﻿Meport 
  of 
  the 
  State 
  Geologist. 
  809 
  

  

  oped. 
  Thus 
  all 
  these 
  genera, 
  from 
  Acrothele 
  to 
  Schizambon 
  

   and 
  Siphonotreta, 
  possess 
  an 
  apical 
  foramen, 
  and 
  the 
  develop- 
  

   ment 
  both 
  of 
  the 
  internal 
  tube 
  and 
  the 
  corresponding 
  external 
  

   groove 
  has 
  been 
  a 
  gradual 
  one. 
  They 
  represent 
  termini 
  of 
  

   slightly 
  divergent 
  series 
  ; 
  consequently 
  they 
  may 
  all 
  be 
  safely 
  

   included 
  under 
  the 
  old 
  family 
  designation 
  introduced 
  by 
  Ketorga 
  

  

  in 
  1848, 
  SlPHONOTRETIDsE. 
  

  

  Crania 
  and 
  its 
  allies 
  (Craniella, 
  Pseudocrania, 
  Pholidops) 
  

   constitute 
  a 
  group 
  in 
  which 
  there 
  is, 
  thus 
  far, 
  no 
  satisfactory 
  evi- 
  

   dence 
  of 
  the 
  existence 
  of 
  the 
  pedicle, 
  and 
  we 
  are 
  left 
  to 
  the 
  infer- 
  

   ence 
  that 
  this 
  organ 
  became 
  atrophied 
  at 
  a 
  very 
  early 
  growth- 
  

   stage. 
  The 
  study 
  of 
  recent 
  Cranias 
  has 
  not 
  yet 
  determined 
  this 
  

   point, 
  but 
  this 
  will 
  probably 
  be 
  ultimately 
  accomplished. 
  At 
  

   whatever 
  stage 
  of 
  growth 
  the 
  pedicle 
  was 
  lost, 
  we 
  may 
  infer 
  that 
  

   its 
  disappearance 
  in 
  Crania, 
  and 
  generally 
  in 
  Craniella, 
  was 
  

   directly 
  followed 
  by 
  a 
  solid 
  fixation 
  of 
  the 
  animal 
  by 
  the 
  sub- 
  

   stance 
  of 
  one 
  of 
  the 
  valves. 
  In 
  Pholidops 
  there 
  was 
  no 
  such 
  

   cementation, 
  but 
  at 
  a 
  correspondingly 
  early 
  stage 
  the 
  shell 
  became 
  

   wholly 
  independent. 
  All 
  these 
  shells 
  with 
  central 
  or 
  subcentral 
  

   beaks 
  have 
  an 
  external 
  resemblance 
  to 
  Orbiculoidea 
  ; 
  the 
  forma- 
  

   tion 
  of 
  the 
  secondary 
  growth 
  of 
  the 
  valves 
  behind 
  the 
  apices 
  or 
  

   position 
  of 
  the 
  protoconch, 
  is 
  a 
  further 
  substantial 
  agreement 
  

   with 
  the 
  Diacaulia 
  as 
  contrasted 
  with 
  the 
  abbreviated 
  posterior 
  

   peripheral 
  shell-growth 
  in 
  the 
  Mesocaijlia 
  (Llngula, 
  Obolus)* 
  

   It 
  is 
  nevertheless 
  to 
  be 
  observed 
  that 
  no 
  trace 
  of 
  a 
  former 
  ped- 
  

   icle-slit, 
  incision 
  or 
  perforation 
  is 
  found 
  on 
  mature 
  or 
  immature 
  

   shells, 
  and 
  it 
  would 
  be 
  difficult 
  to 
  comprehend 
  in 
  what 
  manner 
  

   such 
  an 
  essential 
  modification 
  of 
  the 
  shell 
  could 
  be 
  wholly 
  con- 
  

   cealed 
  by 
  later 
  growth.* 
  Were 
  the 
  pedicle 
  marginal 
  in 
  prim- 
  

   itive 
  growth-stages, 
  and 
  subsequently 
  atrophied, 
  the 
  obliteration 
  

   of 
  the 
  marginal 
  opening 
  by 
  later 
  resorption 
  and 
  growth 
  would 
  be 
  

   a 
  readily 
  intelligible 
  process. 
  There 
  is, 
  hence, 
  in 
  this 
  default 
  of 
  

   evidence, 
  a 
  good 
  reason 
  to 
  doubt 
  the 
  close 
  affinities 
  of 
  Crania 
  

   and 
  Pholidops 
  to 
  the 
  Diacaulia. 
  Present 
  knowledge 
  would 
  

   seem 
  to 
  indicate 
  that 
  they 
  were 
  primarily 
  of 
  the 
  type 
  of 
  the 
  

   Mesocaijlia, 
  and 
  that 
  their 
  resemblance 
  to 
  the 
  Diacaulia 
  is 
  

  

  * 
  Quite 
  earJy 
  conditions 
  of 
  Crania 
  siluriana 
  and 
  Craniella 
  Hamilton-ice-, 
  from 
  1.5 
  to 
  .5 
  mm. 
  

   in 
  diameter, 
  are 
  fully 
  cemented. 
  Examples 
  of 
  Pholidops 
  Hamiltonice, 
  not 
  above 
  .5 
  mm. 
  in 
  

   diameter, 
  give 
  no 
  indication 
  of 
  a 
  pedicle-passage 
  or 
  surface 
  characters 
  not 
  present 
  in 
  the 
  adult. 
  

  

  102 
  

  

  