﻿Report 
  of 
  the 
  State 
  Geologist. 
  825 
  

  

  of 
  Oktbis 
  in 
  its 
  restricted 
  meaning, 
  such 
  as 
  0. 
  callactis, 
  0. 
  

   costalis, 
  etc.) 
  is 
  maintained 
  in 
  all 
  the 
  numerous 
  subdivisions 
  of 
  

   the 
  genus, 
  with 
  the 
  exception 
  of 
  Khipidomella 
  in 
  which 
  there 
  is 
  

   a 
  great 
  expansion 
  of 
  the 
  muscular 
  scars, 
  similar 
  to 
  that 
  in 
  the 
  

   Stbophomenidje 
  and 
  to 
  which 
  reference 
  has 
  just 
  been 
  made. 
  

   Otherwise 
  the 
  sessile 
  condition 
  of 
  the 
  spondylium 
  is 
  not 
  modified 
  

   throughout 
  the 
  entire 
  history 
  of 
  this 
  group. 
  

  

  The 
  elevation 
  of 
  the 
  spondylioid 
  plate 
  or 
  the 
  base 
  of 
  the 
  

   pedicle-cavity 
  into 
  a 
  true 
  spondylium, 
  is 
  a 
  phenomenon 
  of 
  equally 
  

   early 
  age 
  to 
  the 
  two 
  conditions 
  already 
  discussed. 
  It 
  appears 
  in 
  

   a 
  highly-developed 
  state 
  in 
  conjunction 
  with 
  the 
  unmodified 
  

   deltidium, 
  first 
  in 
  Protorthis 
  of 
  the 
  Cambrian, 
  then 
  in 
  Poly- 
  

   tcechia, 
  Syntrophia, 
  CLirAMBoxiTEs 
  and 
  Scentditjm 
  of 
  the 
  early 
  

   and 
  later 
  Silurian 
  and 
  the 
  Devonian. 
  

  

  A 
  parallel 
  line 
  of 
  development 
  is 
  exhibited 
  by 
  spondylium- 
  

   bearing 
  forms 
  in 
  which 
  the 
  deltidium 
  disappeared 
  at 
  a 
  very 
  early 
  

   period, 
  and 
  the 
  shells 
  possess 
  a 
  trihedral, 
  generally 
  coarsely 
  

   plicated 
  and 
  decidedly 
  rhynchonelloid 
  exterior. 
  It 
  seems 
  highly 
  

   probable 
  that 
  this 
  line 
  was 
  differentiated 
  in 
  the 
  early 
  Cambrian, 
  

   as 
  indications 
  of 
  this 
  structure 
  are 
  observable 
  in 
  some 
  primordial 
  

   species, 
  as 
  Carrier 
  ella? 
  minor, 
  "Walcott, 
  and 
  StricMindinia 
  ? 
  

   JBalcletchensis, 
  Davidson; 
  in 
  the 
  Silurian 
  it 
  is 
  represented 
  by 
  

   Camarella 
  and 
  Parastrophia, 
  also 
  by 
  the 
  more 
  rotund 
  and 
  more 
  

   finely 
  plicate 
  shells, 
  Axastrophia, 
  Porambonites, 
  Lycophoria 
  and 
  

  

  J^GETLIKGIA. 
  

  

  The 
  genera 
  last 
  named 
  are 
  not 
  homogeneous 
  with 
  the 
  others 
  in 
  

   the 
  phases 
  of 
  development 
  which 
  they 
  represent, 
  all 
  retaining 
  the 
  

   cardinal 
  areas 
  more 
  or 
  less 
  distinctly, 
  while 
  Lycophoria 
  and 
  

   NasTLmGiA 
  also 
  possess 
  a 
  cardinal 
  process 
  in 
  the 
  brachial 
  valve. 
  

  

  The 
  presence 
  of 
  the 
  cardinal 
  area 
  in 
  such 
  early 
  structures 
  must 
  

   be 
  regarded 
  as 
  a 
  retention 
  rather 
  than 
  a 
  resumption 
  of 
  a 
  primi- 
  

   tive 
  character. 
  

  

  "Whatever 
  may 
  be 
  the 
  oscillation 
  in 
  form 
  and 
  the 
  variation 
  in 
  

   secondary 
  characters 
  presented 
  by 
  Camarella, 
  Parastrophia 
  

   and 
  their 
  allies, 
  present 
  evidence 
  indicates 
  that 
  they 
  must 
  be 
  

   regarded 
  as 
  the 
  genetic 
  precursors, 
  as 
  they 
  are 
  the 
  temporal 
  pre- 
  

   cedents 
  of 
  the 
  great 
  group 
  of 
  true 
  pentameroids 
  (PENTAMHRirg, 
  

   Capelllnia, 
  Co.nchidium, 
  Barrandella, 
  Sieberella, 
  Pentamur- 
  

   ella, 
  Gypjdtjla, 
  Stricklandinia, 
  Amphigeiha) 
  and, 
  indeed, 
  the 
  

   lt>4 
  

  

  