﻿Report 
  of 
  the 
  State 
  Geologist. 
  1109 
  

  

  group, 
  Discina 
  and 
  Discinisca. 
  All 
  these 
  forms 
  are 
  conven- 
  

   iently 
  grouped 
  under 
  the 
  family 
  term 
  Discinidje. 
  

  

  In 
  AcEOTEETA, 
  CoNOTEETA, 
  LlNNAESSONfA, 
  AcEOTHELE 
  and 
  

  

  Iphidea 
  the 
  pedicle-aperture 
  is 
  persistently 
  located 
  at 
  the 
  apex 
  of. 
  

   the 
  pedicle- 
  valve. 
  This 
  group 
  of 
  genera 
  is 
  one 
  of 
  very 
  early 
  date, 
  

   for 
  the 
  most 
  part 
  contemporaneous 
  with 
  Patebina, 
  and 
  the 
  exist- 
  

   ing 
  evidence 
  would 
  indicate 
  that 
  it 
  was 
  not 
  directly 
  ancestral 
  to 
  

   the 
  line 
  of 
  Teematis-Oebicttloidea 
  (Discinidm). 
  The 
  incipi- 
  

   ent 
  formation 
  of 
  an 
  internal 
  foraminal 
  tube 
  is 
  seen 
  in 
  several 
  of 
  

   these 
  genera 
  (Aceoteeta, 
  Aceothele, 
  Linnaessonia), 
  and 
  this 
  

   feature 
  attains 
  its 
  maximum 
  in 
  the 
  true 
  Siphonoteeta 
  of 
  the 
  

   Lower 
  Silurian, 
  where 
  the 
  foramen 
  is 
  still 
  apical 
  and 
  the 
  tube 
  

   wholly 
  internal. 
  Hence 
  Siphonoteeta 
  appears 
  to 
  be 
  a 
  normal 
  

   termination 
  of 
  this 
  line 
  of 
  descent. 
  Schizambon, 
  in 
  the 
  compre- 
  

   hensive 
  meaning 
  of 
  the 
  term 
  ascribed 
  to 
  it 
  in 
  this 
  work, 
  has 
  the 
  

   pedicle-passage 
  superficial, 
  and 
  in 
  such 
  shells 
  as 
  Schizambon 
  Jlssus, 
  

   Kutorga, 
  and 
  var. 
  Canadensis, 
  Ami, 
  the 
  condition 
  of 
  this 
  passage 
  

   is 
  perfectly 
  analogous 
  to 
  that 
  of 
  Siphonoteeta, 
  the 
  entire 
  dif- 
  

   ference 
  being 
  in 
  the 
  enclosure 
  of 
  the 
  latter. 
  In 
  Schizambon 
  the 
  

   fibers 
  of 
  the 
  pedicle, 
  extending 
  through 
  the 
  foramen 
  near 
  the 
  

   middle 
  of 
  the 
  pedicle-valve, 
  were 
  directed 
  toward 
  the 
  apex 
  of 
  that 
  

   valve, 
  and 
  along 
  the 
  concave 
  floor 
  of 
  the 
  external 
  pedicle-groove. 
  

   The 
  inner 
  aperture 
  of 
  the 
  pedicle-tube 
  in 
  Siphonoteeta, 
  corre- 
  

   sponds 
  to 
  the 
  " 
  foramen 
  " 
  of 
  Schizambon, 
  and 
  the 
  outer 
  aperture, 
  

   or 
  true 
  foramen, 
  of 
  the 
  former 
  to 
  the 
  grooved 
  umbo 
  of 
  the 
  

   pedicle-valve 
  in 
  the 
  latter. 
  Hence 
  in 
  Schizambon, 
  thus 
  considered, 
  

   there 
  is 
  no 
  evidence 
  of 
  a 
  progress 
  of 
  the 
  external 
  aperture, 
  or 
  

   true 
  foramen, 
  anteriorly 
  beyond 
  the 
  apex 
  of 
  the 
  pedicle-valve. 
  

   These 
  two 
  genera 
  are 
  but 
  slight 
  departures 
  from 
  the 
  same 
  type 
  

   of 
  structure, 
  but 
  it 
  would 
  appear 
  that 
  this 
  deviation 
  took 
  place 
  

   during 
  primordial 
  times, 
  as 
  the 
  typical 
  Schizambon 
  (S. 
  typicalis, 
  

   "Walcott) 
  is 
  a 
  primordial 
  fossil. 
  The 
  genus, 
  Teematobolus, 
  

   Matthew 
  (T. 
  insignis, 
  Matthew, 
  type), 
  appears 
  to 
  be 
  another 
  

   primordial 
  representative 
  of 
  this 
  structure, 
  with 
  the 
  tubular 
  

   enclosure 
  of 
  the 
  pedicle 
  more 
  highly 
  developed. 
  Thus 
  all 
  these 
  

   genera, 
  from 
  Aceothele 
  to 
  Schizambon 
  and 
  Siphonoteilta, 
  possess 
  

   an 
  apical 
  foramen, 
  and 
  the 
  development 
  both 
  of 
  the 
  internal 
  tube 
  

   and 
  the 
  corresponding 
  external 
  groove 
  has 
  been 
  a 
  gradual 
  one. 
  

   They 
  represent 
  termini 
  of 
  slightly 
  divergent 
  series, 
  consequently 
  

  

  167 
  

  

  