﻿810 
  Forty- 
  seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  wholly 
  of 
  secondary 
  growth.* 
  Waagen's 
  term 
  for 
  this 
  group 
  

   Gastropegmata 
  or 
  Craniacea 
  may, 
  therefore, 
  prove 
  equivalent 
  

   to 
  each 
  of 
  these 
  other 
  two 
  divisions. 
  

  

  The 
  great 
  gulf 
  which 
  has 
  seemed 
  to 
  exist 
  between 
  the 
  Inar- 
  

   ticulate 
  or 
  Lyopomatous, 
  and 
  the 
  Articulate 
  or 
  Arthropomatous 
  

   divisions 
  of 
  the 
  Class 
  Brachiopoda 
  ; 
  those 
  without 
  teeth 
  and 
  those 
  

   with 
  teeth 
  ; 
  those 
  with 
  a 
  largely 
  corneous 
  shell, 
  and 
  those 
  whose 
  

   shell 
  is 
  essentially 
  calcareous, 
  is 
  not 
  yet 
  fully 
  spanned 
  at 
  many 
  

   points. 
  

  

  These 
  divisions 
  were 
  based 
  upon 
  the 
  study 
  of 
  living 
  brachio- 
  

   pods 
  in 
  which 
  all 
  the 
  characteristic 
  differences 
  are 
  pronounced 
  

   and 
  fixed. 
  We 
  naturally 
  'expect 
  to 
  find, 
  however, 
  among 
  the 
  

   early 
  brachiopods, 
  in 
  which 
  the 
  adjustment 
  of 
  the 
  organism 
  to 
  

   its 
  conditions 
  was 
  highly 
  sensitive, 
  that 
  the 
  oscillation 
  and 
  special- 
  

   ization 
  of 
  characters 
  has 
  been 
  very 
  rapid. 
  The 
  development 
  of 
  

   articulating 
  processes 
  has 
  already 
  been 
  noticed 
  among 
  the 
  lingu- 
  

   loids, 
  in 
  Barroisella, 
  Tomasina 
  and 
  Teimerella, 
  among 
  the 
  

   oboloids 
  in 
  Spondyloboltjs, 
  and 
  among 
  the 
  siphonotretoids 
  in 
  

   Trematobolus. 
  It 
  is 
  known 
  that 
  the 
  shell 
  of 
  many 
  inar- 
  

   ticulates 
  is 
  almost 
  wholly 
  calcareous, 
  as 
  in 
  the 
  Trimer- 
  

   ellidsE 
  and 
  all 
  of 
  the 
  so-termed 
  Gasteropegmata. 
  The 
  altera- 
  

   tion 
  in 
  the 
  nature 
  of 
  the 
  shell-substance 
  from 
  the 
  pro- 
  

   toconch, 
  or 
  its 
  examplar, 
  Paterina, 
  which 
  appears 
  to 
  be 
  

   wholly 
  or 
  essentially 
  corneous, 
  to 
  the 
  typical 
  articulate 
  brachio- 
  

   pod, 
  in 
  which 
  the 
  corneous 
  substance 
  is 
  reduced 
  to 
  a 
  thin 
  epider- 
  

   mal 
  film, 
  is 
  a 
  gradual 
  process 
  whose 
  various 
  stages 
  are 
  well 
  

   understood. 
  In 
  Obolella, 
  Elkania 
  and 
  the 
  early 
  forms 
  of 
  

   Lingula, 
  the 
  deposition 
  of 
  calcareous 
  salts 
  in 
  the 
  shell 
  was 
  

   already 
  advanced, 
  these 
  layers 
  alternating 
  with 
  thinner 
  layers 
  of 
  

   corneous 
  substance. 
  The 
  gradual 
  and 
  eventual 
  predominance 
  of 
  

   the 
  calcareous 
  shell-matter 
  along 
  both 
  of 
  these 
  lines 
  of 
  develop- 
  

   ment 
  is 
  seen 
  in 
  the 
  ponderous 
  Trimerellids 
  of 
  the 
  later 
  Silurian. 
  

   The 
  graduation 
  of 
  the 
  corneous 
  Paterina 
  {Kutorgina 
  Zabrado- 
  

   rica, 
  var. 
  Swantonensis) 
  through 
  Kutorgina 
  Zabradorica, 
  and 
  

   into 
  the 
  true 
  calcareous 
  Kutorginas 
  (K 
  cingulata, 
  K. 
  Whitfieldi), 
  

  

  * 
  Some 
  species 
  of 
  Pholidops 
  (P. 
  arenaria, 
  P. 
  linguloides) 
  have 
  a 
  terminal 
  submarginal 
  apex; 
  

   and 
  their 
  resemblance 
  exteriorly 
  to 
  the 
  oboloids 
  is 
  very 
  striking. 
  This 
  is, 
  however, 
  no 
  more 
  

   than 
  a 
  resemblance, 
  as 
  they 
  show, 
  on 
  the 
  under 
  side, 
  the 
  same 
  mode 
  of 
  peripheral 
  growth 
  

   beneath 
  the 
  beak 
  as 
  the 
  other 
  forms 
  of 
  the 
  genus 
  in 
  which 
  the 
  umbones 
  are 
  more 
  nearly 
  

   central. 
  

  

  