﻿886 
  rilOCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSE 
  CM. 
  vol.xx. 
  

  

  general 
  outline 
  of 
  this 
  end 
  is 
  broadly 
  truncate; 
  the 
  ventral 
  margin 
  is 
  

   somewhat 
  convex 
  but 
  slopes 
  upward 
  toward 
  the 
  posterior 
  end 
  and 
  is 
  

   nearly 
  straight 
  along 
  the 
  middle; 
  the 
  posterior 
  end 
  is 
  short, 
  evenly 
  

   rounded, 
  with 
  the 
  dorsal 
  margin, 
  behind 
  the 
  beaks, 
  strongly 
  incurved; 
  

   the 
  large 
  black 
  ligament 
  which 
  occupies 
  this 
  area 
  is 
  continuous 
  with 
  the 
  

   epidermis, 
  so 
  that 
  its 
  outline 
  forms 
  a 
  curve 
  nearly 
  in 
  line 
  with 
  that 
  of 
  

   the 
  anterior 
  end, 
  and 
  shows 
  but 
  a 
  slight 
  angle, 
  or 
  lobe, 
  at 
  the 
  outer 
  end 
  

   of 
  the 
  dorsal 
  line. 
  The 
  umbos 
  are 
  flattened, 
  and 
  scarcely 
  prominent. 
  

   The 
  whole 
  surface 
  is 
  covered 
  with 
  a 
  thick, 
  smooth, 
  glossy 
  ei)idermis, 
  

   chestnut-brown 
  in 
  the 
  young 
  and 
  brownish 
  black 
  in 
  adult, 
  which 
  ante- 
  

   riorly 
  is 
  divided 
  into 
  several 
  rather 
  broad 
  digitations 
  that 
  are 
  shorter 
  

   and 
  sonjewhat 
  wider 
  ventrally, 
  their 
  length 
  diminishing 
  from 
  the 
  mid- 
  

   dle 
  of 
  the 
  anterior 
  end 
  to 
  the 
  \entral 
  margin, 
  along 
  the 
  middle 
  ot 
  which 
  

   there 
  are 
  no 
  digitations, 
  but 
  short 
  and 
  broad 
  ones 
  again 
  appear 
  pos- 
  

   teriorly. 
  The 
  shell 
  is 
  sculptured 
  by 
  radiating 
  ribs 
  and 
  furrows 
  which 
  

   are 
  but 
  slightly 
  developed 
  on 
  the 
  middle 
  region 
  but 
  become 
  large 
  and 
  

   strong 
  anteriorly 
  and 
  smaller 
  and 
  closer 
  posteriorly. 
  On 
  the 
  anterior 
  

   part 
  these 
  ribs 
  are 
  broad 
  and 
  tlat, 
  separated 
  by 
  wMde, 
  flat-bottomed, 
  

   furrows 
  often 
  nearly 
  as 
  wide 
  as 
  the 
  ribs; 
  on 
  the 
  middle 
  area, 
  the 
  lur- 
  

   rows 
  are 
  shallow 
  and 
  lounded 
  while 
  the 
  intervening 
  spaces 
  are 
  flat, 
  

   sometimes 
  broad, 
  at 
  others 
  narrow, 
  this 
  region 
  api)eariug 
  comparatively 
  

   smooth, 
  some 
  specimens 
  showing 
  but 
  slight 
  indications 
  of 
  grooves 
  and 
  

   ribs; 
  posteriorly 
  these 
  are 
  somewhat 
  more 
  numerous, 
  narrower, 
  often 
  

   about 
  equal 
  in 
  width. 
  In 
  specimens 
  of 
  medium 
  size, 
  there 
  are 
  from 
  six 
  

   to 
  eight 
  of 
  the 
  large 
  anterior 
  furrows 
  and 
  as 
  many 
  ribs; 
  the 
  edges 
  of 
  

   the 
  latter 
  are 
  somewhat 
  elevated 
  above 
  the 
  middle 
  portion 
  and 
  these 
  

   thickened 
  margins 
  extend 
  out 
  along 
  the 
  edges 
  of 
  the 
  digitations 
  which 
  

   otherwise 
  conespond 
  to 
  the 
  furrows. 
  Internally 
  the 
  shell 
  is 
  white, 
  

   moderately 
  thick, 
  the 
  anterior 
  portion 
  oblong 
  with 
  obtusely 
  truncated 
  

   end, 
  the 
  dorsal 
  and 
  ventral 
  margins 
  nearly 
  parallel, 
  and 
  the 
  posterior 
  

   much 
  narrower 
  and 
  tapered 
  to 
  an 
  obtusely 
  rounded 
  end, 
  with 
  the 
  

   dorsal 
  margin 
  excavated 
  for 
  the 
  ligamental 
  area. 
  The 
  surface 
  shows 
  

   distinct 
  but 
  not 
  very 
  prominent 
  grooves 
  and 
  ridges 
  corresponding 
  to 
  

   the 
  exterior 
  ones; 
  at 
  the 
  anteiior 
  end 
  the 
  margin 
  shows 
  slight 
  lobes, 
  

   corres[)onding 
  to 
  the 
  intervals 
  between 
  the 
  epidermal 
  digitations. 
  The 
  

   anterior 
  muscular 
  scar 
  is 
  large 
  and 
  rounded, 
  the 
  posterior 
  one 
  is 
  

   smaller 
  and 
  subovate. 
  Anteriorly 
  the 
  hinge-margin 
  is 
  thickened 
  in 
  

   both 
  valves, 
  running 
  from 
  near 
  the 
  beak 
  nearly 
  to 
  the 
  end; 
  posteriorly 
  

   it 
  is 
  more 
  strongly 
  thickened 
  by 
  a 
  sinuous 
  callus 
  to 
  which 
  the 
  liga- 
  

   ment 
  is 
  attached, 
  while 
  under 
  and 
  Just 
  in 
  front 
  of 
  the 
  beak 
  the 
  margin 
  

   is 
  excavated 
  for 
  the 
  reception 
  of 
  the 
  cartilage 
  which 
  continues 
  forward 
  

   in 
  a 
  groove 
  and 
  is 
  continuous 
  with 
  the 
  broad, 
  dorsal, 
  epidermal 
  mar- 
  

   gin 
  which 
  unites 
  the 
  two 
  valves 
  throughout 
  their 
  length. 
  The 
  posterior 
  

   ligament 
  and 
  anterior 
  cartilage 
  appear 
  to 
  blend 
  Just 
  beneath 
  the 
  beaks; 
  

   the 
  commencement 
  of 
  the 
  cartilage 
  is, 
  however, 
  indicated 
  by 
  a 
  slight 
  

   notch 
  in 
  the 
  callus-margin, 
  in 
  both 
  valves, 
  and 
  the 
  ligament 
  appears 
  

  

  