they 
  have 
  vermiculated 
  divergent 
  

   riblets 
  when 
  young. 
  Auricles 
  small, 
  symmetrical, 
  nearly 
  equal. 
  

   Byssal 
  notch 
  small, 
  simple. 
  I'ectinidial 
  teeth 
  generally 
  obsolete, 
  except 
  

   when 
  young. 
  'No 
  internal 
  ribs. 
  Inner 
  surface 
  often 
  with 
  more 
  or 
  less 
  

   pearly 
  luster 
  and 
  a 
  crystalline 
  structure. 
  Hinge-plate 
  with 
  two 
  feeble, 
  

   sliglitly 
  divergent 
  ribs 
  on 
  each 
  end, 
  crossed 
  by 
  fine 
  transverse 
  inci- 
  

   sions. 
  The 
  foot 
  ' 
  is 
  well 
  developed, 
  oblique, 
  slightly 
  narrowed 
  distally 
  

   and 
  enlarged 
  at 
  the 
  end, 
  where 
  it 
  is 
  divided 
  into 
  two 
  lobes 
  by 
  a 
  rather 
  

   deep, 
  oblique, 
  longitudinal 
  fissure, 
  so 
  that 
  the 
  lobes 
  can 
  be 
  spread 
  

   apart 
  or 
  closed 
  at 
  will, 
  thus 
  resembling 
  somewhat 
  the 
  foot 
  of 
  Ledida'. 
  

   Toward 
  the 
  base, 
  on 
  the 
  anterior 
  side, 
  there 
  is 
  also 
  a 
  short, 
  deep 
  byssal 
  

   slit, 
  terminating 
  at 
  a 
  prominent 
  tubercle 
  about 
  the 
  middle 
  of 
  the 
  front 
  

   side. 
  

  

  1 
  Trans. 
  Conn. 
  Aca«l., 
  X, 
  i)l. 
  xx, 
  fig. 
  8. 
  

  

  