﻿Report 
  of 
  the 
  State 
  Geologist. 
  1113 
  

  

  Stropheodonta, 
  Strophonella 
  and 
  Leptostrophla, 
  where 
  it 
  has 
  

   become 
  almost, 
  and 
  sometimes 
  quite 
  obliterated, 
  and 
  the 
  entire 
  

   pedicle 
  and 
  umbonal 
  cavity 
  rilled 
  with 
  testaceous 
  secretions. 
  

   Such 
  filling 
  can 
  occur 
  only 
  in 
  a 
  discarded 
  and 
  useless 
  space, 
  after 
  

   the 
  pedicle 
  has 
  ceased 
  to 
  be 
  functional. 
  A 
  morphological 
  con- 
  

   sideration 
  of 
  much 
  importance 
  presents 
  itself 
  here, 
  as 
  well 
  as 
  in 
  

   many 
  other 
  groups 
  of 
  genera 
  where 
  the 
  shells 
  attain 
  great 
  size. 
  

   The 
  evidence 
  is 
  very 
  direct 
  from 
  the 
  study 
  of 
  the 
  structural 
  

   features 
  as 
  given 
  above, 
  that 
  the 
  entire 
  muscular 
  system 
  on 
  the 
  

   ventral 
  side 
  of 
  the 
  body 
  is, 
  in 
  primitive 
  forms, 
  inserted 
  upon 
  the 
  

   base 
  of 
  the 
  pedicle-cavity. 
  This 
  is 
  apparent 
  from 
  a 
  study 
  of 
  such 
  

   a 
  shell 
  as 
  Orthis 
  callactis, 
  where 
  it 
  is 
  perfectly 
  clear 
  that 
  no 
  

   muscular 
  bands 
  were 
  attached 
  to 
  the 
  pedicle-valve 
  outside 
  the 
  

   limits 
  of 
  this 
  strong 
  and 
  condensed 
  posterior 
  area, 
  which 
  is 
  but 
  

   a 
  sessile 
  spondylium. 
  The 
  contraction 
  of 
  this 
  pedicle-cavity 
  is 
  

   accompanied 
  by 
  (whether 
  in 
  relation 
  of 
  cause 
  to 
  effect 
  may 
  not 
  

   be 
  stated) 
  a 
  diffusion 
  of 
  the 
  area 
  of 
  muscular 
  attachment, 
  and 
  

   when 
  the 
  shells 
  are 
  large, 
  as 
  in 
  Strophomena, 
  Rafinesquina, 
  

   Stropheodonta, 
  Orthothetes, 
  Deebya, 
  etc., 
  the 
  necessity 
  for 
  

   powerful 
  muscles 
  or 
  some 
  similar 
  cause 
  magnifies 
  this 
  expansion 
  

   of 
  the 
  muscular 
  area 
  until 
  the 
  original 
  contents 
  of 
  the 
  pedicle- 
  

   cavity 
  may 
  be 
  represented 
  by 
  enormous 
  muscles 
  whose 
  scars 
  

   extend 
  almost 
  to 
  the 
  anterior 
  margin 
  of 
  the 
  valve, 
  as 
  in 
  

   Hipparionyx 
  and 
  E-hipidomella. 
  

  

  In 
  this 
  great 
  group 
  of 
  genera 
  there 
  are 
  two 
  types 
  of 
  contour, 
  

   one, 
  as 
  in 
  Lept^ena 
  3 
  being 
  normally 
  convexo-concave, 
  that 
  is, 
  

   with 
  the 
  pedicle-valve 
  convex 
  and 
  the 
  brachial 
  valve 
  parallel 
  to 
  

   it 
  and 
  concave 
  ; 
  the 
  other, 
  as 
  in 
  Strophomena, 
  having 
  this 
  con- 
  

   tour 
  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 
  Atrypa, 
  many 
  KhynchoneHas, 
  etc., 
  

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

   investigated, 
  but 
  an 
  unequal 
  peripheral 
  growth 
  of 
  the 
  two 
  valves 
  

   seems 
  to 
  be 
  a 
  sufficient 
  explanation 
  of 
  its 
  existence. 
  As 
  either 
  

   the 
  presence 
  or 
  absence 
  of 
  this 
  reversal 
  is 
  a 
  normal 
  secondary 
  

   140 
  1T1 
  

  

  