﻿Geology 
  and 
  Mineralogy. 
  2'27 
  

  

  Keokuk, 
  this 
  progressive 
  differentiation 
  is 
  most 
  striking, 
  the 
  

   genera 
  departing 
  more 
  widely 
  from 
  each 
  other 
  than 
  in 
  any 
  

   previous 
  epoch. 
  The 
  most 
  notable 
  character 
  of 
  these 
  faunas 
  is 
  

   the 
  loss 
  of 
  the 
  prismatic 
  form 
  in 
  many 
  genera 
  and 
  the 
  assump- 
  

   tion 
  of 
  flowing 
  outlines. 
  Many 
  Upper 
  Devonian 
  species 
  show 
  

   the 
  tendency 
  toward 
  this 
  feature 
  in 
  the 
  smooth 
  upper 
  part 
  of 
  the 
  

   vase 
  in 
  old 
  individuals. 
  The 
  loss 
  of 
  the 
  prismatic 
  angles 
  is 
  a 
  

   result 
  of 
  equalization 
  of 
  growth 
  among 
  several 
  of 
  the 
  higher 
  

   orders 
  of 
  spicular 
  bundles. 
  While 
  this 
  tendency 
  is 
  an 
  approach 
  

   to 
  the 
  simple 
  primitive 
  type, 
  peculiar 
  modifications 
  have 
  taken 
  

   place 
  in 
  other 
  directions. 
  In 
  the 
  earlier, 
  nodose 
  forms, 
  the 
  

   nodes 
  were 
  clearly 
  determined 
  by 
  the 
  intersection 
  of 
  primary, 
  

   vertical 
  spicular 
  bundles 
  and 
  annular 
  enlargements. 
  

  

  In 
  the 
  Lower 
  Carboniferous 
  genus 
  Cleodictya, 
  on 
  the 
  contrary, 
  

   a 
  whorl 
  of 
  rounded 
  obtuse 
  nodes 
  persists 
  near 
  the 
  base, 
  but 
  

   these 
  are 
  no 
  longer 
  determined, 
  as 
  formerly, 
  by 
  prominent 
  spicu- 
  

   lar 
  bundles, 
  the 
  entire 
  reticulum 
  being 
  a 
  uniform 
  fine 
  meshwork. 
  

   It 
  is 
  an 
  interesting 
  instance 
  of 
  the 
  persistence 
  of 
  a 
  feature 
  after 
  

   the 
  determining 
  cause 
  had 
  vanished. 
  Another 
  noteworthy 
  modi- 
  

   fication 
  appearing 
  for 
  the 
  first 
  time 
  is 
  the 
  checkerboard 
  surface 
  of 
  

   Physospongia. 
  In 
  this 
  genus, 
  every 
  alternate 
  square 
  is 
  raised, 
  

   blister-like, 
  above 
  the 
  general 
  surface 
  of 
  the 
  network, 
  and 
  the 
  

   intermediate 
  squares 
  are 
  depressed 
  an 
  equal 
  amount. 
  The 
  

   effect 
  is 
  that 
  of 
  a 
  checkerboard 
  in 
  which 
  the 
  squares 
  of 
  

   one 
  color 
  should 
  be 
  raised 
  and 
  those 
  of 
  the 
  other 
  color 
  sunk 
  

   below 
  the 
  general 
  level 
  of 
  the 
  dividing 
  lines. 
  Although 
  this 
  

   gives 
  a 
  nodose 
  appearance, 
  it 
  is 
  seen 
  to 
  have 
  no 
  relation 
  to 
  the 
  

   nodose 
  forms 
  of 
  the 
  earlier 
  genera. 
  

  

  Still 
  another 
  line 
  of 
  modification 
  is 
  seen 
  in 
  Clathrospongia. 
  

   Here 
  the 
  spicular 
  bundles 
  become 
  horizontal 
  and 
  vertical 
  plates, 
  

   perpendicular 
  to 
  the 
  body-wall, 
  changing 
  what 
  was 
  originally 
  a 
  

   network 
  into 
  a 
  series 
  of 
  pigeon 
  holes, 
  in 
  which 
  primary, 
  secondary, 
  

   and 
  tertiary 
  partitions 
  can 
  still 
  be 
  recognized. 
  The 
  depth 
  of 
  the 
  

   wall 
  increased 
  until 
  it 
  finally 
  equalled 
  a 
  quarter 
  of 
  the 
  radius 
  of 
  

   the 
  vase. 
  

  

  It 
  yet 
  remains 
  to 
  trace 
  the 
  subsequent 
  history 
  of 
  this 
  ancient 
  

   family 
  of 
  sponges 
  and 
  to 
  work 
  out 
  their 
  relationships 
  with 
  the 
  

   Dictyonine 
  Hexactinellida 
  of 
  later 
  periods. 
  Although 
  in 
  some 
  

   respects 
  these 
  ancient 
  vase-like 
  sponges 
  were 
  quite 
  ornate 
  and 
  

   showed 
  much 
  differentiation, 
  their 
  primitive 
  character 
  is 
  seen 
  in 
  

   the 
  skeletal 
  details 
  and 
  general 
  form. 
  

  

  The 
  spicules 
  were 
  not 
  fused 
  together 
  as 
  in 
  the 
  majority 
  of 
  

   modern 
  Hexactinellids, 
  but 
  even 
  in 
  the 
  spicular 
  bundles 
  maintain 
  

   their 
  individuality. 
  In 
  the 
  mature 
  stages 
  of 
  Euplectella 
  and 
  cer- 
  

   tain 
  other 
  existing 
  genera 
  there 
  is 
  a 
  terminal 
  sieve-like 
  plate 
  

   extending 
  across 
  the 
  upper 
  part 
  of 
  the 
  cup 
  which 
  does 
  not 
  show 
  

   a 
  direct 
  derivation 
  from 
  the 
  walls. 
  In 
  the 
  Dictyospongidoi, 
  there 
  

   is 
  no 
  evidence 
  of 
  such 
  a 
  modified 
  protective 
  structure. 
  The 
  most 
  

   specialized 
  feature 
  of 
  this 
  family 
  is 
  the 
  basal 
  disk 
  of 
  Phragmo- 
  

   dictya. 
  In 
  this 
  genus 
  the 
  body-wall 
  is 
  cylindrical 
  and 
  the 
  base 
  is 
  

  

  