﻿Report 
  of 
  ihe 
  State 
  Geologist. 
  1123 
  

  

  derivation. 
  This 
  latter 
  question 
  must 
  long 
  be 
  a 
  matter 
  of 
  legiti- 
  

   mate 
  speculation, 
  and 
  in 
  view 
  of 
  this 
  fact 
  few 
  arguments 
  of 
  such 
  

   a 
  nature 
  in 
  this 
  place 
  will 
  be 
  permissible. 
  The 
  living 
  representa- 
  

   tives 
  of 
  Rhtnchonella 
  and 
  Teeebeatula 
  are 
  animals 
  in 
  which 
  a 
  

   very 
  considerable 
  part 
  of 
  the 
  brachia 
  does 
  not 
  become 
  sufficiently 
  

   spiculized 
  to 
  form 
  a 
  continuous 
  calcareous 
  support. 
  In 
  H. 
  (Semi- 
  

   thyris) 
  psittacea, 
  for 
  example, 
  the 
  brachia 
  are 
  as 
  highly 
  developed 
  

   in 
  the 
  form 
  of 
  coiled 
  spiral 
  arms 
  as 
  they 
  could 
  have 
  been 
  in 
  most 
  

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

   only 
  the 
  short 
  lamella? 
  known 
  as 
  the 
  crural 
  processes. 
  All 
  of 
  the 
  

   living 
  Ancylobbachia 
  which 
  possess 
  a 
  long 
  recurved 
  loop 
  like 
  

   that 
  of 
  Ceyptonella 
  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 
  condition 
  

   of 
  the 
  brachia 
  in 
  their 
  nearest 
  living 
  representatives, 
  it 
  becomes 
  

   necessary 
  to 
  assume 
  that 
  on 
  the 
  one 
  hand 
  the 
  palaeozoic 
  rhyn- 
  

   chonellids 
  possessed 
  long 
  coiled 
  spiral 
  arms, 
  and, 
  on 
  the 
  other, 
  

   that 
  Djelasma 
  and 
  its 
  palaeozoic 
  allies 
  and 
  affines, 
  when 
  mature, 
  

   were 
  provided 
  with 
  the 
  unpaired 
  coiled 
  arm 
  of 
  Teeebeatella. 
  

   This 
  assumption, 
  in 
  the 
  first 
  place, 
  totally 
  destroys 
  the 
  inference 
  

   above 
  made 
  as 
  to 
  the 
  primitive 
  relation 
  of 
  the 
  rhynchonellids 
  to 
  

   the 
  Ancylobbachia 
  and 
  Helicopegmata 
  ; 
  and 
  secondly, 
  would 
  

   seem 
  to 
  necessitate 
  a 
  novel 
  and 
  unexpected 
  interpretation 
  of 
  the 
  

   brachial 
  structure 
  in 
  all 
  the 
  spire-bearers. 
  If 
  Dielasma 
  possessed 
  

   the 
  median 
  arm, 
  supported 
  at 
  its 
  base 
  by 
  the 
  transverse 
  band 
  of 
  

   the 
  loop, 
  which 
  corresponds 
  to 
  the 
  jugum 
  or 
  the 
  spire-bearers, 
  

   then 
  in 
  the 
  Dielasma 
  stage 
  of 
  Zygospiea 
  and 
  other 
  spiriferous 
  

   shells, 
  where 
  this 
  stage 
  was 
  well 
  defined, 
  there 
  must 
  also 
  have 
  

   been 
  a 
  median 
  coiled 
  arm 
  of 
  some 
  extent. 
  This 
  median 
  arm, 
  in 
  

   living 
  forms, 
  is 
  due, 
  as 
  shown 
  by 
  Beechee, 
  to 
  the 
  necessity 
  of 
  

   finding 
  room 
  for 
  the 
  cilia 
  or 
  tentacles 
  multiplying 
  at 
  the 
  extremi- 
  

   ties 
  of 
  the 
  brachia. 
  The 
  mere 
  presence 
  of 
  the 
  transverse 
  band 
  in 
  

   Dielasma 
  and 
  the 
  DiELASMA-stage 
  of 
  Zygospira 
  implies 
  a 
  similar 
  

   extension 
  of 
  the 
  brachia, 
  and 
  from 
  the 
  analogy, 
  a 
  median 
  arm. 
  

   The 
  subsequent 
  growth 
  of 
  the 
  brachia 
  in 
  Zygospiea, 
  carrying 
  the 
  

   calcareous 
  ribbon 
  forward, 
  beyond 
  the 
  bases 
  of 
  the 
  loop 
  and 
  into 
  

   lateral 
  spiral 
  cones, 
  would 
  not 
  of 
  itself 
  afford 
  sufficient 
  rea- 
  

  

  181 
  

  

  