﻿THE 
  TERTIARY 
  FORMATIONS 
  OF 
  YIRGINIA. 
  323 
  

  

  they 
  are 
  found. 
  Thus 
  the 
  corresponding 
  valves 
  are 
  very 
  often 
  found 
  

   together 
  and 
  closely 
  shut. 
  Many 
  of 
  the 
  smaller 
  shells, 
  such 
  as 
  Area 
  

   centenaria. 
  Area 
  incile, 
  Nuculae, 
  Venericardia 
  alticosta, 
  and 
  Chama 
  

   congregata, 
  which 
  are 
  most 
  usually 
  thus 
  found, 
  are 
  often 
  either 
  en- 
  

   tirely 
  empty, 
  or 
  contain 
  a 
  small 
  quantity 
  of 
  clay 
  that 
  is 
  quite 
  impal- 
  

   pable 
  ; 
  indicating 
  plainly 
  that 
  they 
  have 
  been 
  exposed 
  to 
  no 
  violence, 
  

   and 
  that 
  only 
  such 
  solid 
  matter 
  as 
  could 
  pass 
  between 
  the 
  edges 
  of 
  the 
  

   closed 
  valves 
  had 
  obtained 
  access 
  to 
  the 
  interior. 
  Whenever 
  such 
  

   shells, 
  however, 
  have 
  been 
  previously 
  drilled, 
  as 
  is 
  very 
  frequently 
  the 
  

   case 
  even 
  with 
  the 
  largest 
  and 
  thickest 
  shells, 
  the 
  interior 
  is 
  found 
  

   entirely 
  filled 
  with 
  sand, 
  clay, 
  green 
  sand, 
  and 
  small 
  fragments 
  of 
  shell. 
  

   In 
  most 
  cases 
  the 
  larger 
  species 
  of 
  shells, 
  even 
  when 
  their 
  valves 
  appear 
  

   to 
  be 
  in 
  accurate 
  juxtaposition, 
  is 
  thus 
  filled, 
  and 
  in 
  this 
  case 
  it 
  cannot 
  

   be 
  supposed 
  that 
  the 
  contained 
  matter 
  has 
  entered 
  through 
  the 
  holes 
  

   thus 
  drilled, 
  since 
  in 
  many 
  instances 
  shells 
  of 
  considerable 
  magnitude 
  

   are 
  found 
  imprisoned 
  within. 
  Such 
  shells, 
  no 
  doubt, 
  after 
  the 
  death 
  

   of 
  the 
  animal, 
  remained 
  open, 
  or 
  at 
  least 
  partially 
  so, 
  and 
  received 
  the 
  

   sand, 
  clay 
  and 
  other 
  materials 
  which 
  they 
  contain 
  by 
  the 
  gentle 
  action 
  

   of 
  the 
  waves. 
  The 
  ligament 
  at 
  the 
  hinge 
  in 
  the 
  mean 
  time 
  would 
  

   decay, 
  until 
  at 
  length 
  yielding 
  to 
  the 
  pressure 
  of 
  the 
  accumulating 
  

   matter 
  above 
  the 
  shell, 
  in 
  favourable 
  circumstances 
  would 
  collapse 
  into 
  

   its 
  natural 
  closed 
  condition. 
  

  

  The 
  very 
  common 
  occurrence 
  of 
  the 
  valves 
  in 
  juxtaposition, 
  is 
  a 
  

   striking 
  proof 
  that 
  during 
  or 
  subsequent 
  to 
  their 
  deposition 
  they 
  

   have 
  not 
  been 
  exposed 
  to 
  violent 
  agencies. 
  This 
  becomes 
  even 
  more 
  

   remarkable 
  in 
  the 
  case 
  of 
  such 
  shells 
  as 
  the 
  Panopea 
  reflexa, 
  which 
  

   almost 
  in 
  every 
  instance 
  is 
  found 
  with 
  the 
  valves 
  properly 
  united. 
  

   The 
  connection 
  between 
  the 
  two 
  valves 
  in 
  this 
  shell 
  is 
  the 
  slightest 
  

   imaginable 
  after 
  the 
  destruction 
  of 
  the 
  natural 
  organic 
  bond, 
  and 
  an 
  

   inconsiderable 
  force 
  would 
  have 
  sufficed 
  to 
  separate 
  and 
  break 
  the 
  

   valves. 
  

  

  10. 
  The 
  admirable 
  preservation 
  of 
  the 
  shells 
  in 
  many 
  cases 
  is 
  also 
  

   an 
  interesting 
  fact, 
  and 
  affords 
  another 
  evidence 
  of 
  the 
  absence 
  of 
  all 
  

   violent 
  agencies 
  at 
  this 
  period. 
  The 
  most 
  fragile 
  species 
  of 
  Natica, 
  

   delicate 
  Tellinae, 
  Mactra 
  tellinoides, 
  the 
  shell 
  and 
  processes 
  of 
  the 
  

   Crepidula, 
  the 
  minute 
  and 
  sharp 
  angles 
  of 
  the 
  Fusus 
  tetricus, 
  the 
  thin 
  

  

  