﻿FROM 
  THE 
  CAMBRIAN 
  STRATA 
  OF 
  NORTH 
  AMERICA, 
  ETC. 
  145 
  

  

  than 
  10 
  millim. 
  in 
  depth. 
  The 
  outer 
  surface 
  is 
  rough 
  and 
  uneven, 
  

   showing 
  no 
  other 
  structure 
  beyond 
  traces 
  of 
  infilled 
  canals 
  apparently 
  

   directed 
  towards 
  the 
  summit. 
  Minute 
  radial 
  canals 
  can 
  be 
  faintly 
  

   seen 
  in 
  some 
  places 
  in 
  the 
  interior 
  of 
  the 
  wall. 
  The 
  body 
  of 
  the 
  

   sponge 
  appears 
  to 
  have 
  been 
  entirely 
  filled 
  with 
  the 
  skeletal 
  tissue, 
  

   but 
  the 
  spicules 
  are 
  now 
  to 
  a 
  great 
  extent 
  obliterated 
  ; 
  they 
  are 
  

   variable 
  in 
  form, 
  but 
  chiefly 
  with 
  a 
  straight 
  or 
  curved 
  axial 
  portion, 
  

   and 
  tuberculated 
  and 
  slightly 
  branched 
  extremities, 
  thus 
  of 
  the 
  

   JRJiizomorina-tj^e 
  (PL 
  Y. 
  fig. 
  15). 
  

  

  The 
  general 
  build 
  of 
  this 
  sponge 
  is 
  clearly 
  distinct 
  from 
  that 
  of 
  

   the 
  genus 
  Calathium, 
  in 
  which 
  Mr. 
  Billings 
  provisionally 
  placed 
  it. 
  

  

  The 
  type 
  species 
  is 
  from 
  the 
  Calciferous 
  Formation 
  of 
  the 
  Mingan 
  

   Islands, 
  Lower 
  St. 
  Lawrence. 
  

  

  Genus 
  TRicHosroisrGiA, 
  Billings. 
  

  

  (1865. 
  Pal. 
  Foss. 
  vol. 
  i. 
  p. 
  357.) 
  

  

  This 
  genus 
  occurs 
  in 
  depressed 
  cup-shaped 
  masses, 
  now 
  of 
  granular 
  

   silica, 
  in 
  which 
  the 
  original 
  structure 
  is 
  largely 
  destroyed. 
  Partially 
  

   detached 
  rod-shaped 
  or 
  acerate 
  spicules 
  are 
  present 
  in 
  the 
  porous 
  

   portion 
  of 
  the 
  mass, 
  and 
  traces 
  of 
  similar 
  spicules 
  in 
  thin 
  sections 
  of 
  

   the 
  more 
  compact 
  portions. 
  Some 
  of 
  the 
  spicules 
  have 
  apparently 
  

   a 
  subparallel 
  arrangement, 
  but 
  no 
  definite 
  fibres 
  can 
  be 
  made 
  out. 
  

   As 
  no 
  other 
  than 
  monactinellid 
  spicules 
  are 
  present, 
  the 
  genus 
  may 
  

   be 
  regarded 
  as 
  belonging 
  to 
  the 
  suborder 
  Monactinellidge. 
  Only 
  

   one 
  species, 
  T. 
  sericea, 
  Bill., 
  is 
  known, 
  it 
  is 
  from 
  the 
  Calciferous 
  

   Formation 
  of 
  the 
  Mingan 
  Islands. 
  

  

  It 
  is 
  worthy 
  of 
  remark 
  that 
  the 
  Calciferous 
  Formation 
  of 
  the 
  

   Mingan 
  Islands 
  is 
  the 
  lowest 
  geological 
  horizon 
  in 
  which 
  sponges 
  

   are 
  met 
  with 
  in 
  sufficient 
  numbers 
  to 
  constitute 
  an 
  appreciable 
  

   element 
  in 
  the 
  fossil 
  fauna. 
  Below 
  this 
  horizon 
  only 
  the 
  hexacti- 
  

   nellid 
  genus 
  Protospongia 
  is 
  known, 
  and 
  specimens 
  of 
  this 
  are 
  of 
  com- 
  

   paratively 
  rare 
  occurrence. 
  In 
  the 
  Mingan 
  strata 
  two 
  genera 
  of 
  

   Lithistid 
  sponges, 
  Archwoscyphia 
  and 
  Nipterella, 
  are 
  the 
  earliest 
  

   known 
  representatives 
  of 
  this 
  important 
  group, 
  whilst 
  Trichospongia 
  

   is 
  the 
  earliest 
  Monactinellid 
  genus 
  yet 
  described 
  ; 
  and, 
  judging 
  from 
  

   the 
  variety 
  of 
  detached 
  acuate 
  and 
  acerate 
  spicules 
  in 
  the 
  same 
  

   rocks 
  *, 
  other 
  forms 
  of 
  the 
  same 
  group 
  are 
  associated 
  with 
  it. 
  

   Another 
  genus, 
  Rhahdaria, 
  is 
  likewise 
  mentioned 
  by 
  Mr. 
  Billings 
  f 
  ; 
  

   but 
  the 
  spicular 
  structure 
  of 
  this 
  is 
  not 
  yet 
  known. 
  It 
  is 
  also 
  

   noticeable 
  that 
  the 
  presence 
  of 
  siliceous 
  sponges 
  in 
  these 
  rocks 
  is 
  

   associated 
  with 
  the 
  occurrence 
  in 
  them 
  of 
  many 
  nodules 
  and 
  patches 
  

   of 
  chert 
  J 
  ; 
  also 
  in 
  the 
  Durness 
  Limestones, 
  in 
  which 
  Archceo- 
  

   scyphia 
  and 
  Calathium 
  have 
  been 
  recognized, 
  and 
  other 
  siliceous 
  

  

  * 
  Billings, 
  Pal. 
  Foss. 
  vol. 
  i. 
  p. 
  355, 
  fig. 
  344 
  ; 
  Hinde, 
  Geol. 
  Mag. 
  vol. 
  v. 
  

   (1888), 
  p. 
  226, 
  fig. 
  1, 
  a-d. 
  

   t 
  Pal. 
  Foss. 
  vol. 
  i. 
  p. 
  357. 
  

   X 
  Geological 
  Survej 
  of 
  Canada, 
  Keport 
  of 
  Progress, 
  1863, 
  p. 
  120. 
  

  

  Q. 
  J. 
  G. 
  S. 
  No. 
  177. 
  l 
  

  

  