﻿Report 
  of 
  the 
  State 
  Geologist, 
  1119 
  

  

  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. 
  Hallina, 
  Protozyga, 
  

   Cyclospira 
  of 
  the 
  Lower 
  Silurian 
  possess 
  brachidia 
  which 
  make 
  

   a 
  little 
  less 
  than 
  one 
  or 
  two 
  volutions 
  of 
  the 
  calcified 
  lamellae, 
  

   with 
  a 
  slight 
  inclination 
  toward 
  each 
  other, 
  and 
  to 
  the 
  median 
  

   axis 
  of 
  the 
  shell. 
  Zygospira 
  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 
  transverse 
  axis 
  of 
  the 
  shell, 
  the 
  apices 
  being 
  

   inclined 
  somewhat 
  toward 
  the 
  brachial 
  valve, 
  and 
  this 
  tendency 
  

   to 
  lateral 
  evolution 
  in 
  the 
  spiral 
  cones 
  is 
  carried 
  to 
  its 
  extreme 
  in 
  

   the 
  genus 
  Alrypa 
  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 
  

   spiral 
  cones 
  exists. 
  It 
  is 
  true 
  that 
  there 
  is 
  a 
  difference 
  of 
  18° 
  

   in 
  the 
  position 
  of 
  the 
  axes 
  of 
  the 
  spiral 
  cones 
  in 
  Cyclospira 
  and 
  

   Spirifer, 
  but 
  the 
  spirals 
  have 
  never, 
  by 
  gradual 
  changes, 
  revolved 
  

   from 
  their 
  inverted 
  position 
  in 
  the 
  former 
  to 
  their 
  everted 
  

   position 
  in 
  the 
  latter. 
  Such 
  a 
  process 
  might 
  have 
  been 
  possible, 
  

   but 
  had 
  it 
  actually 
  occurred 
  the 
  forms 
  resulting 
  would 
  have 
  been 
  

   totally 
  different 
  in 
  structure 
  from 
  any 
  now 
  known. 
  Instead 
  of 
  

   having 
  the 
  primary 
  lamellae 
  and 
  jugum 
  on 
  the 
  dorsal 
  side 
  as 
  in 
  

   all 
  shells 
  with 
  everted 
  spirals, 
  these 
  parts 
  would 
  lie 
  on 
  the 
  ventral 
  

   side 
  of 
  the 
  shell. 
  It 
  must 
  hence 
  be 
  inferred 
  that 
  the 
  Spiriferidm, 
  

   the 
  Athyridm, 
  the 
  Meristibm, 
  and 
  all 
  genera 
  with 
  everted 
  

  

  177 
  

  

  