﻿132 
  DE. 
  0. 
  J. 
  HIXDE 
  OK 
  AECHiEOCrATHUS 
  AXD 
  OTHER 
  GEXERA. 
  

  

  septa 
  are 
  fairly 
  even 
  in 
  thickness 
  and 
  in 
  their 
  distance 
  apart 
  ; 
  new 
  

   septa 
  are 
  introduced 
  at 
  intervals 
  with 
  the 
  increase 
  in 
  width 
  of 
  the 
  

   specimen. 
  The 
  septa 
  are 
  perforated 
  by 
  numerous, 
  small, 
  circular 
  or 
  

   oval 
  apertures, 
  disposed 
  regularly 
  in 
  oblique 
  rows: 
  the 
  dissepiments, 
  

   however, 
  appear 
  to 
  be 
  imperforate 
  (PL 
  Y. 
  figs. 
  4, 
  5). 
  The 
  walls 
  

   and 
  septa 
  are 
  of 
  compact, 
  homogeneous 
  limestone; 
  they 
  do 
  not 
  

   seem 
  to 
  be 
  porous 
  or 
  canalicular 
  ; 
  occasionally 
  there 
  are 
  lighter 
  and 
  

   darker 
  layers, 
  indicating 
  secondary 
  thickenings 
  or 
  layers 
  of 
  material. 
  

  

  The 
  Canadian 
  examples 
  of 
  this 
  genus 
  which 
  I 
  have 
  examined 
  

   (PI. 
  Y. 
  figs. 
  1, 
  2) 
  are 
  all 
  from 
  the 
  so-called 
  Potsdam 
  Limestone 
  of 
  

   the 
  coast 
  of 
  Labrador 
  ; 
  one 
  appears 
  to 
  be 
  the 
  original 
  of 
  the 
  figure 
  

   of 
  A. 
  profundus 
  (Pal. 
  Fossils, 
  vol. 
  i. 
  fig. 
  2, 
  p. 
  4). 
  The 
  specimens 
  

   are 
  all 
  imperfect 
  ; 
  they 
  are 
  either 
  weathered 
  and 
  worn 
  on 
  the 
  

   exterior, 
  or 
  else 
  partly 
  enclosed 
  in 
  the 
  limestone 
  matrix, 
  so 
  that 
  in 
  

   no 
  case 
  is 
  the 
  natural 
  unworn 
  exterior 
  surface 
  of 
  the 
  fossil 
  exposed. 
  

   They 
  vary 
  considerably 
  in 
  size 
  ; 
  a 
  subcylindrical 
  individual 
  is 
  37 
  

   millim. 
  in 
  height 
  by 
  20 
  millim. 
  across 
  at 
  the 
  summit 
  ; 
  a 
  cup-shaped 
  

   specimen 
  of 
  about 
  the 
  same 
  height 
  is 
  42 
  millim. 
  wide 
  above, 
  and 
  

   the 
  wall 
  varies 
  from 
  4 
  to 
  6*5 
  millim. 
  in 
  thickness 
  ; 
  and 
  the 
  surface 
  

   of 
  a 
  saucer-shaped 
  individual 
  is, 
  in 
  one 
  direction, 
  60 
  millim. 
  across, 
  

   and 
  the 
  wall 
  is 
  from 
  10 
  to 
  15 
  millim. 
  thick. 
  

  

  The 
  character 
  of 
  the 
  exterior 
  and 
  inner 
  surfaces 
  of 
  the 
  wall 
  in 
  

   these 
  forms 
  can 
  only 
  be 
  known 
  from 
  sections 
  of 
  specimens 
  which 
  

   are 
  enclosed 
  in 
  matrix, 
  and 
  even 
  in 
  these 
  the 
  structures 
  are 
  not 
  

   uniformly 
  preserved, 
  but 
  appear 
  to 
  have 
  been 
  partially 
  destroyed 
  

   previous 
  to 
  fossilization. 
  In 
  the 
  best 
  of 
  the 
  Canadian 
  examples 
  of 
  

   A. 
  profundus 
  the 
  outer 
  surface 
  of 
  the 
  wall 
  consists 
  of 
  a 
  very 
  thin 
  

   delicate 
  lamina, 
  apparently 
  non-perforate, 
  and 
  immediately 
  within 
  

   this 
  is 
  a 
  thin 
  layer 
  of 
  perforate 
  or 
  vesicular 
  tissue. 
  It 
  is 
  only 
  in 
  

   places 
  that 
  the 
  surface 
  or 
  epithecal 
  lamina 
  has 
  been 
  preserved 
  ; 
  

   where 
  it 
  is 
  present 
  the 
  interspaces 
  in 
  the 
  perforated 
  tissues 
  within 
  

   are 
  filled 
  with 
  calcite 
  ; 
  but 
  where 
  it 
  is 
  wanting 
  the 
  matrix 
  has 
  rilled 
  

   the 
  cavities, 
  showing 
  in 
  this 
  latter 
  case 
  open 
  communication 
  with 
  

   the 
  interior 
  of 
  the 
  wall. 
  1 
  have 
  not 
  found 
  any 
  trace 
  of 
  a 
  similar 
  

   epithecal 
  lamina 
  in 
  any 
  of 
  the 
  Sardinian 
  specimens 
  of 
  Archceocyatlms 
  ; 
  

   but 
  this 
  may 
  perhaps 
  arise 
  from 
  the 
  fact 
  that 
  they 
  are 
  by 
  no 
  means 
  

   so 
  well 
  preserved 
  as 
  the 
  forms 
  from 
  Canada. 
  The 
  outer 
  surface 
  of 
  

   the 
  wall 
  in 
  the 
  Sardinian 
  forms 
  consists 
  of 
  an 
  extremely 
  delicate 
  

   perforate 
  membrane, 
  the 
  pores 
  in 
  which 
  are 
  arranged 
  alternately 
  in 
  

   vertical 
  rows, 
  of 
  which 
  there 
  are 
  from 
  two 
  to 
  six 
  between 
  a 
  pair 
  of 
  

   septa 
  : 
  these 
  pores 
  open 
  directly 
  into 
  the 
  interseptal 
  loculi 
  (PI. 
  Y. 
  

   fig. 
  6). 
  

  

  In 
  the 
  Canadian 
  specimens 
  the 
  surface 
  of 
  the 
  wall 
  next 
  the 
  cup 
  

   has 
  also 
  a 
  delicate 
  lamina, 
  like 
  that 
  above 
  described 
  on 
  the 
  exterior, 
  

   and 
  beneath 
  or 
  within 
  this 
  is 
  a 
  layer 
  of 
  perforated 
  vesicular 
  tissue 
  

   of 
  varying 
  thickness. 
  In 
  the 
  casts 
  of 
  Arcliceocyathus 
  acutus 
  from 
  

   Sardinia 
  the 
  inner 
  plate 
  of 
  the 
  wall 
  is 
  directly 
  perforated 
  by 
  closely 
  

   set 
  circular 
  apertures 
  regularly 
  arranged 
  in 
  quincunx, 
  which 
  open 
  

   into 
  the 
  cup, 
  and 
  with 
  slight 
  modifications 
  the 
  same 
  structure 
  of 
  the 
  

  

  