﻿828 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  only 
  the 
  crura 
  and 
  in 
  the 
  Strophomenibm 
  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 
  the 
  loop, 
  or 
  to 
  employ 
  a 
  term 
  more 
  appropriate 
  

   in 
  view 
  of 
  the 
  homologies 
  of 
  the 
  spire-, 
  and 
  loop-bearing 
  shells, 
  

   the 
  jugum, 
  however, 
  which 
  is 
  subject 
  to 
  the 
  most 
  frequent 
  

   variation 
  in 
  form 
  and 
  which 
  serves 
  as 
  the 
  generic 
  index. 
  

   "When 
  the 
  spirals 
  are 
  directed 
  outward 
  toward 
  the 
  lateral 
  mar- 
  

   gins 
  of 
  the 
  valves, 
  the 
  jugum 
  seems 
  to 
  be 
  much 
  more 
  variable 
  

   than 
  in 
  shells 
  where 
  the 
  spirals 
  are 
  introverted 
  or 
  take 
  some 
  

   intermediate 
  position. 
  In 
  the 
  latter 
  there 
  is 
  a 
  much 
  greater 
  

   variation 
  in 
  the 
  position 
  of 
  the 
  loop 
  upon 
  the 
  primary 
  lamellae 
  

   than 
  occurs 
  in 
  the 
  former. 
  

  

  The 
  earliest 
  spire-bearing 
  shells 
  yet 
  discovered 
  are 
  the 
  simplest 
  

   in 
  the 
  structure 
  of 
  the 
  brachidium. 
  Halllna, 
  Pbotozyga, 
  

   Cyclospira 
  of 
  the 
  Lower 
  Silurian 
  possess 
  brachidia 
  which 
  are 
  little 
  

   else 
  than 
  one 
  or 
  two 
  volutions 
  of 
  the 
  calcified 
  lamellae 
  with 
  a 
  

   slight 
  inclination 
  toward 
  each 
  other 
  and 
  the 
  median 
  axis 
  of 
  the 
  

   shell. 
  Zygospika 
  and 
  Glassia, 
  the 
  contemporaries 
  and 
  successors 
  

   of 
  these 
  primitive 
  structures 
  show 
  progressed 
  conditions 
  of 
  the 
  

   same 
  form 
  of 
  brachidium. 
  In 
  these 
  genera, 
  however, 
  there 
  is 
  a 
  

   slight 
  deviation 
  in 
  the 
  vertical 
  axes 
  of 
  the 
  spirals 
  from 
  the 
  trans- 
  

   verse 
  axis 
  of 
  the 
  shell, 
  the 
  apices 
  being 
  inclined 
  somewhat 
  

   toward 
  the 
  brachial 
  valve, 
  and 
  this 
  tendency 
  to 
  lateral 
  revo- 
  

   lution 
  in 
  the 
  spiral 
  cones 
  is 
  carried 
  to 
  its 
  extreme 
  in 
  the 
  

   genus 
  Ateypa, 
  where 
  the 
  multispiral 
  cones 
  of 
  the 
  fully 
  

   matured 
  forms 
  of 
  the 
  Devonian 
  may 
  sometimes 
  have 
  their 
  axes 
  

   nearly 
  parallel. 
  This 
  is 
  the 
  termination 
  of 
  all 
  revolution 
  of 
  the 
  

   cones, 
  a 
  change 
  through 
  an 
  arc 
  of 
  less 
  than 
  90°, 
  probably 
  due 
  in 
  

   a 
  large 
  degree 
  to 
  alterations 
  in 
  the 
  form 
  of 
  the 
  internal 
  cavity 
  of 
  

   the 
  valves 
  ; 
  and 
  the 
  fact 
  that 
  this 
  revolution 
  here 
  ceases 
  strictly 
  

   delimits 
  the 
  group 
  of 
  forms 
  bearing 
  spirals 
  of 
  this 
  type. 
  

   (Atrypidm) 
  

  

  It 
  is 
  well 
  to 
  emphasize 
  the 
  fact, 
  lest 
  misconceptions 
  already 
  set 
  

   on 
  foot 
  should 
  become 
  prevalent, 
  that 
  no 
  wider 
  revolution 
  of 
  the 
  

  

  