﻿Report 
  of 
  the 
  State 
  Geologist. 
  1105 
  

  

  or 
  primitive 
  deltidial 
  plate 
  formed 
  upon 
  the 
  body 
  of 
  the 
  embryo 
  

   (prodeltidium). 
  

  

  The 
  transverse 
  plate 
  or 
  platform 
  in 
  the 
  brachial 
  valve 
  of 
  the 
  

   Particulates 
  is 
  evidently 
  homologous 
  in 
  origin 
  and 
  function 
  with 
  

   that 
  of 
  the 
  pedicle-valve 
  in 
  the 
  same 
  group. 
  On 
  the 
  other 
  hand. 
  

   the 
  spoon-shaped 
  process 
  or 
  cruraliwr/i 
  in 
  the 
  brachial 
  valve 
  of 
  

   the 
  Articulates 
  I 
  Pextameeus, 
  etc.), 
  is 
  a 
  totally 
  different 
  structure 
  

   from 
  the 
  spondylium, 
  similar 
  in 
  function 
  but 
  of 
  more 
  fugitive 
  

   valve. 
  It 
  has 
  originated 
  from 
  the 
  convergence 
  and 
  union 
  of 
  

   the 
  crural 
  plates, 
  and 
  it 
  may, 
  like 
  the 
  spondylium. 
  rest 
  upon 
  the 
  

   inner 
  surface 
  of 
  the 
  valve 
  or 
  be 
  supported 
  by 
  a 
  median 
  septum. 
  

  

  The 
  cardinal 
  area 
  is 
  a 
  feature 
  of 
  which 
  we 
  find 
  a 
  trace 
  spas- 
  

   modically 
  among 
  the 
  Inarticulates. 
  and. 
  in 
  the 
  Articulates, 
  is 
  more 
  

   generally 
  developed 
  in 
  the 
  deltidium-bearing 
  genera, 
  though 
  it 
  

   may 
  be 
  said 
  that 
  in 
  this 
  group 
  also 
  it 
  is 
  very 
  irregular 
  in 
  its 
  

   appearance. 
  The 
  genus 
  Spirifee 
  furnishes 
  a 
  striking 
  instance 
  of 
  

   its 
  persistence 
  in 
  the 
  deltarium-bearing 
  shells. 
  It 
  is 
  probable 
  that 
  

   the 
  existence 
  of 
  this 
  area 
  has 
  little 
  fundamental 
  connexion 
  with 
  

   the 
  condition 
  of 
  the 
  pedicle-coverings. 
  It 
  is 
  a 
  very 
  palpable 
  fact 
  

   that 
  there 
  is 
  a 
  much 
  more 
  intimate 
  relation 
  between 
  this 
  area 
  

   and 
  the 
  general 
  form 
  of 
  the 
  shell 
  ; 
  thus 
  in 
  the 
  elongate 
  shells 
  like 
  

   the 
  terebratuloids, 
  meristoids. 
  retzioids 
  and 
  the 
  pentameroids 
  for 
  

   the 
  most 
  part, 
  there 
  is 
  no 
  such 
  area 
  present. 
  ^Vhere 
  the 
  form 
  of 
  

   the 
  shell 
  is 
  more 
  generally 
  transverse 
  as 
  among 
  the 
  Orthijd^;, 
  

   in 
  Stbophomexa, 
  Clitamboxites, 
  Deebya, 
  Spieieee, 
  etc.. 
  the 
  area 
  

   is 
  highly 
  developed. 
  

  

  It 
  has 
  been 
  suggested 
  that 
  this 
  area 
  may 
  have 
  originated 
  in 
  an 
  

   obstruction 
  to 
  the 
  peripheral 
  growth 
  of 
  the 
  valves 
  upon 
  their 
  

   posterior 
  margins, 
  and 
  would, 
  therefore, 
  be 
  greatest 
  in 
  shells 
  

   whose 
  pedicle 
  was 
  short 
  and 
  necessitated 
  a 
  close 
  attachment 
  (see 
  

   pp. 
  155, 
  163). 
  

  

  In 
  this 
  presentation 
  of 
  the 
  characters 
  of 
  the 
  generic 
  groups, 
  

   the 
  genus 
  LrxorLA 
  has 
  been 
  taken 
  as 
  a 
  starting 
  point, 
  more 
  for 
  

   the 
  reason 
  that 
  it 
  is 
  a 
  widely 
  distributed 
  and 
  well-known 
  type 
  

   than 
  because 
  we 
  have 
  any 
  reason 
  to 
  regard 
  it 
  as 
  primitive. 
  

  

  The 
  nearest 
  approach 
  to 
  the 
  radicle 
  of 
  the 
  entire 
  class 
  is 
  

  

  expressed 
  by 
  the 
  genus 
  Pateeixa. 
  Beecher 
  (p. 
  247). 
  which 
  bears 
  

  

  at 
  maturity 
  the 
  form 
  and 
  structure 
  of 
  the 
  primitive 
  shell 
  or 
  pro- 
  

  

  tegulum. 
  It 
  is 
  orbicular 
  in 
  form, 
  with 
  a 
  straight 
  or 
  arcuate 
  

  

  139 
  163 
  

  

  