﻿1122 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  that 
  the 
  spire-bearing 
  forms 
  have 
  derived 
  their 
  brachidia 
  from 
  a 
  

   primitive 
  terebratuloid 
  condition, 
  and 
  this 
  derivation 
  has 
  been 
  

   effected 
  by 
  growth 
  with 
  accompanying 
  resorption. 
  The 
  pro- 
  

   gressive 
  modification 
  of 
  the 
  loop 
  in 
  the 
  recent 
  terebratellids 
  by 
  

   resorption 
  of 
  calcareous 
  tissue 
  in 
  the 
  growth 
  of 
  the 
  individual, 
  is 
  

   a 
  well-known 
  fact 
  which 
  has 
  invited 
  the 
  study 
  of 
  many 
  investi- 
  

   gators. 
  In 
  such 
  forms 
  this 
  modification 
  is 
  extreme 
  and 
  is 
  

   unquestionably 
  complicated 
  by 
  the 
  intimate 
  connexion 
  of 
  the 
  

   loop 
  with 
  the 
  median 
  septum 
  of 
  the 
  brachial 
  valve. 
  Among 
  the 
  

   palaeozoic 
  genera 
  there 
  is, 
  with 
  the 
  single 
  exception 
  of 
  Tro- 
  

   pidoleptus, 
  no 
  clear 
  evidence 
  that 
  the 
  median 
  septum 
  has 
  shared 
  

   in, 
  or 
  contributed 
  to 
  the 
  growth-modifications 
  of 
  the 
  brachial 
  

   supports; 
  nevertheless, 
  the 
  outcome 
  and 
  final 
  result 
  of 
  this 
  

   growth 
  with 
  modification 
  in 
  the 
  most 
  progressed 
  forms 
  of 
  

   Terebratella 
  and 
  such 
  palaeozoic 
  genera 
  as 
  Dielasma, 
  Crypto- 
  

   nella, 
  Harttina, 
  etc., 
  is 
  the 
  same. 
  

  

  Progressive 
  modification 
  of 
  the 
  brachial 
  supports 
  in 
  both 
  the 
  

   Helicopegmata 
  and 
  palaeozoic 
  Ancylobrachia 
  being 
  now 
  fully 
  

   established, 
  it 
  is 
  interesting 
  to 
  observe 
  that 
  the 
  primitive 
  condition 
  

   of 
  the 
  loop, 
  as 
  in 
  Dielasma 
  turgida, 
  is 
  one 
  of 
  simple 
  apposition 
  of 
  

   the 
  two 
  short 
  brachial 
  processes, 
  at 
  their 
  expanded 
  anterior 
  

   extremities; 
  having 
  the 
  expression 
  of 
  the 
  mature 
  loop 
  in 
  the 
  

   genera 
  Centronella, 
  Kenssel^eria, 
  Selenella, 
  etc. 
  A 
  simple 
  

   step 
  further 
  back 
  would 
  afford 
  a 
  condition 
  in 
  which 
  the 
  brachial 
  

   processes 
  with 
  their 
  expanded 
  extremities 
  are 
  not 
  as 
  yet 
  united 
  

   but 
  discrete 
  as 
  in 
  the 
  rhynchonellids. 
  A 
  more 
  primitive 
  condition 
  

   than 
  that 
  in 
  Centronella 
  or 
  the 
  centronellid 
  stage 
  in 
  Dielasma, 
  

   could 
  not 
  be 
  different 
  from 
  this. 
  On 
  the 
  ground 
  of 
  these 
  differ- 
  

   ences 
  in 
  the 
  conditions 
  of 
  the 
  brachidium 
  and 
  the 
  phyletic 
  

   stages 
  corresponding 
  thereto, 
  it 
  would 
  seem 
  fair 
  to 
  infer 
  that 
  

   of 
  the 
  rhynchonellids, 
  the 
  terebratuloids 
  and 
  the 
  spire-bearers, 
  

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

   mate 
  derivatives 
  of 
  that 
  stock, 
  through 
  the 
  terebratuloids, 
  or 
  

   both 
  of 
  the 
  latter 
  derived 
  along 
  divergent 
  lines 
  from 
  the 
  

   rhychonellids. 
  This 
  conclusion, 
  however 
  coherent 
  and 
  con- 
  

   sistent 
  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 
  sup- 
  

   ports, 
  have 
  passed 
  through 
  corresponding 
  phases 
  of 
  growth 
  and 
  

  

  180 
  

  

  