﻿FROM 
  THE 
  CAMBRIAN 
  STRATA 
  OP 
  NORTH 
  AMERICA, 
  ETC. 
  137 
  

  

  microscopic 
  sections, 
  and 
  these 
  show 
  such 
  peculiar 
  structural 
  cha- 
  

   racters, 
  that 
  they 
  seem 
  to 
  me 
  to 
  justify 
  placing 
  it 
  in 
  a 
  distinct 
  

   genus. 
  The 
  type 
  specimen 
  is 
  subcylindrical 
  or 
  elongate-conical, 
  the 
  

   extreme 
  base 
  and 
  summit 
  are 
  wanting, 
  so 
  that 
  the 
  portion 
  preserved 
  

   is 
  only 
  about 
  58 
  millim., 
  or 
  2- 
  1 
  - 
  inches 
  in 
  length 
  *, 
  18 
  millim. 
  in 
  

   diameter 
  near 
  the 
  base, 
  and 
  28 
  millim. 
  at 
  the 
  top. 
  It 
  has 
  been 
  

   sectioned 
  transversely 
  near 
  the 
  base, 
  and 
  the 
  upper 
  portion 
  divided 
  

   by 
  a 
  median 
  longitudinal 
  section. 
  The 
  specimen 
  is 
  in 
  a 
  matrix 
  of 
  

   hard, 
  compact, 
  reddish 
  limestone, 
  which 
  has 
  been 
  almost 
  entirely 
  

   removed 
  from 
  the 
  outer 
  surface, 
  but 
  completely 
  fills 
  the 
  interior 
  

   cavity 
  and 
  also 
  some 
  of 
  the 
  interspaces 
  in 
  the 
  wall 
  ; 
  other 
  inter- 
  

   spaces 
  are 
  occupied 
  by 
  calcite. 
  The 
  outer 
  surface 
  is 
  apparently 
  

   smooth, 
  and 
  exhibits 
  obscure 
  annular 
  ridges 
  and 
  wide 
  shallow 
  

   furrows 
  between 
  them. 
  The 
  walls 
  and 
  substance 
  of 
  the 
  fossil 
  are 
  

   of 
  limestone 
  of 
  a 
  dull 
  yellowish- 
  white 
  tint 
  ; 
  by 
  transmitted 
  light, 
  in 
  

   thin 
  sections, 
  it 
  shows 
  the 
  same 
  minute 
  granular 
  texture 
  which 
  has 
  

   been 
  already 
  described 
  in 
  Archceocyathus. 
  

  

  The 
  smooth 
  outer 
  lamina 
  of 
  the 
  wall 
  appears 
  to 
  have 
  been 
  per- 
  

   forated 
  by 
  minute, 
  closely-set, 
  subcircular 
  pores, 
  about 
  -2 
  millim. 
  in 
  

   width, 
  giving 
  it 
  a 
  delicately 
  reticulate 
  appearance. 
  This 
  structure 
  

   can 
  only 
  be 
  distinguished 
  on 
  certain 
  portions 
  of 
  the 
  surface 
  by 
  a 
  

   strong 
  lens, 
  and 
  it 
  is 
  not 
  shown 
  in 
  transverse 
  sections, 
  possibly 
  

   because 
  the 
  pores 
  may 
  have 
  been 
  subsequently 
  filled 
  up. 
  

  

  The 
  walls 
  bounding 
  the 
  inner 
  tubular 
  cavity 
  vary 
  from 
  8 
  to 
  10 
  

   millim. 
  in 
  thickness, 
  whilst 
  the 
  tube 
  itself 
  is 
  from 
  10 
  to 
  12 
  millim. 
  

   wide, 
  or 
  somewhat 
  more 
  than 
  one 
  third 
  the 
  diameter 
  of 
  the 
  fossil. 
  

   The 
  plates 
  or 
  laminae 
  composing 
  the 
  wall 
  are 
  very 
  irregularly 
  

   arranged 
  ; 
  sometimes 
  there 
  are 
  traces 
  of 
  a 
  radial 
  disposition 
  (PI. 
  V. 
  

   fig. 
  8), 
  but 
  more 
  frequently 
  they 
  curve 
  and 
  inosculate 
  with 
  each 
  other 
  

   so 
  as 
  to 
  form 
  a 
  series 
  of 
  closed 
  loops 
  (PI. 
  V. 
  figs. 
  9, 
  10). 
  The 
  outer 
  

   and 
  inner 
  laminae, 
  though 
  much 
  thicker, 
  are 
  of 
  precisely 
  the 
  same 
  

   structure 
  as 
  the 
  intermediate 
  plates. 
  The 
  canals 
  and 
  lacunae 
  thus 
  

   enclosed 
  by 
  the 
  wall-plates 
  are 
  very 
  irregular 
  in 
  disposition 
  and 
  

   size 
  ; 
  frequently 
  they 
  are 
  much 
  contracted 
  by 
  the 
  thickening 
  of 
  

   the 
  plates, 
  and 
  occasionally 
  are 
  quite 
  filled 
  up. 
  The 
  wall-plates 
  

   are 
  likewise 
  connected 
  by 
  slender 
  calcareous 
  threads, 
  circular 
  in 
  

   section, 
  and 
  about 
  *025 
  millim. 
  in 
  thickness 
  ; 
  some 
  of 
  these 
  fibres 
  

   also 
  irregularly 
  bridge 
  over 
  the 
  axial 
  tube 
  of 
  the 
  fossil 
  and 
  connect 
  

   opposite 
  sides 
  of 
  the 
  wall. 
  

  

  The 
  primary 
  or 
  central 
  layer 
  of 
  the 
  wall-laminae 
  is 
  approxi- 
  

   mately 
  even, 
  about 
  *12 
  millim. 
  in 
  thickness 
  ; 
  it 
  is 
  distinctly 
  marked 
  

   off 
  from 
  the 
  secondary 
  layers 
  by 
  marginal 
  lines 
  (PI. 
  Y. 
  fig. 
  10), 
  but 
  

   the 
  substance 
  of 
  this 
  layer 
  is 
  of 
  the 
  same 
  nature, 
  though 
  of 
  a 
  lighter 
  

   tint, 
  by 
  transmitted 
  light 
  than 
  the 
  enclosing 
  secondary 
  layers. 
  

   These 
  vary 
  in 
  number, 
  but 
  each 
  is 
  more 
  or 
  less 
  distinctly 
  bounded 
  

   by 
  a 
  marginal 
  line, 
  darker 
  when 
  seen 
  in 
  thin 
  section. 
  The 
  wall- 
  

   plates 
  vary 
  from 
  *4 
  to 
  '8 
  millim. 
  in 
  thickness 
  : 
  that 
  bounding 
  the 
  

   inner 
  tube 
  reaches 
  in 
  places 
  2 
  millim. 
  in 
  thickness, 
  from 
  the 
  depo- 
  

  

  * 
  In 
  the 
  Pal. 
  Fosb. 
  vol. 
  i. 
  p. 
  6, 
  the 
  fragment 
  is 
  stated 
  to 
  be 
  4| 
  in. 
  in 
  length 
  . 
  

   but 
  this 
  seems 
  to 
  be 
  a 
  mistake. 
  

  

  