﻿Report 
  of 
  the 
  State 
  Geologist. 
  1103 
  

  

  In 
  the 
  chapter 
  upon 
  the 
  " 
  Development 
  of 
  the 
  Shell 
  " 
  it 
  has 
  

   been 
  shown 
  that 
  the 
  deltidium 
  and 
  deltidial 
  plates, 
  though 
  

   similar 
  in 
  function, 
  are 
  profoundly 
  distinct 
  both 
  in 
  origin 
  and 
  

   structure. 
  Tae 
  former 
  is 
  primitive 
  and 
  fundamental, 
  the 
  latter 
  

   wholly 
  secondary 
  ; 
  a 
  replacement 
  of, 
  but 
  never 
  a 
  derivative 
  from 
  

   the 
  former. 
  They 
  may 
  resemble 
  each 
  other, 
  as 
  the 
  so-called 
  

   pseudodeltidium 
  of 
  Cye'ina 
  is 
  externally 
  like 
  the 
  true 
  deltidium 
  

   of 
  Clitamboxites, 
  Strophomena 
  and 
  Rafjnesquina, 
  but 
  this 
  is 
  

   purely 
  a 
  superficial 
  similarity. 
  The 
  pseudodeltidium 
  consists, 
  in 
  

   its 
  early 
  condition, 
  of 
  discrete 
  deltidial 
  plates 
  and 
  in 
  its 
  mature 
  

   state 
  it 
  expresses 
  simply 
  a 
  firm 
  coalescence 
  of 
  those 
  plates. 
  The 
  

   term 
  pseudodeltidium 
  is 
  a 
  convenient 
  one 
  to 
  employ 
  for 
  this 
  

   peculiar 
  phase 
  of 
  the 
  deltidial 
  plates; 
  but 
  we 
  have 
  proposed 
  to 
  

   distinguish 
  the 
  latter 
  generally 
  by 
  the 
  use 
  of 
  the 
  term 
  deltarium 
  

   in 
  application 
  to 
  the 
  parts 
  as 
  a 
  whole, 
  whether 
  coalesced 
  

   (Cyrtina, 
  Nttcleospira, 
  Retzia, 
  etc.), 
  or 
  discrete, 
  and 
  deltaria 
  in 
  

   referring 
  to 
  the 
  component 
  plates. 
  The 
  reader 
  is 
  again 
  referred 
  

   to 
  the 
  chapter 
  already 
  cited 
  for 
  the 
  full 
  explanation 
  of 
  the 
  

   differences 
  in 
  the 
  deltidium 
  and 
  deltarium 
  as 
  worked 
  out 
  

   by 
  Beecher, 
  in 
  which 
  it 
  is 
  demonstrated 
  that 
  the 
  former 
  is, 
  

   in 
  a 
  certain 
  sense, 
  a 
  third 
  valve, 
  not 
  forming 
  on 
  the 
  

   mantle 
  lobes 
  of 
  the 
  young 
  as 
  do 
  the 
  true 
  valves, 
  but 
  upon 
  

   the 
  body 
  of 
  the 
  embryo, 
  enclosing 
  the 
  cardinal 
  space 
  between 
  

   the 
  latter. 
  This 
  solid, 
  continuous, 
  never 
  divided 
  plate 
  or 
  pedicle- 
  

   sheath 
  remains 
  throughout 
  all 
  growth-stages 
  in 
  a 
  large 
  division 
  

   of 
  the 
  Articulate 
  genera 
  mostly 
  of 
  early 
  age, 
  while 
  in 
  a 
  coexten- 
  

   sive 
  group 
  predominating 
  throughout 
  the 
  later 
  history 
  of 
  the 
  

   class, 
  this 
  primitive 
  structure 
  is 
  resorbed 
  at 
  an 
  early 
  stage 
  of 
  

   growth, 
  and 
  the 
  enclosure 
  of 
  the 
  pedicle 
  effected 
  by 
  the 
  forma- 
  

   tion 
  of 
  discrete 
  secondary 
  plates 
  which 
  originate 
  from 
  the 
  mantle 
  

   lobes 
  and 
  not 
  from 
  the 
  body 
  of 
  the 
  shell. 
  

  

  The 
  term 
  spondylium 
  has 
  been 
  applied 
  to 
  the 
  spoon-shaped 
  

   plate 
  which 
  frequently 
  occurs 
  in 
  ihe 
  pedicle- 
  valve 
  of 
  many 
  of 
  the 
  

   Articulates 
  (Pentamerus, 
  Clitamb 
  >nites). 
  A 
  plate 
  of 
  similar 
  

   character 
  appearing 
  in 
  the 
  brachial 
  valve 
  of 
  the 
  same 
  groups 
  of 
  

   genera 
  has 
  been 
  referred 
  to 
  under 
  the 
  same 
  name, 
  but 
  it 
  is 
  found 
  

   that 
  these 
  plates 
  in 
  the 
  different 
  valves 
  are 
  similar 
  neither 
  in 
  

   origin 
  or 
  function 
  ; 
  hence 
  that 
  of 
  the 
  brachial 
  valve 
  is 
  now 
  distin- 
  

   guished 
  by 
  the 
  term 
  cruralium. 
  In 
  the 
  Inarticulate 
  Brachio- 
  

  

  161 
  

  

  