﻿818 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  spondylium 
  attains 
  its 
  greatest 
  development, 
  the 
  deltidium 
  is- 
  

   usually 
  lost, 
  but 
  when 
  retained 
  is 
  very 
  thin 
  and 
  has 
  a 
  concave 
  

   exterior, 
  a 
  form 
  doubtless 
  largely 
  due 
  to 
  the 
  arching 
  of 
  the 
  umbo- 
  

   of 
  the 
  pedicle-valve 
  over 
  the 
  full, 
  procumbent 
  beak 
  of 
  the 
  brachial 
  

   valve. 
  The 
  spondylium 
  occurs 
  in 
  various 
  modified 
  conditions 
  ; 
  

   in 
  cases 
  where 
  the 
  teeth 
  are 
  wholly 
  without 
  dental 
  lamellae, 
  or 
  

   where 
  such 
  lamellae 
  do 
  not 
  extend 
  to 
  the 
  bottom 
  of 
  the 
  valve, 
  it 
  

   seems 
  necessary 
  to 
  regard 
  them 
  as 
  instances 
  of 
  degeneracy 
  or 
  

   resorption 
  of 
  the 
  primitive 
  spondylium. 
  As 
  the 
  growth, 
  modifi- 
  

   cation 
  and 
  disappearance 
  of 
  the 
  differential 
  parts 
  of 
  the 
  prodel- 
  

   tidium 
  do 
  not 
  progress 
  pari 
  passu, 
  there 
  will 
  frequently 
  be 
  

   examples 
  of 
  one 
  being 
  retained 
  when 
  the 
  other 
  has 
  disappeared. 
  

   A 
  remarkable 
  illustration 
  of 
  this 
  fact 
  is 
  afforded 
  by 
  the 
  genus 
  

   Camarophoeia, 
  which 
  possesses 
  a 
  highly 
  developed 
  spondylium, 
  

   while 
  the 
  deltidium 
  has 
  been 
  resorbed 
  and 
  secondary 
  deltidial 
  

   plates 
  or 
  deltaria 
  formed 
  about 
  the 
  pedicle-passage. 
  

  

  In 
  the 
  fundamental 
  division 
  of 
  the 
  Articulata 
  two 
  groups 
  will 
  

   be 
  recognized, 
  one 
  embracing 
  those 
  forms 
  in 
  which 
  the 
  prodel- 
  

   tidium 
  is 
  represented 
  by 
  the 
  deltidium 
  and 
  spondylium, 
  one 
  or 
  

   both 
  ; 
  the 
  other 
  a 
  group 
  in 
  which 
  the 
  prodeltidium 
  has 
  been 
  fully 
  

   modified, 
  resorbed 
  or 
  replaced. 
  The 
  former 
  group 
  is 
  equivalent 
  

   to 
  Waagen's 
  suborder, 
  Aphaneeopegmata 
  (1883), 
  with 
  the 
  addi- 
  

   tion 
  of 
  Thecidea 
  and 
  its 
  allies, 
  and 
  to 
  Beechee's 
  Peotemata 
  

   (1891), 
  excepting 
  the 
  genus 
  Teopidoleptus. 
  So 
  deep-seated 
  does 
  

   this 
  difference 
  in 
  these 
  groups 
  of 
  genera 
  appear 
  to 
  be, 
  that 
  exam- 
  

   ples 
  of 
  such 
  combinations 
  of 
  primary 
  and 
  secondary 
  conditions 
  

   as 
  shown 
  by 
  Camarophoeia, 
  are 
  of 
  the 
  rarest 
  occurrence. 
  

  

  The 
  spoon- 
  shaped 
  process 
  of 
  the 
  brachial- 
  valve, 
  which 
  has 
  been 
  

   termed 
  the 
  cruralium, 
  is 
  a 
  feature 
  of 
  more 
  fugitive 
  value. 
  It 
  is 
  

   formed 
  by 
  the 
  convergence 
  or 
  union 
  of 
  the 
  crural 
  plates, 
  and 
  it 
  

   may 
  rest 
  upon 
  the 
  inner 
  surface 
  of 
  the 
  valve, 
  or 
  like 
  the 
  spondy- 
  

   lium, 
  be 
  supported 
  by 
  a 
  median 
  septum. 
  More 
  often 
  the 
  crural 
  

   plates, 
  when 
  highly 
  developed, 
  stand 
  erect 
  upon 
  the 
  valve 
  and 
  

   do 
  not 
  unite, 
  but 
  their 
  position 
  is 
  highly 
  variable, 
  and 
  it 
  has 
  been 
  

   shown 
  that 
  in 
  Pentameeus, 
  Conch 
  idium, 
  and 
  their 
  allied 
  forms, 
  the 
  

   union 
  of 
  these 
  plates 
  is 
  not 
  of 
  first 
  importance 
  as 
  a 
  generic 
  char- 
  

   acter. 
  When 
  the 
  crural 
  plates 
  extend 
  to 
  the 
  bottom 
  of 
  the 
  valve 
  as 
  

   distinct 
  septa, 
  they 
  simply 
  inclose 
  an 
  extension 
  of 
  the 
  median 
  incis- 
  

   ion 
  of 
  the 
  hinge-plate. 
  It 
  has 
  become 
  evident, 
  from 
  a 
  study 
  of 
  the 
  

  

  