﻿HAAS 
  : 
  ON 
  UNIONID^ 
  IJS" 
  THE 
  THAMES 
  YALLEY. 
  

  

  107 
  

  

  anatomical 
  and 
  embryological 
  characters 
  of 
  tbe 
  two 
  genera 
  in 
  question 
  

   tabulated 
  in 
  parallel 
  columns: 
  — 
  

  

  IInio. 
  Margaritana. 
  

  

  Animal 
  having 
  the 
  inner 
  bran- 
  

   chiae 
  free 
  from 
  the 
  abdominal 
  sac 
  

   for 
  from 
  one-half 
  to 
  their 
  entire 
  

   length 
  ; 
  gilU 
  united 
  to 
  the 
  mantle 
  

   heJiind 
  to 
  their 
  extreme 
  points 
  or 
  

   very 
  nearly 
  so 
  ; 
  papillce 
  on 
  bran- 
  

   chial 
  and 
  a?ial 
  openings 
  unhranched; 
  

   su'peranal 
  openings 
  alivays 
  closed 
  

   heloio} 
  

  

  Animal 
  Avith 
  very 
  long 
  gills, 
  

   inner 
  wider 
  in 
  front, 
  free 
  for 
  the 
  

   greater 
  part 
  of 
  their 
  length 
  from 
  

   the 
  abdominal 
  sac, 
  the 
  two 
  pans 
  

   united 
  to 
  their 
  posterior 
  ends, 
  ivhich 
  

   project 
  backivards 
  for 
  some 
  distance 
  

   unconnected 
  with 
  the 
  mantle 
  ; 
  bran- 
  

   chial 
  opening 
  having 
  crowded, 
  often 
  

   arborescent 
  papillm 
  arranged 
  in 
  

   folds 
  ; 
  anal 
  opening 
  smooth 
  or 
  

   crenulate 
  ; 
  superanal 
  opening 
  not 
  

   closed 
  below." 
  

  

  Eggs 
  developed 
  both 
  in 
  the 
  

   outer 
  and 
  inner 
  gills 
  of 
  the 
  female 
  

   into 
  aglochidium 
  only 
  0*0475mm. 
  

   in 
  length, 
  and 
  possessing 
  rounded, 
  

   extremely 
  convex 
  shells, 
  which, 
  

   when 
  closed, 
  give 
  it 
  a 
  globular 
  

   form 
  . 
  

  

  The 
  tivo 
  sides 
  of 
  the 
  shells 
  are 
  

   almost 
  equal. 
  True 
  hooks 
  are 
  

   ivanting. 
  In 
  their 
  place 
  one 
  

   observes 
  at 
  the 
  apex 
  of 
  the 
  shell 
  

   6-7 
  attenuated 
  hooks 
  bent 
  for- 
  

   ward 
  and 
  inward. 
  

  

  Larval 
  threads 
  (byssus 
  threads) 
  

   present.^ 
  

  

  Eggs 
  developed 
  only 
  in 
  the 
  

   outer 
  gills 
  of 
  the 
  female 
  into 
  

   a 
  glochidium 
  0'29mm. 
  in 
  length, 
  

   possessing 
  two 
  triangularly 
  

   rounded 
  flattened 
  shells 
  con- 
  

   nected 
  by 
  means 
  of 
  an 
  inner 
  basal 
  

   ligament 
  [adductor 
  muscle]. 
  

  

  Of 
  the 
  two 
  sides 
  of 
  the 
  triangle 
  

   the 
  anterior 
  is 
  the 
  greater. 
  At 
  

   the 
  apex 
  of 
  each 
  occurs 
  a 
  strong 
  

   hook, 
  bent 
  imvards, 
  which 
  is 
  pro- 
  

   vided 
  with 
  numerous 
  minute 
  

   teeth. 
  

  

  Larval 
  threads 
  (byssus 
  threads) 
  

   present.^ 
  

  

  To 
  return 
  to 
  ITtiio 
  sinuatus. 
  A 
  true 
  Unio, 
  the 
  hinge 
  being 
  provided 
  

   with 
  cardinals 
  and 
  laterals 
  and 
  described 
  in 
  1819 
  by 
  Lamarck* 
  as 
  

   follows: 
  " 
  C^w'o 
  testa 
  ovato-oblonga, 
  superne 
  coarctato-sinuata, 
  crassa; 
  

   natibus 
  subprominulis 
  ; 
  dente 
  cardinali 
  crasso, 
  lobato, 
  striate." 
  

  

  Mya 
  margaritiferai^), 
  Linne: 
  Gmelin, 
  p. 
  3219; 
  Lister, 
  Conch., 
  pl.cxlix, 
  

  

  fig. 
  4; 
  Knorr, 
  Vergn., 
  vol. 
  iv, 
  pi. 
  xxv, 
  fig. 
  2; 
  Schrciter, 
  Flussc, 
  

  

  pi. 
  iv, 
  fig. 
  I 
  ; 
  Drap., 
  Hist. 
  Moll., 
  p. 
  132, 
  pi. 
  x, 
  figs. 
  8, 
  16, 
  19 
  ; 
  

  

  Ency., 
  pi. 
  ccxlviii, 
  figs. 
  Iff, 
  b. 
  

  

  In 
  the 
  diagnosis 
  quoted 
  above 
  the 
  lateral 
  lamellae 
  are 
  not 
  mentioned. 
  

  

  This 
  circumstance, 
  together 
  with 
  the 
  citation 
  of 
  the 
  figures 
  of 
  

  

  Margaritana 
  margaritifera 
  of 
  Lister, 
  Schroter, 
  and 
  Draparnaud, 
  leads 
  

  

  to 
  the 
  conclusion 
  that 
  Unio 
  sinuatus 
  was 
  considered 
  to 
  be 
  related 
  

  

  to 
  TI. 
  ehngatus^ 
  (a 
  true 
  Margaritana 
  being, 
  in 
  fact, 
  the 
  narrow 
  

  

  1 
  After 
  Simpson, 
  1900, 
  Proc. 
  U.S. 
  Nat. 
  Museum, 
  vol. 
  xxii, 
  p. 
  680. 
  

  

  '^ 
  After 
  Simpson, 
  ibid., 
  p. 
  674. 
  

  

  » 
  After 
  Harms, 
  1909, 
  Zool. 
  Jahrb., 
  Abth. 
  fur 
  Anat. 
  u. 
  Ont., 
  Band 
  xxviii, 
  Heft 
  ii. 
  

  

  * 
  Lamarck, 
  1819, 
  Hist. 
  Xat. 
  Anim. 
  sans 
  Yert., 
  vol. 
  vi, 
  p. 
  70. 
  

  

  ^ 
  Loc. 
  cit., 
  p. 
  70. 
  

  

  