﻿Report 
  of 
  the 
  State 
  Geologist. 
  829 
  

  

  spiral 
  cones 
  exists. 
  It 
  is 
  true 
  that 
  there 
  is 
  a 
  difference 
  of 
  180° 
  in 
  

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

   Splrifee, 
  but 
  the 
  spirals 
  have 
  never, 
  by 
  gradual 
  changes, 
  re 
  vol 
  ed 
  

   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 
  knovn. 
  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 
  Sfiriferid^j, 
  the 
  

   A 
  thyridm, 
  the 
  Meristidm 
  and 
  all 
  genera 
  with 
  everted 
  brachidia, 
  

   are 
  related 
  to 
  the 
  Atrypibje 
  only 
  through 
  their 
  early 
  ancestral 
  

   forms. 
  

  

  The 
  Lower 
  Silurian 
  faunas 
  have 
  furnished 
  no 
  evidence 
  of 
  

   Helicopegmata 
  with 
  everted 
  spirals, 
  and 
  this 
  hiatus 
  in 
  our 
  knowl- 
  

   edge 
  forbids 
  any 
  satisfactory 
  deductions 
  as 
  to 
  the 
  source 
  or 
  deri- 
  

   vation 
  of 
  these 
  forms. 
  It 
  is 
  true 
  in 
  a 
  general 
  sense 
  that 
  the 
  

   eversion 
  of 
  the 
  spirals 
  is 
  accompanied 
  by 
  a 
  convexity 
  of 
  both 
  

   valves, 
  just 
  as 
  the 
  inverted 
  spirals 
  of 
  the 
  Atrypibm 
  are 
  associated 
  

   with 
  valves 
  of 
  notably 
  unequal 
  depth. 
  Still, 
  among 
  the 
  latter, 
  

   Glassia 
  possesses 
  biconvex 
  valves, 
  while 
  of 
  the 
  former, 
  the 
  group 
  

   composed 
  of 
  Coelospika, 
  Anoplotheca, 
  Koninceina 
  and 
  Amphi- 
  

   cllna 
  is 
  characterized 
  by 
  convexo-plane 
  or 
  convexo-concave 
  

   valves. 
  In 
  this 
  group 
  also 
  the 
  apices 
  of 
  the 
  spirals 
  are 
  not 
  

   directed 
  toward 
  the 
  lateral 
  commissures 
  of 
  the 
  valves, 
  but 
  

   toward 
  the 
  lateral 
  slopes 
  of 
  the 
  pedicle-valve, 
  such 
  a 
  form 
  and 
  

   direction 
  being 
  a 
  necessary 
  outcome 
  of 
  the 
  contracted 
  interior 
  

   space. 
  

  

  We 
  are 
  at 
  present 
  disposed 
  to 
  believe 
  that 
  among 
  the 
  early 
  

   Silurian 
  species 
  will 
  be 
  found 
  some 
  whose 
  spiral 
  ribbon 
  deviates 
  

   outwardly 
  from 
  the 
  vertical 
  plane 
  to 
  the 
  same 
  degree 
  as 
  it 
  is 
  

   found 
  to 
  incline 
  inwardly 
  in 
  Ctclospira 
  and 
  Pbotozyga. 
  Indeed 
  

   in 
  Cyclosjpira 
  bisulcata 
  itself 
  the 
  spiral 
  sometimes 
  lies 
  so 
  nearly 
  

   in 
  the 
  vertical 
  plane 
  that 
  the 
  inward 
  inclination 
  of 
  the 
  apices 
  is 
  

   not 
  always 
  positive. 
  Only 
  some 
  such 
  form 
  of 
  the 
  earlier 
  faunas 
  

   could 
  have 
  been 
  the 
  progenitor 
  of 
  the 
  everted 
  spirals. 
  

  

  In 
  the 
  Atrypibm 
  possibilities 
  of 
  variation 
  in 
  the 
  form 
  of 
  

   the 
  jugum 
  were 
  much 
  restricted; 
  in 
  the 
  other 
  groups 
  of 
  

   the 
  Helicopegmata 
  these 
  were 
  very 
  great 
  and 
  resulted 
  in 
  

  

  