﻿Report 
  of 
  the 
  State 
  Geologist 
  827 
  

  

  theless 
  the 
  early 
  forms 
  of 
  the 
  Silurian, 
  such 
  as 
  Oeihoehynchula 
  

   and 
  Peotoehyncha, 
  rarely 
  show 
  any 
  indication 
  of 
  deltaria 
  at 
  

   maturity, 
  but 
  the 
  delthyrium 
  in 
  its 
  final 
  stage 
  is 
  unobstructed 
  

   and 
  simple, 
  as 
  in 
  young 
  conditions 
  of 
  later 
  rhynchonellids 
  in 
  

   which 
  the 
  deltaria 
  fully 
  develop. 
  

  

  "We 
  may 
  look 
  upon 
  the 
  Rhynchonellids 
  as 
  a 
  family 
  whose 
  

   characters 
  became 
  established 
  very 
  early 
  and 
  have 
  been 
  perpetu- 
  

   ated 
  up 
  to 
  the 
  present 
  without 
  wide 
  departure 
  at 
  any 
  time 
  from 
  

   the 
  early 
  derived 
  type. 
  

  

  In 
  the 
  study 
  of 
  the 
  multifold 
  variations 
  of 
  the 
  articulates 
  bear- 
  

   ing 
  calcified 
  spiral 
  brachial 
  supports, 
  the 
  Helicopegmata 
  (Waagen, 
  

   1883), 
  the 
  conclusion 
  has 
  enforced 
  itself 
  that 
  the 
  degree 
  of 
  solidi- 
  

   fication 
  of 
  the 
  brachia 
  in 
  this 
  group 
  is 
  to 
  be 
  regarded 
  as 
  an 
  index 
  

   of 
  differentiation. 
  To 
  illustrate 
  : 
  there 
  is 
  no 
  evidence 
  for 
  assum- 
  

   ing 
  that 
  the 
  single 
  evolution 
  made 
  by 
  the 
  spiral 
  in 
  Peotozyga 
  

   and 
  Hallina 
  represents 
  an 
  incomplete 
  spiculization 
  of 
  the 
  

   brachia, 
  or 
  that 
  the 
  spiniform 
  and 
  discrete 
  jugal 
  processes 
  in 
  

   Spibifee, 
  persisting 
  throughout 
  the 
  genus, 
  do 
  not 
  fully 
  exemplify 
  

   the 
  adult 
  condition 
  of 
  the 
  jugum 
  (= 
  loop) 
  in 
  these 
  shells. 
  The 
  

   mode 
  of 
  spiculation 
  of 
  the 
  brachia 
  in 
  such 
  of 
  the 
  living 
  tere- 
  

   bratuloids, 
  in 
  which 
  the 
  solidification 
  is 
  direct 
  or 
  without 
  compli- 
  

   cated 
  metamorphoses, 
  is, 
  on 
  the 
  whole, 
  confirmatory 
  of 
  this 
  

   inference 
  ; 
  but 
  as 
  there 
  is 
  no 
  living 
  representative 
  of 
  the 
  Helico- 
  

   pegmata, 
  the 
  evidence 
  in 
  regard 
  to 
  the 
  mode 
  and 
  degree 
  of 
  

   spiculation 
  in 
  the 
  group, 
  derived 
  from 
  the 
  existing 
  loop-bearing 
  

   shells 
  in 
  which 
  the 
  brachial 
  supports 
  pass 
  through 
  highly 
  

   complicated 
  metamorphoses, 
  is 
  not 
  altogether 
  germane. 
  In 
  such 
  

   intricate 
  structures 
  as 
  the 
  brachidia 
  of 
  Athyeis, 
  Kayseeia, 
  

   Koninckina, 
  etc., 
  there 
  can 
  be 
  little 
  doubt 
  that 
  the 
  calcified 
  

   apparatus 
  represents 
  the 
  full 
  extent 
  of 
  the 
  fleshy 
  brachia, 
  simply 
  

   if 
  for 
  no 
  other 
  reason, 
  because 
  the 
  further 
  expansion 
  of 
  the 
  

   brachial 
  lamellae 
  would 
  not 
  be 
  possible. 
  Moreover, 
  in 
  the 
  spicu- 
  

   lation 
  of 
  the 
  spirals 
  in 
  all 
  these 
  old 
  shells 
  there 
  have 
  been 
  no 
  

   changes 
  of 
  form 
  in 
  later 
  growth, 
  except 
  those 
  proceeding 
  from 
  the 
  

   normal 
  process 
  of 
  resorption 
  and 
  deposition 
  necessary 
  for 
  increase 
  

   in 
  size 
  and 
  length. 
  The 
  reason 
  why, 
  in 
  the 
  Helicopegmata, 
  

   spiculation 
  should 
  be 
  complete, 
  while 
  in 
  the 
  Ancylobbachia 
  or 
  

   terebratuloids, 
  it 
  does 
  not 
  extend 
  beyond 
  the 
  loop 
  or 
  the 
  lateral 
  

   extensions 
  of 
  the 
  brachia, 
  in 
  the 
  Rhtnchonellidm 
  affects 
  

  

  