﻿1118 
  Forty- 
  seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  of 
  solidification 
  of 
  the 
  brachia 
  in 
  this 
  group 
  is 
  to 
  be 
  regarded 
  as 
  

   an 
  index 
  of 
  differentiation. 
  To 
  illustrate 
  : 
  There 
  is 
  no 
  evidence 
  

   for 
  assuming 
  that 
  the 
  single 
  revolution 
  made 
  by 
  the 
  spiral 
  in 
  

   Protozyga 
  and 
  Hallina 
  represents 
  an 
  incomplete 
  spiculation 
  of 
  

   the 
  brachia, 
  or 
  that 
  the 
  spiniform 
  and 
  discrete 
  jugal 
  processes 
  in 
  

   Spirifer, 
  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 
  terebratu- 
  

   loids, 
  in 
  which 
  the 
  solidification 
  is 
  direct 
  or 
  without 
  complicated 
  

   metamorphoses, 
  is 
  on 
  the 
  whole 
  confirmatory 
  of 
  this 
  inference 
  ; 
  

   but 
  as 
  there 
  is 
  no 
  living 
  representative 
  of 
  the 
  spire-bearing 
  forms 
  

   evidence 
  in 
  regard 
  to 
  the 
  mode 
  and 
  degree 
  of 
  spiculation 
  in 
  this 
  

   group 
  to 
  be 
  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 
  Athyris, 
  Kayseria, 
  Koninckina, 
  

   etc., 
  there 
  can 
  be 
  little 
  doubt 
  that 
  the 
  calcified 
  apparatus 
  repre- 
  

   sents 
  the 
  full 
  extent 
  of 
  the 
  (fleshy 
  brachia 
  simply 
  because, 
  if 
  for 
  

   no 
  other 
  reason, 
  the 
  further 
  expansion 
  of 
  the 
  brachial 
  lamellae 
  

   would 
  not 
  be 
  possible 
  for 
  want 
  of 
  space. 
  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 
  the 
  spiculation 
  

   should 
  be 
  complete 
  in 
  the 
  spire-bearing 
  forms, 
  while 
  in 
  the 
  Ancy- 
  

   lobrachia 
  or 
  the 
  terebratuloids 
  it 
  does 
  not 
  extend 
  beyond 
  the 
  

   loop 
  and 
  the 
  lateral 
  extensions 
  of 
  the 
  brachia, 
  but 
  in 
  the 
  Rhyn- 
  

   chonellidjE 
  affects 
  only 
  the 
  crura, 
  and 
  in 
  the 
  Strophomenidjb 
  

   does 
  not 
  occur, 
  even 
  in 
  the 
  most 
  elementary 
  condition, 
  is 
  for 
  

   future 
  investigations 
  to 
  ascertain. 
  

  

  The 
  form 
  of 
  the 
  paired 
  spirals 
  varies 
  but 
  little 
  except 
  under 
  

   the 
  necessity 
  of 
  conforming 
  to 
  the 
  interior 
  cavity 
  of 
  the 
  valves. 
  

   Their 
  inclination 
  and 
  direction 
  is 
  a 
  feature 
  of 
  much 
  significance 
  

   when 
  considered 
  with 
  reference 
  to 
  the 
  development 
  of 
  the 
  entire 
  

   shell. 
  It 
  is, 
  however, 
  the 
  loop, 
  or 
  to 
  employ 
  a 
  term 
  more 
  appro- 
  

   priate 
  in 
  view 
  of 
  the 
  homologies 
  of 
  the 
  spire-bearing 
  and 
  loop- 
  

   bearing 
  shells, 
  the 
  jugum, 
  which 
  is 
  subject 
  to 
  the 
  most 
  frequent 
  

   variations 
  in 
  form, 
  and 
  which 
  serves 
  as 
  the 
  generic 
  index. 
  When 
  

   the 
  spirals 
  are 
  directed 
  outward 
  toward 
  the 
  lateral 
  margins 
  of 
  the 
  

  

  176 
  

  

  