﻿Report 
  of 
  the 
  State 
  Geologist. 
  823 
  

  

  €on 
  cave 
  ; 
  the 
  other, 
  as 
  in 
  Steophomena, 
  having 
  this 
  contour 
  

   reversed, 
  the 
  pedicle-valve 
  at 
  first 
  convex, 
  but 
  subsequently 
  

   and 
  through 
  all 
  later 
  growth- 
  stages 
  concave, 
  while 
  the 
  

   brachial 
  valve 
  becomes 
  correspondingly 
  convex. 
  In 
  both 
  cases, 
  

   as 
  in 
  other 
  brachiopods, 
  the 
  primitive 
  and 
  post-embryonic 
  valves 
  

   are 
  both 
  convex. 
  The 
  peculiar 
  reversal 
  of 
  contour, 
  which 
  is 
  never 
  

   more 
  extremely 
  manifested 
  than 
  in 
  this 
  group, 
  but 
  nevertheless 
  

   occurs 
  in 
  other 
  genera, 
  such 
  as 
  Ateypa, 
  many 
  Rhynchonella-, 
  etc., 
  

   is 
  a 
  purely 
  secondary 
  condition. 
  Its 
  causes 
  have 
  not 
  been 
  fully 
  

   investigated, 
  but 
  an 
  unequal 
  peripheral 
  growth 
  of 
  the 
  two 
  valves 
  

   arising 
  from 
  inequality 
  in 
  the 
  size 
  of 
  the 
  ventral 
  and 
  dorsal 
  

   mantle 
  lobes, 
  seems 
  to 
  be 
  a 
  partial 
  if 
  not 
  sufficient 
  explanation 
  of 
  

   its 
  existence. 
  As 
  eitheT 
  the 
  presence 
  or 
  absence 
  of 
  this 
  reversal 
  

   is 
  a 
  normal 
  secondary 
  condition, 
  it 
  is 
  not 
  possible 
  to 
  give 
  it 
  great 
  

   weight 
  in 
  a 
  broader 
  grouping 
  of 
  the 
  genera, 
  for 
  we 
  find 
  that 
  

   Steophonella 
  is 
  but 
  a 
  reversed 
  Steopheodonta, 
  passing 
  through 
  

   similar 
  phases 
  ; 
  Amphisteophia 
  is 
  a 
  reversed 
  Beachypeion, 
  both 
  

   existing 
  in 
  faunas 
  of 
  the 
  same 
  age, 
  and 
  Steophomena 
  is 
  a 
  reversed 
  

   Rafinesqtjina, 
  both 
  similarly 
  coexistent. 
  

  

  With 
  this 
  presentation 
  of 
  the 
  subject 
  it 
  seems 
  neither 
  necessary 
  

   nor 
  desirable 
  to 
  propose 
  any 
  broad 
  division 
  of 
  this 
  group 
  of 
  

   genera. 
  In 
  1846 
  King 
  proposed 
  to 
  embrace 
  Steophomina 
  and 
  its 
  

   allies, 
  in 
  the 
  family 
  Strophomemd^e. 
  The 
  large 
  number 
  of 
  gen- 
  

   eric 
  values 
  allied 
  to 
  Strophomena, 
  which 
  have 
  been 
  determined 
  

   since 
  that 
  date, 
  make 
  this 
  comprehensive 
  family 
  divisible 
  ad 
  

   libitum, 
  seel 
  non 
  in 
  majorem 
  Dei 
  gloriam. 
  

  

  The 
  calcareous 
  fixation 
  of 
  the 
  pedicle-valve 
  to 
  extraneous 
  

   bodies 
  after 
  the 
  closure 
  of 
  the 
  pedicle-passage 
  and 
  atrophy 
  of 
  the 
  

   pedicle 
  itself, 
  is 
  repeatedly 
  manifested 
  by 
  these 
  shells. 
  This, 
  as 
  

   already 
  shown, 
  is 
  a 
  pre 
  adult 
  condition 
  in 
  Oethothetes, 
  Derbya 
  

   and 
  Steeptoehynchus, 
  the 
  shell 
  becoming 
  wholly 
  free 
  before 
  

   full 
  growth 
  was 
  attained 
  ; 
  but 
  in 
  Lept^enisca 
  and 
  Davidsonia 
  the 
  

   attachment 
  was 
  maintained 
  throughout 
  the 
  later 
  existence 
  of 
  the 
  

   shell. 
  

  

  The 
  impressions 
  left 
  by 
  the 
  spiral 
  arms 
  upon 
  the 
  interior 
  of 
  

   the 
  valves 
  in 
  Dayidsonia 
  and 
  Leptjenisca, 
  and 
  also 
  observed 
  by 
  

   Davidson 
  in 
  specimens 
  of 
  Bafinesquina 
  Jukesi 
  and 
  Zeptcena 
  rhom- 
  

   boidalis, 
  show 
  a 
  complete 
  correspondence 
  in 
  the 
  direction 
  and 
  

   curvature 
  of 
  the 
  coils, 
  and 
  we 
  are 
  left 
  to 
  infer 
  that 
  other 
  members 
  

  

  