﻿1120 
  Forty-seventh 
  Report 
  on 
  the 
  State 
  Museum. 
  

  

  brachidia 
  are 
  related 
  to 
  the 
  Atrtpidm 
  only 
  through 
  their 
  early- 
  

   ancestral 
  forms. 
  

  

  The 
  Lower 
  Silurian 
  faunas 
  have 
  furnished 
  no 
  evidence 
  of 
  

   species 
  with 
  everted 
  spirals, 
  and 
  this 
  hiatus 
  in 
  our 
  knowledge 
  

   forbids 
  any 
  satisfactory 
  deductions 
  as 
  to 
  the 
  source 
  or 
  derivation 
  

   of 
  these 
  forms. 
  It 
  is 
  true 
  in 
  a 
  general 
  sense 
  that 
  the 
  e 
  version 
  of 
  

   the 
  spirals 
  is 
  accompanied 
  by 
  a 
  convexity 
  of 
  both 
  valves, 
  just 
  as 
  

   the 
  inverted 
  spirals 
  of 
  the 
  Atrypibm 
  are 
  associated 
  with 
  valves 
  

   of 
  notably 
  unequal 
  depth. 
  Still, 
  among 
  the 
  latter, 
  Glassia 
  

   possesses 
  biconvex 
  valves, 
  while 
  of 
  the 
  former 
  the 
  group 
  composed 
  

   of 
  Ccelospira, 
  Anoplotheca, 
  Koninckina 
  and 
  Amphiclina 
  is 
  

   characterized 
  by 
  convexo-plane 
  or 
  convexo-concave 
  valves. 
  In 
  

   this 
  group 
  also 
  the 
  apices 
  of 
  the 
  spirals 
  are 
  not 
  directed 
  toward 
  

   the 
  lateral 
  commissures 
  of 
  the 
  valves, 
  but 
  toward 
  the 
  lateral 
  

   slopes 
  of 
  the 
  pedicle-valve, 
  such 
  a 
  form 
  and 
  direction 
  being 
  a 
  

   necessary 
  outcome 
  of 
  the 
  contracted 
  interior 
  space. 
  From 
  present 
  

   evidence 
  it 
  would 
  seem 
  probable 
  that 
  among 
  the 
  early 
  Silurian 
  

   species 
  will 
  be 
  found 
  some 
  form 
  whose 
  spiral 
  ribbon 
  deviates 
  

   outwardly 
  from 
  the 
  vertical 
  plane 
  to 
  the 
  same 
  degree 
  as 
  it 
  inclines 
  

   inwardly 
  in 
  Cyclospira 
  and 
  Frotozyga. 
  Indeed, 
  in 
  Cyclospira 
  

   hisulcata 
  itself, 
  the 
  spiral 
  sometimes 
  lies 
  so 
  nearly 
  in 
  the 
  vertical 
  

   plane 
  that 
  the 
  inward 
  inclination 
  of 
  the 
  apices 
  is 
  not 
  always 
  

   positive. 
  Only 
  some 
  such 
  form 
  of 
  the 
  earliest 
  faunas 
  could 
  have 
  

   been 
  the 
  progenitor 
  of 
  the 
  everted 
  spirals. 
  

  

  In 
  the 
  Atrtpidm 
  possibilities 
  of 
  variation 
  in 
  the 
  form 
  of 
  the 
  

   jugum 
  were 
  much 
  restricted; 
  in 
  the 
  other 
  groups 
  of 
  the 
  spire 
  

   bearers 
  they 
  were 
  very 
  great, 
  and 
  resulted 
  in 
  the 
  production 
  of 
  

   a 
  wonderful 
  series 
  of 
  modidcations 
  whose 
  relations 
  it 
  is 
  not 
  neces- 
  

   sary 
  to 
  rehearse 
  here. 
  The 
  extreme 
  range 
  of 
  these 
  modifications 
  

   is 
  seen 
  in 
  the 
  simple 
  termination 
  of 
  the 
  jugum 
  in 
  Whitfieldella, 
  

   Khynchospira, 
  etc. 
  ; 
  the 
  bifurcate 
  extremity 
  in 
  Mekistina, 
  Eume- 
  

   tria 
  and 
  Retzia, 
  these 
  terminal 
  branches 
  in 
  Kayseria, 
  Diplospi- 
  

   rella, 
  etc., 
  finally 
  becoming 
  coextensive 
  with 
  the 
  lamellae 
  of 
  the 
  

   primary 
  spirals 
  and 
  thus 
  forming 
  a 
  second 
  pair 
  of 
  spiral 
  cones. 
  

   This 
  complication 
  of 
  the 
  brachidium 
  is 
  effected 
  only 
  late 
  in 
  the 
  

   history 
  of 
  the 
  various 
  groups 
  producing 
  them. 
  Koninckina 
  and 
  

   Amphiclina 
  are 
  double-spiraled 
  convexo-concave 
  shells 
  which 
  are 
  

   the 
  post-palaeozoic 
  and 
  final 
  representatives 
  of 
  Anoplotheca 
  and 
  

   Ccelospira. 
  Pexidella 
  and 
  Diplospirella, 
  of 
  the 
  St. 
  Cassian 
  

  

  178 
  

  

  