﻿Report 
  of 
  the 
  State 
  Geologist. 
  817 
  

  

  enclosed 
  by 
  the 
  growth 
  of 
  the 
  pedicle-valve, 
  and 
  further 
  modified 
  

   by 
  the 
  development 
  of 
  articulating 
  processes 
  where 
  it 
  comes 
  in 
  

   contact 
  with 
  the 
  brachial 
  valve. 
  It 
  therefore 
  follows, 
  as 
  a 
  natural 
  

   inference, 
  that 
  wherever 
  the 
  spondylium 
  is 
  present, 
  whether 
  in 
  

   the 
  incipient 
  condition 
  or 
  in 
  the 
  more 
  advanced 
  stage 
  of 
  develop- 
  

   ment 
  in 
  which 
  it 
  supports 
  all 
  the 
  muscles 
  of 
  the 
  valve, 
  it 
  is, 
  or 
  

   has, 
  at 
  some 
  period 
  of 
  growth, 
  been 
  accompanied 
  by 
  the 
  external 
  

   portion 
  of 
  the 
  sheath, 
  which 
  is 
  termed 
  the 
  deltidium. 
  Thus 
  the 
  

   spondylium 
  appears 
  to 
  be 
  but 
  the 
  complement 
  of 
  the 
  deltidium, 
  

   or 
  the 
  original 
  plate 
  formed 
  upon 
  the 
  body 
  of 
  the 
  embryo, 
  

   and 
  that 
  portion 
  of 
  the 
  adult 
  shell 
  to 
  which 
  the 
  term 
  deltidium 
  

   has 
  been 
  applied, 
  is 
  the 
  other 
  part 
  of 
  the 
  original 
  or 
  primitive 
  

   deltidial 
  plate 
  or 
  pedicle 
  sheath. 
  Here 
  again 
  our 
  terminology 
  

   seems 
  at 
  fault 
  and 
  should 
  be 
  further 
  adapted 
  to 
  the 
  proper 
  con- 
  

   ception 
  of 
  these 
  structures. 
  Should 
  the 
  term 
  prodeltidium 
  be 
  

   employed 
  for 
  the 
  primitive 
  body 
  plate 
  or 
  the 
  pedicle 
  sheath 
  in 
  

   its 
  entirety, 
  we 
  shall 
  then 
  have 
  the 
  terms 
  spondylium 
  and 
  delti- 
  

   dium 
  applied 
  to 
  corresponding 
  and 
  equivalent 
  modified 
  parts 
  of 
  

   this 
  plate, 
  the 
  former 
  internal, 
  the 
  latter 
  external. 
  

  

  The 
  adult 
  condition 
  of 
  the 
  shell 
  does 
  not 
  always 
  furnish 
  com- 
  

   plete, 
  and 
  sometimes 
  not 
  even 
  suggestive 
  evidence 
  of 
  the 
  relations 
  

   of 
  the 
  spondylium 
  and 
  deltidium. 
  For 
  example, 
  in 
  the 
  genus 
  

   Orthis 
  and 
  its 
  various 
  subdivisions, 
  the 
  delthyrium 
  is 
  almost 
  

   always 
  open 
  at 
  maturity 
  and 
  indeed 
  all 
  through 
  the 
  later 
  growth- 
  

   stages 
  of 
  the 
  shell. 
  The 
  deltidium 
  unquestionably 
  existed 
  at 
  an 
  

   early 
  stage 
  and 
  has 
  usually 
  become 
  resorbed 
  long 
  before 
  evidences 
  

   of 
  maturity 
  in 
  other 
  respects 
  are 
  assumed 
  ; 
  the 
  spondylium, 
  also, 
  

   does 
  not 
  pass 
  beyond 
  a 
  condition 
  which 
  makes 
  the 
  pedicle-cavity 
  

   a 
  clearly 
  defined 
  feature 
  of 
  the 
  interior. 
  In 
  more 
  elementary 
  or 
  

   less 
  modified 
  orthoid 
  structures 
  like 
  Billinosella, 
  Pr 
  >torthis, 
  

   and 
  the 
  Orthis 
  deflecta 
  and 
  0. 
  loricula 
  (see 
  Plate 
  V 
  a, 
  figs. 
  30, 
  31), 
  

   the 
  deltidium 
  is 
  fully 
  retained 
  at 
  maturity, 
  while 
  the 
  spondylium 
  

   remains 
  in 
  its 
  condition 
  of 
  a 
  simple 
  pedicle-cavity. 
  The 
  coexist- 
  

   ence 
  of 
  both 
  features 
  with 
  a 
  high 
  degree 
  of 
  development, 
  as 
  in 
  

   Clttamb 
  »nites, 
  Polytcechia, 
  etc., 
  indicates 
  a 
  more 
  primitive 
  con- 
  

   dition 
  than 
  in 
  Orthis, 
  though 
  in 
  such 
  cases 
  the 
  extension 
  of 
  the 
  

   spondylium 
  to 
  such 
  a 
  degree 
  as 
  to 
  carry 
  all 
  the 
  muscular 
  bands 
  

   of 
  the 
  pedicle-valve 
  must 
  be 
  regarded 
  as 
  a 
  secondary 
  modification 
  

   of 
  this 
  organ. 
  In 
  Pentamertts 
  and 
  allied 
  genera, 
  where 
  the 
  

   ]<-3 
  

  

  