﻿1108 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum, 
  

  

  Some 
  of 
  the 
  accessory 
  causes 
  contributing 
  to 
  the 
  production 
  of 
  

   the 
  vaulted 
  plates 
  seem 
  to 
  have 
  been 
  efficient 
  at 
  various 
  times 
  

   throughout 
  the 
  history 
  of 
  the 
  entire 
  class, 
  and 
  we 
  find 
  them 
  

   manifested 
  in 
  the 
  Articulate 
  Brachiopods 
  by 
  the 
  marginal 
  eleva- 
  

   tion 
  of 
  the 
  muscular 
  area 
  in 
  Douvtllina 
  and 
  Lept^cta; 
  they 
  

   probably 
  also 
  served 
  to 
  render 
  more 
  complete 
  the 
  isolation 
  and 
  

   perfection 
  of 
  the 
  spondylium, 
  but 
  the 
  true 
  platform 
  does 
  not 
  

   again 
  appear. 
  (For 
  suggestions 
  as 
  to 
  the 
  organic 
  causes 
  of 
  

   such 
  elevations 
  see 
  p. 
  194.) 
  With 
  the 
  knowledge 
  of 
  so 
  many 
  

   transition 
  genera 
  between 
  the 
  oboloids 
  and 
  linguloids 
  and 
  

   trimerellids, 
  it 
  is 
  a 
  pure 
  convention 
  to 
  accord 
  recognition 
  to 
  the 
  

   families 
  Obolibm, 
  Ling 
  ulidje 
  and 
  Trimerellibm. 
  

  

  This 
  entire 
  group 
  of 
  genera 
  is 
  characterized 
  by 
  the 
  presence 
  of 
  

   an 
  unenclosed 
  marginal 
  pedicle. 
  They 
  compose 
  the 
  Mesocattlia- 
  

   or 
  Lingulacea 
  of 
  Waag-en 
  (Atremata 
  of 
  Beecher). 
  % 
  

  

  The 
  second 
  main 
  division 
  of 
  the 
  Inarticulate 
  genera 
  is 
  com- 
  

   posed 
  of 
  those 
  in 
  which 
  the 
  pedicle- 
  aperture, 
  in 
  immature 
  stages 
  

   or 
  in 
  primitive 
  adult 
  conditions 
  takes 
  the 
  form 
  of 
  a 
  marginal 
  

   incision 
  of 
  the 
  pedicle-valve, 
  but 
  becomes 
  enclosed 
  in 
  the 
  shell- 
  

   substance 
  in 
  later 
  stages 
  of 
  growth. 
  To 
  this 
  group 
  Waagen 
  

   applied 
  the 
  term 
  Diacattlia* 
  or 
  Discinacea 
  (1883). 
  

  

  In 
  the 
  genus 
  Orbiculoidea, 
  early 
  stages 
  of 
  the 
  shell 
  have 
  the 
  

   pedicle-passage 
  or 
  open 
  triangular 
  incision 
  extending 
  with 
  widen- 
  

   ing 
  margins 
  from 
  the 
  apex 
  of 
  the 
  pedicle-valve 
  to 
  the 
  periphery. 
  

   With 
  advance 
  of 
  age 
  this 
  passage 
  becomes 
  contracted 
  and 
  

   normally 
  closed 
  at 
  the 
  posterior 
  margin, 
  and 
  may 
  be 
  variously 
  

   modified 
  by 
  the 
  deposition 
  of 
  adventitious 
  deposits 
  about 
  it. 
  In 
  

   the 
  Cambrian 
  genus 
  Discinolepis 
  the 
  open 
  slit 
  is 
  a 
  mature 
  char- 
  

   acter, 
  and 
  it 
  also 
  appears 
  in 
  several 
  similar 
  genera 
  of 
  later 
  date, 
  

   sometimes 
  slightly 
  modified, 
  e. 
  g., 
  Trematis, 
  Schizobolus, 
  

   GEhlertella, 
  in 
  others 
  quite 
  primitive 
  and 
  unchanged, 
  e. 
  g., 
  

   Schizocrania. 
  These 
  are 
  all 
  orbicular 
  and 
  highly 
  chitinous 
  

   shells; 
  the 
  line 
  along 
  which 
  they 
  have 
  been 
  derived 
  departed 
  

   at 
  an 
  early 
  period 
  from 
  the 
  radicle 
  stock, 
  apparently 
  attained 
  

   its 
  diverse 
  variations 
  in 
  Palaeozoic 
  time, 
  and 
  has 
  been 
  continued 
  

   to 
  the 
  present 
  in 
  the 
  most 
  highly 
  modified 
  representatives 
  of 
  the 
  

  

  * 
  This 
  name 
  was 
  originally 
  printed 
  Daikaulia, 
  probably 
  a 
  typographical 
  error 
  in 
  the 
  spelling 
  

   of 
  the 
  first 
  syllables. 
  

  

  166 
  

  

  