﻿Report 
  of 
  the 
  State 
  Geologist, 
  1117 
  

  

  deltaria. 
  Every 
  now 
  and 
  then 
  specimens 
  will 
  show 
  a 
  clearly- 
  

   developed 
  cardinal 
  area 
  ; 
  always 
  in 
  Stkicklandinia, 
  f 
  requeni 
  ly 
  

   and 
  normally 
  in 
  Gypidoxa, 
  rarely 
  in 
  Pentamerella. 
  Stkick- 
  

   landinia 
  possesses 
  so 
  straight 
  and 
  long 
  a 
  hinge, 
  so 
  sharply 
  defined 
  

   an 
  area 
  and 
  so 
  short 
  a 
  spondylium, 
  that 
  it 
  is 
  more 
  natural 
  to 
  

   regard 
  this 
  genus 
  as 
  the 
  accompaniment, 
  rather 
  than 
  the 
  close 
  

   organic 
  kin 
  of 
  the 
  other 
  pentameroids, 
  deriving 
  its 
  differentials 
  

   directly 
  from 
  those 
  long 
  and 
  straight 
  hinged 
  shells 
  of 
  the 
  early 
  

   Silurian, 
  which 
  constitute 
  the 
  genus 
  Syntrophia. 
  

  

  It 
  will 
  not 
  now 
  appear 
  a 
  matter 
  of 
  inexplicable 
  aberrancy 
  that 
  

   the 
  spondylium 
  presents 
  itself 
  in 
  the 
  great 
  secondary 
  groups 
  com- 
  

   prising 
  the 
  rhynchonellids, 
  and 
  those 
  shells 
  with 
  calcified 
  

   brachidia. 
  Hence 
  we 
  meet 
  with 
  it 
  in 
  Cyrtlna 
  and 
  Camarospira 
  

   in 
  a 
  highly-developed 
  state, 
  and 
  in 
  Camarotcechia 
  in 
  a 
  less 
  

   advanced 
  condition, 
  while 
  Ampuigen 
  a 
  presents 
  the 
  remarkable 
  

   combination 
  of 
  a 
  spondylium 
  coexistent 
  with 
  a 
  shell 
  of 
  completely 
  

   rensselseroid 
  aspect 
  (that 
  is, 
  in 
  respect 
  to 
  form, 
  contour, 
  muscular 
  

   markings 
  and 
  articulating 
  apparatus) 
  and 
  with 
  rhynchonelloid 
  

   brachial 
  supports. 
  

  

  Attention 
  has 
  already 
  been 
  directed 
  to 
  the 
  fact 
  that 
  some 
  of 
  

   the 
  Rhynchonellids, 
  early 
  in 
  their 
  history, 
  occasionally 
  

   retain 
  a 
  well-defined 
  cardinal 
  area 
  and 
  that, 
  in 
  default 
  of 
  other 
  

   evidence, 
  the 
  presence 
  of 
  this 
  character 
  may 
  be 
  regarded 
  as 
  

   indicative 
  of 
  the 
  common 
  origin 
  of 
  Orthis, 
  the 
  Stropho- 
  

   menids, 
  and 
  the 
  Rhynchonellas. 
  The 
  earliest 
  phyletic 
  stages 
  

   of 
  the 
  rhynchonellids 
  must 
  have 
  been 
  highly 
  accelerated, 
  for 
  

   there 
  is 
  no 
  evidence 
  of 
  any 
  form 
  which 
  has 
  shown 
  the 
  

   slightest 
  trace 
  of 
  deltidium. 
  Nevertheless 
  the 
  early 
  forms 
  

   of 
  the 
  Silurian, 
  such 
  as 
  ORTeoRHi^cnuLA 
  and 
  Pr 
  >to- 
  

   rhyncha, 
  rarely 
  show 
  any 
  indication 
  of 
  dtltaria 
  at 
  matu- 
  

   rity, 
  but 
  the 
  delthryium, 
  in 
  its 
  final 
  stage, 
  is 
  unobstructed 
  

   and 
  simple, 
  as 
  in 
  young 
  conditions 
  of 
  later 
  rhynchonellids 
  in 
  

   which 
  the 
  deltaria 
  fully 
  develop. 
  We 
  may 
  look 
  upon 
  the 
  Rhyn- 
  

   chonellids 
  &s 
  a 
  family 
  whose 
  characters 
  became 
  established 
  very 
  

   early 
  and 
  have 
  been 
  perpetuated 
  up 
  to 
  the 
  present 
  without 
  wide 
  

   departure, 
  at 
  any 
  time, 
  from 
  the 
  early 
  derived 
  type. 
  

  

  In 
  the 
  study 
  of 
  the 
  multifold 
  variations 
  of 
  the 
  articulates 
  bear- 
  

   ing 
  calcified 
  spiral 
  brachial 
  supports, 
  the 
  Helicopegmata 
  of 
  

   Waagen 
  (1883), 
  the 
  conclusion 
  has 
  enforced 
  itself 
  that 
  the 
  degree 
  

  

  175 
  

  

  