﻿826 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  last 
  of 
  these 
  pentameroids, 
  Camarophorta, 
  of 
  the 
  Carboniferous 
  

   and 
  Permian 
  faunas, 
  is 
  an 
  exemplification 
  of, 
  in 
  fact 
  a 
  return 
  to 
  

   the 
  rhynchonelloid 
  exterior 
  and 
  the 
  camerellid 
  aspect, 
  with 
  the 
  

   addition 
  of 
  deltaria 
  in 
  the 
  delthyrium. 
  

  

  While 
  considering 
  in 
  detail 
  the 
  pentameroid 
  genera 
  mentioned 
  

   above 
  we 
  have 
  seen 
  that 
  in 
  certain 
  of 
  them, 
  as 
  Pentamerus 
  and 
  

   Concbidium, 
  a 
  true 
  deltidium 
  1 
  is 
  often 
  retained, 
  though 
  it 
  is 
  a 
  

   fragile 
  structure 
  rendered 
  concave 
  by 
  the 
  arched 
  growth 
  of 
  the 
  

   umbones 
  of 
  the 
  valves, 
  and 
  is 
  generally 
  lost. 
  In 
  others 
  (Gypiduxa, 
  

   Pen 
  tamerell 
  a) 
  there 
  are 
  occasionally 
  evidences 
  of 
  lateral, 
  erect 
  

   or 
  convex 
  growths 
  upon 
  the 
  margins 
  of 
  the 
  delthyrium, 
  which 
  

   may 
  be 
  interpreted 
  either 
  as 
  remnants 
  of 
  a 
  resorbed 
  convex 
  

   deltidium, 
  or 
  as 
  highly 
  accelerated 
  secondary 
  deltaria 
  Every 
  

   now 
  and 
  then 
  specimens 
  will 
  show 
  a 
  clearly 
  developed 
  cardinal 
  

   area, 
  always 
  in 
  Stricklakdinia, 
  frequently 
  and 
  normally 
  in 
  

   Gtypidula, 
  rarely 
  in 
  Pentamerella. 
  Stricklandinia 
  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 
  have 
  been 
  placed 
  under 
  the 
  genus 
  Stntrophia. 
  

  

  It 
  will 
  now 
  appear 
  not 
  a 
  matter 
  of 
  inexplicable 
  aberrancy 
  that 
  

   the 
  spondylium 
  presents 
  itself 
  in 
  the 
  great 
  secondary 
  groups 
  com- 
  

   prising 
  the 
  rhynchonellids 
  and 
  those 
  shells 
  with 
  calcified 
  bra- 
  

   chidia. 
  Hence 
  we 
  meet 
  with 
  it 
  in 
  Cyrtina 
  and 
  Camarosplra 
  in 
  a 
  

   highly 
  developed 
  state, 
  and 
  in 
  Camarotcechia 
  in 
  a 
  less 
  advanced 
  

   condition, 
  while 
  Amphioenia 
  presents 
  the 
  remarkable 
  combina- 
  

   tion 
  of 
  a 
  spondylium 
  coexistent 
  with 
  a 
  completely 
  rensselserioid 
  

   shell 
  structure 
  (that 
  is, 
  form, 
  contour, 
  muscular 
  markings 
  and 
  

   articulating 
  apparatus) 
  and 
  rhynchonelloid 
  brachial 
  supports. 
  

  

  Attention 
  has 
  already 
  been 
  directed 
  to 
  the 
  fact 
  that 
  the 
  Rhtn- 
  

   chonellib^e, 
  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 
  Strophomenidm 
  and 
  the 
  Khyn- 
  

   chonellas. 
  The 
  earliest 
  phyletic 
  stages 
  of 
  the 
  rhynchonellids 
  

   must 
  have 
  been 
  highly 
  accelerated, 
  for 
  there 
  is 
  no 
  evidence 
  of 
  any 
  

   which 
  has 
  yet 
  shown 
  the 
  slightest 
  trace 
  of 
  a 
  deltidium. 
  Never- 
  

  

  