﻿1124 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  son 
  for 
  assuming 
  that 
  the 
  growth 
  of 
  the 
  brachia 
  at 
  their 
  

   extremities, 
  which 
  produced 
  the 
  median 
  arm, 
  was 
  necessarily 
  

   discontinued, 
  but 
  rather 
  that 
  this 
  median 
  unpaired 
  arm 
  coexisted 
  

   with 
  the 
  lateral 
  paired 
  spirals. 
  This 
  course 
  of 
  argument, 
  though 
  

   seemingly 
  logical, 
  appears 
  to 
  be 
  be 
  based 
  on 
  insufficient 
  premises. 
  

   The 
  brachiopods 
  with 
  which 
  we 
  have 
  to 
  deal 
  in 
  the 
  Palaeozoic 
  are 
  

   essentially 
  primitive 
  structures, 
  whether 
  rhynchonellids, 
  tere- 
  

   bratuloids 
  or 
  spire-bearers. 
  If 
  the 
  living 
  Rhynchonella 
  and 
  

   Terebratella 
  possess 
  in 
  their 
  mature 
  condition 
  extensive 
  

   free 
  arms, 
  it 
  does 
  not 
  necessarily 
  follow 
  that 
  their 
  early 
  

   palaeozoic 
  representatives 
  were 
  provided 
  with 
  similar 
  uncalcified 
  

   extensions 
  ; 
  on 
  the 
  contrary, 
  it 
  would 
  be 
  much 
  more 
  reasonable 
  

   and 
  in 
  accordance 
  with 
  our 
  knowledge 
  of 
  natural 
  laws 
  to 
  infer 
  

   that 
  in 
  these 
  early 
  forms 
  the 
  adult 
  condition 
  of 
  the 
  brachia 
  was 
  

   more 
  nearly 
  that 
  of 
  immature 
  conditions 
  of 
  these 
  organs 
  in 
  their 
  

   living 
  representatives. 
  There 
  is 
  a 
  primitive 
  condition 
  of 
  develop- 
  

   ment 
  in 
  the 
  Ancylobrachia 
  in 
  which 
  the 
  loop 
  is 
  coextensive 
  with 
  

   the 
  brachia. 
  There 
  is 
  reason 
  to 
  believe 
  that 
  such 
  has 
  been 
  the 
  

   relation 
  of 
  these 
  parts 
  in 
  the 
  primitive 
  terebratuloids, 
  as 
  Centro- 
  

   nella, 
  Renssel^eria, 
  Cryptonella, 
  Dielasma, 
  etc. 
  ; 
  in 
  Tropi- 
  

   doleptus, 
  which 
  has 
  been 
  shown 
  to 
  represent 
  a 
  highly 
  primitive 
  

   phyletic 
  condition 
  of 
  the 
  Terebratellid^e 
  ; 
  and, 
  also 
  in 
  the 
  

   earliest 
  spire-bearers 
  and 
  rhynchonellids. 
  Hence 
  the 
  conclusion 
  

   above 
  expressed 
  as 
  to 
  the 
  successive 
  phyletic 
  relations 
  of 
  the 
  primi- 
  

   tive 
  rhynchonellids, 
  terebratuloids 
  and 
  spire-bearers 
  and 
  based 
  upon 
  

   the 
  relations 
  and 
  modifications 
  in 
  the 
  form 
  of 
  their 
  brachial 
  sup- 
  

   ports, 
  is 
  fairly 
  substantiated 
  by 
  the 
  evidence 
  drawn 
  from 
  other 
  data. 
  

  

  The 
  divergence 
  from 
  the 
  ancestral 
  rhynchonellid 
  stock 
  was 
  very 
  

   early 
  and 
  the 
  differentiation 
  undoubtedly 
  consisted, 
  to 
  a 
  large 
  extent 
  

   in 
  rapid 
  acceleration 
  of 
  growth 
  in 
  the 
  brachia, 
  and 
  obstruction 
  to 
  

   the 
  coextensive 
  development 
  of 
  the 
  fleshy 
  arms 
  and 
  their 
  supports. 
  

  

  Finally, 
  it 
  is 
  desirable 
  to 
  again 
  recall 
  the 
  intimate 
  similarity 
  

   between 
  Rensseljeri 
  a 
  and 
  the 
  pentameroid 
  genus 
  Amphigenia; 
  

   genera 
  in 
  which 
  the 
  essential 
  distinction 
  between 
  the 
  typical 
  

   forms 
  of 
  each 
  lies 
  in 
  the 
  simple 
  loop 
  of 
  the 
  former 
  and 
  the 
  long, 
  

   expanded 
  but 
  still 
  discrete 
  crural 
  processes 
  of 
  the 
  latter. 
  Atten- 
  

   tion 
  has 
  been 
  directed 
  to 
  these 
  similarities 
  and 
  differences, 
  and 
  it 
  

   has 
  also 
  been 
  pointed 
  out 
  that 
  the 
  spondylium 
  in 
  Amphigenia 
  

   elongata 
  is 
  at 
  times 
  almost 
  reproduced 
  in 
  specimens 
  of 
  Rensselceria 
  

   ovoides 
  where 
  the 
  dental 
  lamellae 
  are 
  highly 
  developed. 
  

  

  182 
  

  

  