﻿1116 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  chia, 
  Syntrophia, 
  Clitambonites 
  and 
  Scenidiem 
  of 
  the 
  early 
  and 
  

   later 
  Silurian 
  and 
  of 
  the 
  Devonian. 
  

  

  A 
  parallel 
  line 
  of 
  development 
  is 
  exhibited 
  by 
  spondylium- 
  

   bearing 
  forms 
  in 
  which 
  the 
  deltidium 
  disappeared 
  at 
  a 
  very 
  early 
  

   period, 
  and 
  the 
  shells 
  possess 
  a 
  trihedral, 
  generally 
  coarsely 
  

   plicated 
  and 
  decidedly 
  rhynchonelloid 
  exterior. 
  It 
  seems 
  highly 
  

   probable 
  that 
  this 
  line 
  was 
  differentiated 
  in 
  the 
  early 
  Cambrian, 
  

   as 
  indications 
  of 
  this 
  structure 
  are 
  observable 
  in 
  some 
  primordial 
  

   species, 
  as 
  Camarella 
  f 
  minor, 
  Walcott, 
  and 
  Stricklandinia 
  f 
  

   Balcletchensis, 
  Davidson; 
  in 
  the 
  Silurian 
  it 
  is 
  represented 
  by 
  

   Camarella 
  and 
  Parastr 
  phia, 
  by 
  the 
  more 
  rotund 
  and 
  more 
  

   finely 
  plicate 
  shells, 
  An 
  astro 
  phia, 
  Porambonites, 
  Lycophoria 
  and 
  

   N<e 
  lingia. 
  The 
  last-named 
  genera 
  are 
  not 
  homogeneous 
  with 
  

   the 
  others 
  in 
  the 
  phases 
  of 
  development 
  which 
  they 
  represent, 
  all 
  

   of 
  them 
  retaining 
  the 
  cardinal 
  areas 
  more 
  or 
  less 
  distinctly, 
  while 
  

   Lyc 
  PHORI4 
  and 
  .Ncstlingta 
  also 
  possess 
  a 
  cardinal 
  process 
  in 
  the 
  

   brachial 
  valve. 
  The 
  presence 
  of 
  the 
  cardinal 
  area 
  in 
  such 
  early 
  

   structures 
  must 
  be 
  regarded 
  as 
  a 
  retention, 
  rather 
  than 
  a 
  resump- 
  

   tion 
  of 
  a 
  primitive 
  character. 
  

  

  Whatever 
  may 
  be 
  the 
  oscillation 
  in 
  form 
  and 
  the 
  variation 
  in 
  

   secondary 
  characters 
  presented 
  by 
  Camarella, 
  Parastrophia 
  

   and 
  their 
  allies, 
  present 
  evidence 
  indicates 
  that 
  they 
  must 
  be 
  

   regarded 
  as 
  the 
  genetic 
  precursors, 
  as 
  they 
  are 
  the 
  secular 
  prece- 
  

   dents 
  of 
  the 
  great 
  group 
  of 
  true 
  pentameroids 
  (Pentamerus, 
  

   Capellinia, 
  Conchidium, 
  Barrandella, 
  Sieberella, 
  Pentame- 
  

   rella, 
  Gyp 
  dula, 
  Stricklandinia, 
  Amphioenta) 
  ; 
  and, 
  indeed 
  the 
  

   last 
  of 
  these 
  pentameroids, 
  Camarophoria, 
  of 
  the 
  Carboniferous 
  

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

   to 
  the 
  rhynchonelloid 
  exterior 
  and 
  the 
  camarellid 
  aspect, 
  with 
  the 
  

   addition 
  of 
  deltaria 
  in 
  the 
  delthyrium. 
  

  

  "While 
  considering 
  in 
  detail 
  the 
  pentameroid 
  genera 
  mentioned 
  

   above, 
  it 
  has 
  been 
  shown 
  that 
  in 
  certain 
  of 
  them, 
  as 
  Pentamerds 
  

   and 
  Conchidium, 
  a 
  true 
  deltidium 
  is 
  often 
  retained, 
  though 
  it 
  is 
  a 
  

   fragile 
  structure 
  rendered 
  concave 
  by 
  the 
  arched 
  growth 
  of 
  the 
  

   umbones 
  of 
  the 
  valves, 
  and 
  is 
  generally 
  absent. 
  In 
  others, 
  as 
  

   Gyp 
  dula 
  and 
  Pentamerella, 
  there 
  are 
  occasionally 
  evidences 
  of 
  

   lateral, 
  erect 
  or 
  convex 
  growths 
  upon 
  the 
  margins 
  of 
  the 
  del- 
  

   thyrium, 
  which 
  may 
  be 
  interpreted 
  either 
  as 
  remnants 
  of 
  a 
  

   resorbed 
  convex 
  deltidium, 
  or 
  as 
  highly 
  accelerated 
  secondary 
  

  

  174 
  

  

  