﻿142 
  DE. 
  G. 
  J. 
  HIXDE 
  OX 
  AECBLEOCYATB7CS 
  AXD 
  OTHEE 
  GENERA 
  

  

  the 
  other 
  genera 
  with 
  which 
  it 
  is 
  associated 
  ; 
  and 
  it 
  seems 
  desirable, 
  

   by 
  describing 
  the 
  microscopic 
  structure 
  of 
  these 
  forms, 
  to 
  show 
  

   their 
  real 
  characters, 
  and 
  at 
  the 
  same 
  time 
  the 
  radical 
  differences 
  

   between 
  them 
  and 
  Archceocyathus 
  proper 
  and 
  its 
  allied 
  genera. 
  I 
  

   have 
  likewise 
  been 
  permitted 
  by 
  the 
  Director-General 
  of 
  the 
  

   Geological 
  Surrey 
  of 
  Great 
  "Britain 
  to 
  examine 
  some 
  fossils 
  from 
  

   the 
  Durness 
  Limestones 
  of 
  the 
  north-west 
  of 
  Scotland, 
  which 
  had 
  

   been 
  referred 
  to 
  ArcJiceocyathus 
  ; 
  and 
  their 
  minute 
  structure 
  proves 
  

   that 
  they 
  are 
  not 
  related 
  to 
  the 
  true 
  genus 
  of 
  this 
  name, 
  though 
  

   they 
  are 
  to 
  Arcliceoscyphia 
  minganensis 
  and 
  Calatlilum. 
  

  

  Genus 
  Aech^osctphia., 
  gen. 
  nov. 
  (Plate 
  Y. 
  figs. 
  12, 
  13, 
  14.) 
  

  

  This 
  genus 
  is 
  proposed 
  to 
  include 
  Arclicrocyatlius 
  minganemis 
  

   with 
  the 
  following 
  generic 
  characters 
  : 
  — 
  

  

  Sponges 
  simple, 
  vasiform 
  or 
  subcylindrical, 
  apparently 
  free. 
  

   Outer 
  surface 
  with 
  strongly 
  marked 
  annular 
  projections. 
  Wall 
  

   robust, 
  the 
  skeleton 
  built 
  up 
  in 
  the 
  form 
  of 
  septum-like 
  longitudinal 
  

   plates, 
  closely 
  arranged 
  with 
  narrow 
  interrupted 
  spaces 
  between. 
  

   The 
  dermal 
  layer 
  smooth, 
  with 
  minute 
  canal 
  apertures, 
  beneath 
  are 
  

   larger 
  canals 
  opening 
  into 
  the 
  intervals 
  between 
  the 
  septum-like 
  

   plates. 
  Xo 
  definite 
  inner 
  wall 
  next 
  the 
  cloacal 
  cavity. 
  The 
  skeleton 
  

   consists 
  of 
  minute 
  siliceous 
  spicules 
  of 
  the 
  tetracladine 
  type 
  ; 
  their 
  

   rays 
  are 
  slightly 
  furcate 
  and 
  branched 
  at 
  their 
  extremities, 
  and 
  they 
  

   interlock 
  without 
  forming 
  prominent 
  nodes. 
  In 
  many 
  spicules 
  only 
  

   three 
  rays 
  are 
  apparently 
  developed. 
  Irregular 
  branching 
  spicules 
  

   are 
  likewise 
  present. 
  

  

  At 
  present 
  only 
  fragments 
  of 
  the 
  type 
  species 
  are 
  known, 
  from 
  

   which 
  Mr. 
  Billings 
  produced 
  a 
  restored 
  figure 
  of 
  the 
  complete 
  sponge, 
  

   which 
  has 
  been 
  often 
  copied 
  in 
  different 
  works. 
  The 
  fragments 
  

   indicate 
  a 
  specimen 
  of 
  at 
  least 
  80 
  millim. 
  in 
  height 
  and 
  50 
  millim. 
  

   in 
  width. 
  They 
  are 
  wholly 
  siliceous, 
  in 
  a 
  somewhat 
  granular 
  con- 
  

   dition, 
  and 
  very 
  unfavourable 
  for 
  preparing 
  sections. 
  The 
  original 
  

   structure 
  of 
  the 
  interior 
  has 
  almost 
  entirely 
  been 
  obliterated 
  in 
  the 
  

   fossilization, 
  so 
  that 
  only 
  indistinct 
  traces 
  of 
  the 
  spicular 
  mesh 
  can 
  

   be 
  recognized 
  in 
  thin 
  sections. 
  The 
  form 
  of 
  the 
  sponge 
  is 
  peculiar, 
  

   from 
  the 
  irregular 
  annular 
  extensions 
  or 
  platforms 
  developed 
  at 
  

   intervals 
  on 
  the 
  outer 
  surface, 
  which 
  in 
  places 
  project 
  from 
  10 
  to 
  

   15 
  millim. 
  beyond 
  the 
  general 
  surface 
  of 
  the 
  wall 
  (PI. 
  Y. 
  fig. 
  12). 
  

   In 
  the 
  intervals 
  between 
  these 
  rings 
  the 
  wall 
  is 
  about 
  6 
  millim. 
  in 
  

   thickness. 
  The 
  septum-like 
  plates 
  are 
  from 
  "5 
  to 
  1 
  millim. 
  in 
  thickness, 
  

   with 
  about 
  the 
  same 
  distance 
  between 
  each. 
  Their 
  connexion 
  with 
  

   the 
  outer 
  portion 
  of 
  the 
  wall 
  is 
  by 
  no 
  means 
  so 
  distinct 
  as 
  it 
  appears 
  

   in 
  Mr. 
  Billings's 
  figures, 
  and 
  the 
  interspaces 
  are 
  now 
  to 
  a 
  great 
  

   extent 
  infilled 
  by 
  siliceous 
  material, 
  but 
  they 
  appear 
  originally 
  to 
  

   have 
  been 
  bridged 
  over 
  by 
  lateral 
  extensions 
  of 
  the 
  skeletal 
  mesh. 
  

   The 
  inner 
  edges 
  of 
  the 
  septum-like 
  plates 
  are 
  free 
  and 
  uncovered, 
  

   and 
  I 
  can 
  see 
  no 
  traces, 
  in 
  the 
  type 
  specimens 
  sent 
  to 
  me, 
  of 
  an 
  inner 
  

   wall 
  (PI. 
  Y. 
  fig. 
  13). 
  

  

  The 
  dermal 
  layer 
  is 
  smooth 
  ; 
  very 
  minute 
  perforations 
  can 
  be 
  seen 
  

  

  