﻿Report 
  of 
  tee 
  State 
  Geologist. 
  833 
  

  

  rhynchonellids 
  to 
  the 
  terebratuloids 
  and 
  spire-bearers 
  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 
  

   of 
  the 
  spire-bearers, 
  then 
  in 
  the 
  Dielasma- 
  stage 
  of 
  Zygospira 
  

   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 
  Beecher, 
  

   to 
  the 
  necessity 
  of 
  finding 
  room 
  for 
  the 
  cilia 
  or 
  tentacles 
  multi- 
  

   plying 
  at 
  the 
  extremities 
  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 
  

   this 
  analogy, 
  a 
  median 
  arm. 
  The 
  subsequent 
  growth 
  of 
  the 
  

   brachia 
  in 
  Zygospira, 
  carrying 
  the 
  calcareous 
  ribbon 
  forward 
  

   beyond 
  the 
  bases 
  of 
  the 
  loop 
  and 
  into 
  the 
  lateral 
  spiral 
  cones, 
  would 
  

   not, 
  of 
  itself, 
  afford 
  sufficient 
  reason 
  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 
  apparently 
  logical, 
  appears 
  to 
  

   be 
  based 
  on 
  insufficient 
  premises. 
  

  

  The 
  brachiopods 
  with 
  which 
  we 
  have 
  to 
  deal 
  in 
  the 
  Palaeozoic 
  

   are 
  essentially 
  primitive 
  structures, 
  whether 
  rhynchonellids, 
  

   terebratuloids 
  or 
  spire-bearers. 
  If 
  the 
  living 
  Rhynchonella 
  

   and 
  Terebratella, 
  in 
  their 
  mature 
  conditions, 
  possess 
  extensive 
  

   unsolidified 
  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 
  terebratuloids 
  in 
  which 
  the 
  loop 
  is 
  coextensive 
  with 
  

   the 
  brachia 
  ; 
  such 
  we 
  believe 
  to 
  have 
  been 
  the 
  relation 
  of 
  these 
  

   parts 
  in 
  the 
  mature 
  phases 
  of 
  the 
  primitive 
  terebratuloids 
  as 
  

   Centronella, 
  Renssel^ria, 
  Cryptcnella, 
  Dielasma, 
  etc.; 
  in 
  

   Tropidoleptus, 
  which 
  has 
  been 
  shown 
  to 
  represent 
  a 
  highly 
  

   105 
  

  

  