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  line 
  of 
  the 
  dorsal 
  surface, 
  and 
  these 
  upper 
  edges 
  are 
  united 
  

   by 
  the 
  middle 
  portion 
  of 
  the 
  primitive 
  shell, 
  which 
  becomes 
  

   converted 
  into 
  the 
  hinge 
  ligament. 
  

  

  In 
  Auodonta, 
  and 
  apparently 
  in 
  Unio, 
  the 
  process 
  is 
  some- 
  

   what 
  similar, 
  but 
  each 
  calcareous 
  valve 
  is 
  formed 
  by 
  the 
  union 
  

   of 
  a 
  number 
  of 
  patches 
  which 
  are 
  deposited 
  on 
  the 
  inner 
  sur- 
  

   face 
  of 
  the 
  embryonic 
  shell 
  at 
  several 
  centres 
  of 
  calcification. 
  

  

  In 
  the 
  oyster 
  the 
  primitive 
  shell 
  appears 
  to 
  be 
  wanting, 
  and 
  

   the 
  two 
  valves 
  of 
  the 
  shell 
  correspond 
  to 
  the 
  two 
  centres 
  of 
  

   calcification 
  which 
  are 
  present 
  in 
  Cyclas. 
  

  

  The 
  fact 
  that 
  the 
  primitive 
  shell 
  of 
  Cyclas 
  or 
  of 
  Anodonta 
  

   closely 
  resembles 
  the 
  embryonic 
  shell 
  of 
  a 
  Gasteropod, 
  a 
  Ptero- 
  

   pod 
  or 
  a 
  Cephalopod 
  in 
  appearance 
  and 
  shape, 
  as 
  well 
  as 
  in 
  

   position 
  and 
  method 
  of 
  formation, 
  would 
  seem 
  to 
  indicate 
  

   that 
  the 
  manner 
  in 
  which 
  the 
  shell 
  of 
  Cyclas 
  is 
  formed 
  is 
  the 
  

   primitive 
  method, 
  and 
  the 
  process, 
  in 
  the 
  oyster, 
  a 
  secondary 
  

   modification. 
  

  

  THE 
  MANTLE, 
  

  

  The 
  manner 
  in 
  which 
  the 
  lobes 
  and 
  cavity 
  of 
  the 
  mantle 
  

   are 
  formed 
  in 
  the 
  oyster 
  appears 
  to 
  diifer 
  slightly 
  from 
  the 
  

   process 
  as 
  it 
  is 
  exhibited 
  during 
  the 
  development 
  of 
  the 
  fresh- 
  

   water 
  Lamellibranchs. 
  

  

  In 
  Anodonta 
  the 
  mantle 
  cavity 
  is 
  formed 
  by 
  an 
  invagina- 
  

   tion 
  of 
  the 
  body 
  wall 
  between 
  the 
  free 
  ventral 
  edges 
  of 
  the 
  

   valves, 
  and 
  in 
  Cyclas 
  the 
  mantle 
  folds 
  off 
  from 
  the 
  sides 
  of 
  

   the 
  body 
  in 
  advance 
  of 
  the 
  growing 
  shell. 
  The 
  process 
  in 
  the 
  

   oyster 
  is 
  about 
  half-way 
  between 
  that 
  in 
  Cyclas 
  and 
  that 
  in 
  

   Anodonta. 
  The 
  mantle 
  is 
  formed 
  as 
  a 
  ridge 
  or 
  fold 
  of 
  the 
  in- 
  

   tegument 
  on 
  each 
  side 
  of 
  the 
  body, 
  but 
  this 
  ridge 
  is 
  situated 
  

   at 
  the 
  ventral 
  edge 
  of 
  the 
  shell, 
  not, 
  as 
  in 
  Cyclas, 
  in 
  advance 
  

   of 
  it. 
  

  

  LIST 
  OF 
  PAPERS 
  QUOTED. 
  

  

  The 
  following 
  is 
  a 
  list 
  of 
  the 
  papers 
  to 
  which 
  I 
  have 
  re- 
  

   ferred, 
  or 
  from 
  which 
  I 
  have 
  quoted, 
  in 
  the 
  preceding 
  paper 
  : 
  

  

  History 
  of 
  the 
  Oyster 
  and 
  Oyster 
  Fisheries. 
  By 
  T. 
  C. 
  

   Eyton. 
  London: 
  18.58. 
  

  

  