﻿832 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  processes, 
  with 
  their 
  expanded 
  extremities, 
  are 
  not 
  as 
  yet 
  united, 
  

   but 
  discrete 
  as 
  in 
  the 
  rhynchonellids. 
  A 
  more 
  primitive 
  condi- 
  

   tion 
  than 
  that 
  in 
  Centronella, 
  or 
  the 
  centroneliid 
  stage 
  in 
  

   Dielasma, 
  could 
  not 
  be 
  different 
  from 
  this. 
  

  

  On 
  the 
  ground 
  of 
  these 
  differences 
  in 
  the 
  conditions 
  of 
  the 
  

   brachidium 
  and 
  the 
  phyletic 
  stages 
  corresponding 
  with 
  them, 
  

   it 
  would 
  seem 
  fair 
  to 
  infer 
  that, 
  of 
  the 
  rhynchonellids, 
  the 
  tere- 
  

   bratuloids 
  and 
  the 
  spire-bearers, 
  the 
  first 
  is 
  the 
  primitive 
  stock, 
  

   and 
  the 
  spire-bearers 
  legitimate 
  derivatives 
  from 
  that 
  stock 
  

   through 
  the 
  terebratuloids, 
  or 
  both 
  of 
  the 
  latter 
  derived 
  along 
  

   divergent 
  lines 
  from 
  the 
  rhynchonellids. 
  

  

  This 
  conclusion, 
  however 
  coherent 
  and 
  consistent 
  with 
  the 
  

   geological 
  evidence, 
  will 
  be 
  found 
  to 
  lack 
  stability 
  until 
  the 
  data 
  

   are 
  sufficient 
  to 
  establish 
  the 
  fact 
  that 
  the 
  brachia 
  themselves, 
  

   and 
  not 
  alone 
  their 
  calcareous 
  supports, 
  have 
  passed 
  through 
  cor- 
  

   responding 
  phases 
  of 
  growth 
  and 
  derivation. 
  This 
  latter 
  ques- 
  

   tion 
  must 
  long 
  be 
  a 
  matter 
  of 
  legitimate 
  speculation, 
  and, 
  in 
  view 
  

   of 
  this 
  fact, 
  a 
  few 
  arguments 
  of 
  such 
  a 
  nature 
  in 
  this 
  place 
  will 
  be 
  

   permissible. 
  

  

  The 
  living 
  representatives 
  of 
  Rhynchonella 
  and 
  TerebRatula 
  

   are 
  animals 
  in 
  which 
  a 
  very 
  considerable 
  part 
  of 
  the 
  brachia 
  does 
  

   not 
  become 
  sufficiently 
  spiculized 
  to 
  form 
  a 
  continuous 
  calcareous 
  

   support. 
  In 
  R. 
  (Hemithyris) 
  psittacea, 
  for 
  example, 
  the 
  brachia 
  

   are 
  as 
  highly 
  developed 
  in 
  the 
  form 
  of 
  coiled 
  spiral 
  arms 
  as 
  in 
  

   most 
  of 
  the 
  ancient 
  spire- 
  bearers, 
  but 
  their 
  calcareous 
  supports 
  

   are 
  only 
  the 
  short 
  lamellae 
  known 
  as 
  the 
  crural 
  processes. 
  All 
  of 
  

   the 
  living 
  Ancyi/ 
  brac 
  ia 
  which 
  possess 
  a 
  long 
  recurved 
  loop 
  like 
  

   that 
  of 
  Cryptonella 
  and 
  Dielasma, 
  of 
  the 
  Palaeozoic, 
  have 
  an 
  

   unsupported 
  median 
  unpaired 
  spiral 
  arm, 
  coiled 
  in 
  a 
  direction 
  

   which 
  is 
  the 
  reverse 
  of 
  that 
  prevailing 
  among 
  the 
  spire-bearers. 
  

   If, 
  now, 
  we 
  are 
  to 
  interpret 
  the 
  condition 
  of 
  the 
  brachia 
  in 
  the 
  

   fossil 
  rhynchonellids 
  and 
  terebratuloids 
  from 
  the 
  adult 
  condi- 
  

   tion 
  of 
  the 
  brachia 
  in 
  their 
  nearest 
  living 
  representatives, 
  it 
  

   becomes 
  necessary 
  to 
  assume 
  that, 
  on 
  the 
  one 
  hand, 
  the 
  palaeo- 
  

   zoic 
  rhynchonellids 
  possessed 
  long, 
  coiled, 
  spiral 
  arms, 
  and, 
  

   on 
  the 
  other, 
  that 
  Dielasma 
  and 
  its 
  palaeozoic 
  allies 
  and 
  

   affines 
  were, 
  when 
  mature, 
  provided 
  with 
  the 
  unpaired 
  coiled 
  

   arm 
  of 
  Teerbratella. 
  This 
  assumption, 
  first, 
  destroys 
  totally 
  

   the 
  inference 
  above 
  made 
  as 
  to 
  the 
  primitive 
  relation 
  of 
  the 
  

  

  