﻿Geology 
  and 
  Mineralogy. 
  225 
  

  

  ing 
  allies 
  are 
  most 
  abundant 
  in 
  the 
  more 
  quiet 
  waters 
  with 
  

   muddy 
  bottoms, 
  which 
  form 
  the 
  deeper 
  parts 
  of 
  the 
  continental 
  

   shelves. 
  

  

  As 
  it 
  is 
  only 
  in 
  the 
  rarest 
  instances 
  that 
  the 
  minute 
  details 
  of 
  

   the 
  spicular 
  network 
  are 
  preserved, 
  the 
  separation 
  of 
  the 
  fossil 
  

   forms 
  into 
  species 
  rests 
  mainly 
  upon 
  more 
  general 
  characteristics. 
  

   For 
  the 
  same 
  reason 
  a 
  long 
  time 
  elapsed 
  before 
  the 
  true 
  nature 
  

   of 
  these 
  fossils 
  became 
  evident. 
  In 
  1842 
  Conrad 
  described 
  

   Hydnoceras 
  as 
  a 
  cephalopod 
  and 
  subsequently 
  these 
  Hexactinel- 
  

   lids 
  were 
  doubtfully 
  grouped 
  with 
  the 
  Alga3. 
  It 
  was 
  not 
  until 
  

   1881, 
  nearly 
  forty 
  years 
  later, 
  that 
  specimens 
  were 
  found 
  at 
  

   Crawfordsville, 
  Indiana, 
  showing 
  the 
  individual 
  spicules 
  and 
  

   their 
  spongious 
  affinities. 
  

  

  Considering 
  their 
  fragile 
  nature, 
  their 
  forms 
  are 
  remarkably 
  

   well 
  preserved. 
  In 
  some 
  instances 
  colonies 
  were 
  overwhelmed 
  by 
  

   a 
  deposit 
  of 
  mud, 
  at 
  other 
  times 
  they 
  were 
  broken 
  loose 
  from 
  

   their 
  moorings 
  before 
  burial, 
  but 
  in 
  both 
  cases 
  perfect 
  molds 
  have 
  

   been 
  left, 
  showing 
  not 
  only 
  the 
  details 
  of 
  form 
  but 
  the 
  general 
  

   arrangement 
  of 
  the 
  spicular 
  network. 
  

  

  The 
  form, 
  which 
  is 
  the 
  most 
  important 
  feature 
  for 
  the 
  specific 
  

   differentiation 
  and 
  generic 
  delimitation 
  of 
  these 
  fossil 
  sponges, 
  is 
  

   subject 
  to 
  variation 
  in 
  several 
  directions. 
  The 
  simplest 
  type, 
  and 
  

   the 
  most 
  primitive 
  as 
  well, 
  is 
  a 
  slender 
  cone, 
  narrow 
  at 
  the 
  base 
  

   and 
  open 
  at 
  the 
  upper 
  margin. 
  This 
  may 
  become 
  almost 
  cylin- 
  

   drical, 
  as 
  in 
  Dictyospongia, 
  or 
  may 
  expand 
  into 
  a 
  wide 
  saucer- 
  

   like 
  shape, 
  as 
  in 
  Hyphantcenia. 
  The 
  next 
  modification 
  in 
  form 
  

   consists 
  in 
  the 
  development 
  of 
  longitudinal 
  ridges, 
  thus 
  breaking 
  

   up 
  the 
  vase 
  into 
  pyramidal 
  or 
  prismatic 
  faces. 
  This 
  prismatic 
  

   type 
  is 
  determined 
  by, 
  and 
  depends 
  upon, 
  the 
  vertical 
  bundles 
  of 
  

   spicules. 
  

  

  The 
  bases 
  of 
  such 
  sponges 
  are 
  conical, 
  but 
  as 
  growth 
  advances 
  

   they 
  pass 
  into 
  four-sided 
  forms, 
  due 
  to 
  four 
  primary 
  spicular 
  

   bundles. 
  Finally, 
  as 
  these 
  diverge, 
  four 
  intermediate 
  secondary 
  

   bundles 
  assume 
  prominence, 
  and 
  the 
  sponge 
  becomes 
  eight-sided, 
  

   which 
  is 
  the 
  normal 
  prismatic 
  form. 
  

  

  The 
  sharpness 
  of 
  the 
  prismatic 
  angles 
  diminishes 
  toward 
  the 
  

   upper 
  part 
  of 
  large 
  specimens 
  and 
  they 
  begin 
  to 
  show 
  again 
  the 
  

   smoothness 
  characteristic 
  of 
  their 
  initial 
  growth. 
  Another 
  varia- 
  

   tion 
  in 
  form 
  is 
  due 
  to 
  ring 
  like 
  contractions 
  at 
  intervals 
  along 
  the 
  

   cone 
  or 
  pyramid, 
  giving 
  a 
  crenulated 
  longitudinal 
  outline. 
  This 
  

   is 
  presumably 
  acquired 
  by 
  an 
  alternating 
  contraction 
  and 
  expan- 
  

   sion 
  of 
  the 
  aperture 
  as 
  it 
  grows 
  upward, 
  but 
  to 
  what 
  extent 
  the 
  

   growth 
  is 
  confined 
  to 
  the 
  upper 
  part, 
  as 
  in 
  corals, 
  or 
  to 
  what 
  

   extent 
  there 
  is 
  a 
  general 
  expansion 
  of 
  the 
  entire 
  structure, 
  is 
  not 
  

   discussed. 
  

  

  The 
  prismatic 
  and 
  annular 
  modifications 
  combined 
  give 
  rise 
  at 
  

   their 
  intersections 
  to 
  nodes, 
  which 
  in 
  some 
  species 
  of 
  Ilydnoceras 
  

   are 
  merely 
  blunt 
  protuberances 
  on 
  the 
  surface, 
  but 
  in 
  others 
  pass 
  

   into 
  sharp-pointed 
  swellings 
  of 
  considerable 
  elevation. 
  The 
  

  

  Am. 
  Jour. 
  Sol— 
  Fourth 
  Series, 
  Yol. 
  IX, 
  Xo. 
  51.— 
  March, 
  1900. 
  

   16 
  

  

  