﻿EROM 
  THE 
  CAMBRIAN 
  STRATA 
  OF 
  NORTH 
  AMERICA, 
  ETC. 
  131 
  

  

  thin 
  sections 
  they 
  are 
  nearly 
  transparent 
  and 
  composed 
  for 
  the 
  most 
  

   part 
  of 
  crystalline 
  calcite. 
  In 
  a 
  few 
  cases 
  they 
  are 
  of 
  minutely 
  

   granular 
  carbonate 
  of 
  lime 
  like 
  those 
  from 
  other 
  places, 
  and 
  even 
  

   part 
  of 
  the 
  same 
  specimen 
  may 
  be 
  granular 
  and 
  part 
  crystalline. 
  

   The 
  rock-matrix 
  is 
  a 
  finely 
  granular 
  carbonate 
  of 
  lime, 
  of 
  a 
  brownish 
  

   or 
  reddish 
  tint 
  *. 
  

  

  From 
  the 
  above 
  it 
  is 
  evident 
  that 
  the 
  mineral 
  constitution 
  of 
  

   ArcliceocyatTius 
  and 
  its 
  allied 
  forms 
  is 
  of 
  minutely 
  granular 
  carbonate 
  

   of 
  lime 
  ; 
  and 
  this 
  is 
  probably 
  their 
  original 
  condition, 
  since 
  if 
  these 
  

   organisms 
  had 
  possessed 
  siliceous 
  skeletons 
  they 
  would 
  not 
  have 
  

   been 
  replaced 
  by 
  carbonate 
  of 
  lime 
  in 
  the 
  granular 
  form, 
  but 
  by 
  

   crystalline 
  calcite. 
  In 
  the 
  few 
  cases 
  where 
  specimens 
  are 
  of 
  crystal- 
  

   line 
  calcite, 
  it 
  is 
  shown 
  that 
  this 
  has 
  replaced 
  a 
  granular 
  condition 
  

   of 
  the 
  mineral 
  and 
  is 
  not 
  a 
  replacement 
  after 
  silica. 
  

  

  3. 
  Mode 
  of 
  Growth 
  and 
  Structure 
  of 
  Archseocyathus 
  and 
  

   allied 
  forms. 
  

  

  These 
  features 
  will 
  be 
  best 
  treated 
  by 
  detailed 
  descriptions, 
  first 
  

   of 
  Arclmocyathus 
  proper 
  and 
  then 
  of 
  Ethmopliyllum, 
  Meek, 
  Coscino- 
  

   cyathus, 
  Bornemann, 
  AnthomorpTia, 
  Bornem., 
  Protopharetr 
  a, 
  Bornem,, 
  

   and 
  Spirocyatlius, 
  gen. 
  nov., 
  based 
  on 
  Archceocyathus 
  atlanticus, 
  

   Bill. 
  

  

  Genus 
  Archceocyathus, 
  Billings. 
  (Plate 
  V. 
  figs. 
  1-6.) 
  

  

  (1861. 
  New 
  Species 
  of 
  Lower 
  Silurian 
  Fossils, 
  p. 
  3. 
  

   1865. 
  Palasozoic 
  Fossils, 
  vol. 
  i. 
  p. 
  3.) 
  

  

  This 
  genus, 
  of 
  which 
  A. 
  profundus, 
  Bill., 
  is 
  taken 
  as 
  the 
  typical 
  

   species, 
  consists 
  of 
  cup-shaped, 
  subcylindrical, 
  or 
  turbinate 
  forms, 
  

   apparently 
  free. 
  The 
  outer 
  surface 
  of 
  the 
  wall 
  appears 
  to 
  be 
  

   generally 
  rugose 
  or 
  with 
  horizontal 
  ridges, 
  sometimes, 
  however, 
  

   smooth 
  ; 
  the 
  inner 
  surface 
  nearly 
  smooth. 
  The 
  basal 
  portion 
  is 
  

   generally 
  inverted-conical 
  in 
  form 
  ; 
  the 
  further 
  growth 
  may 
  be 
  nearly 
  

   cylindrical 
  with 
  a 
  tubular 
  central 
  cavity 
  : 
  or 
  cup-shaped 
  with 
  a 
  deep 
  

   hollow, 
  or 
  open 
  saucer-shaped. 
  The 
  wall 
  of 
  the 
  organism 
  consists 
  

   of 
  an 
  outer 
  and 
  inner 
  lamina 
  or 
  plate, 
  bounding 
  the 
  exterior 
  and 
  

   the 
  inner 
  surface 
  of 
  the 
  cup 
  respectively, 
  and 
  a 
  series 
  of 
  stout 
  radial 
  

   vertical 
  septa, 
  which, 
  like 
  those 
  of 
  a 
  coral, 
  extend 
  from 
  the 
  outer 
  

   to 
  the 
  inner 
  wall-plates 
  (PI. 
  Y. 
  figs. 
  4, 
  5.) 
  The 
  wall-plates 
  both 
  

   of 
  the 
  interior 
  and 
  exterior 
  surfaces 
  are 
  perforate, 
  but 
  in 
  the 
  best- 
  

   preserved 
  specimen 
  there 
  is 
  a 
  delicate 
  enveloping 
  surface-lamina, 
  

   which 
  seems 
  to 
  be 
  imperforate. 
  The 
  interseptal 
  loculi 
  are 
  occupied 
  

   by 
  dissepimental 
  vesicles, 
  like 
  those 
  in 
  many 
  Rugose 
  corals 
  ; 
  these 
  

   are 
  disposed 
  obliquely, 
  with 
  their 
  convexities 
  to 
  the 
  exterior. 
  The 
  

  

  * 
  The 
  specimens 
  of 
  Ethmophyllum 
  from 
  Nevada 
  which 
  I 
  have 
  recently 
  

   received 
  from 
  Mr. 
  Walcott, 
  likewise 
  consist 
  of 
  minutely 
  granular 
  carbonate 
  of 
  

   lime, 
  which 
  in 
  places 
  has 
  been 
  replaced 
  by 
  calcite. 
  They 
  are 
  imbedded 
  in 
  a 
  

   matrix 
  of 
  compact 
  reddish 
  limestone. 
  There 
  is 
  thus 
  a 
  singularly 
  close 
  resem- 
  

   blance 
  in 
  the 
  mineral 
  condition 
  of 
  these 
  fossils 
  from 
  the 
  above-mentioned 
  

   widely 
  separated 
  localities. 
  — 
  (G. 
  J. 
  H., 
  Jan. 
  24, 
  1889.) 
  

  

  k2 
  

  

  