﻿Report 
  of 
  the 
  State 
  Geologist. 
  821 
  

  

  of 
  the 
  Rhynchonellas 
  originated. 
  These 
  are 
  shells 
  which 
  assumed, 
  

   at 
  a 
  very 
  early 
  period, 
  the 
  deltarium 
  or 
  secondary 
  condition 
  of 
  

   the 
  pedicle-covering. 
  It 
  would 
  be 
  presumptuous 
  to 
  assume 
  that 
  

   a 
  single 
  species 
  of 
  this 
  great 
  group 
  developed 
  a 
  cardinal 
  area, 
  

   solely 
  from 
  mechanical 
  causes, 
  such 
  as 
  obstructed 
  growth 
  on 
  the 
  

   posterior 
  margins 
  of 
  the 
  valves. 
  Its 
  presence 
  seems, 
  rather, 
  to 
  

   suggest 
  the 
  perpetuation 
  of 
  an 
  ancestral 
  character 
  indicating 
  that 
  

   these 
  modified 
  shells 
  have 
  been 
  derived 
  from 
  a 
  more 
  primitive 
  

   condition 
  in 
  which 
  the 
  cardinal 
  area 
  was 
  normal, 
  and, 
  no 
  doubt, 
  

   accompanied 
  by 
  a 
  deltidium. 
  In 
  the 
  absence 
  of 
  further 
  evidence 
  

   such 
  a 
  character 
  is 
  of 
  much 
  importance. 
  

  

  Under 
  the 
  guidance 
  of 
  the 
  structural 
  features 
  above 
  considered, 
  

   the 
  main 
  lines 
  of 
  derivation 
  of 
  the 
  Articulate 
  genera 
  are 
  more 
  

   readily 
  apprehended. 
  

  

  The 
  earliest 
  known 
  representatives 
  of 
  a 
  given 
  group 
  of 
  genera 
  

   are 
  not 
  always 
  the 
  most 
  primitive 
  in 
  structure. 
  In 
  the 
  instance 
  

   cited 
  in 
  the 
  preceding 
  paragraph, 
  Orthorhynchula 
  Linneyi 
  is 
  per- 
  

   haps, 
  by 
  itself 
  considered, 
  the 
  closest 
  expression 
  of 
  the 
  fudamen- 
  

   tal 
  stock 
  from 
  which 
  the 
  rhynchonollids 
  have 
  been 
  derived, 
  but 
  

   it 
  is 
  by 
  no 
  means 
  the 
  earliest 
  of 
  the 
  group. 
  It 
  is 
  known 
  only 
  in 
  

   the 
  latest 
  fauna 
  of 
  the 
  Lower 
  Silurian, 
  while 
  in 
  the 
  earlier 
  faunas 
  

   Protorhyncha, 
  Khynchotrema 
  and 
  Camarotcechia 
  have 
  attained 
  

   an 
  abundant 
  development. 
  Orthorhynchula 
  either 
  represents 
  

   a 
  resumption 
  of 
  the 
  primitive 
  type, 
  subsequent 
  to 
  such 
  modifica- 
  

   tions 
  as 
  appear 
  in 
  the 
  earlier 
  rhynchonelloid 
  genera, 
  or 
  a 
  con- 
  

   tinuation 
  of 
  that 
  type, 
  without 
  modification, 
  through 
  pre- 
  

   existing 
  forms 
  as 
  yet 
  unknown. 
  Such 
  instances 
  could 
  be 
  mul- 
  

   tiplied, 
  as 
  facts 
  of 
  similar 
  import 
  are 
  constantly 
  recurring, 
  and 
  a 
  

   careful 
  consideration 
  of 
  the 
  stage 
  of 
  development 
  or 
  decline 
  of 
  

   each 
  separate 
  and 
  individual 
  organ 
  is 
  requisite 
  to 
  determine 
  how 
  

   far 
  the 
  organism 
  in 
  question 
  is 
  a 
  direct 
  or 
  modified 
  outcome 
  of 
  

   the 
  fundamental 
  type 
  ; 
  or 
  a 
  degenerate 
  or 
  senile 
  relapse, 
  after 
  

   modification, 
  to 
  phyletic 
  immaturity. 
  

  

  The 
  most 
  elementary 
  structure, 
  then, 
  observable, 
  among 
  the 
  

   Articulate 
  Brachiopods 
  is 
  the 
  combination 
  of 
  the 
  deltidium 
  with 
  

   a 
  distinct 
  pedicle-cavity, 
  whose 
  anterior 
  margins 
  are 
  not 
  free, 
  

   and 
  whose 
  lateral 
  walls 
  or 
  dental 
  lamellae 
  are 
  not 
  highly 
  devel- 
  

   oped 
  ; 
  these 
  features 
  being 
  accompanied 
  by 
  gently 
  and 
  unequally 
  

  

  