﻿H. 
  ALLEYNE 
  NICHOLSON 
  ON 
  SPECIES 
  OF 
  CHJETEXES. 
  499 
  

  

  38. 
  Descriptions 
  of 
  Species 
  of 
  Cb^tetes 
  from 
  the 
  Lower 
  Silurian 
  

   Rocxs 
  of 
  North 
  America. 
  By 
  H. 
  Alletne 
  Nicholson, 
  M.D.,D.Sc, 
  

   F.R.S.E., 
  F.G.S., 
  Professor 
  of 
  Biology 
  in 
  the 
  College 
  of 
  Physical 
  

   Science, 
  Newcastle-on-Tyne. 
  (Read 
  June 
  24, 
  1874.) 
  

  

  [Plates 
  XXIX. 
  & 
  XXX.] 
  

  

  The 
  Lower 
  Silurian 
  rocks 
  of 
  North 
  America 
  have 
  yielded 
  a 
  very 
  

   large 
  number 
  of 
  small 
  corals, 
  which 
  have 
  been 
  ordinarily 
  referred 
  

   by 
  palaeontologists 
  to 
  one 
  or 
  other 
  of 
  the 
  three 
  genera 
  Chcetetes, 
  

   Monticulipora, 
  and 
  Stenopora. 
  Before 
  considering 
  these, 
  however, 
  

   it 
  would 
  seem 
  advisable 
  to 
  very 
  briefly 
  discuss 
  the 
  value 
  of 
  the 
  

   above-mentioned 
  generic 
  divisions, 
  the 
  boundaries 
  of 
  which 
  are 
  by 
  

   no 
  means 
  yet 
  satisfactorily 
  defined. 
  

  

  The 
  genus 
  Stenopora 
  is 
  defined 
  by 
  Lonsdale 
  as 
  follows 
  : 
  — 
  " 
  A 
  

   ramose, 
  spherical, 
  or 
  amorphous 
  tubular 
  polypidom 
  ; 
  tubes 
  polygonal 
  

   or 
  cylindrical, 
  radiated 
  from 
  a 
  centre 
  or 
  an 
  imaginary 
  axis, 
  contracted 
  

   at 
  irregular 
  distances, 
  but 
  in 
  planes 
  parallel 
  to 
  the 
  surface 
  of 
  the 
  

   specimen 
  ; 
  tubular 
  mouths 
  closed 
  at 
  final 
  periods 
  of 
  growth 
  ; 
  ridge 
  

   bounding 
  the 
  mouths 
  granulated 
  or 
  tuberculated 
  ; 
  additional 
  tubes 
  

   interpolated." 
  (Lonsdale 
  in 
  Strzelecki's 
  ' 
  Physical 
  Description 
  of 
  

   New 
  South 
  Wales 
  and 
  Van 
  Diemen's 
  Land,' 
  p. 
  262, 
  1845.) 
  

  

  The 
  chief 
  features 
  in 
  the 
  above 
  description 
  which 
  are 
  relied 
  upon 
  

   as 
  characterizing 
  the 
  genus 
  are 
  the 
  constriction 
  of 
  the 
  corallites 
  at 
  

   irregular 
  intervals 
  in 
  planes 
  parallel 
  with 
  the 
  surface, 
  the 
  granula- 
  

   tion 
  or 
  tuberculation 
  of 
  the 
  ridges 
  bounding 
  the 
  calices, 
  and 
  the 
  

   interpolation 
  of 
  the 
  young 
  tubes 
  between 
  the 
  old 
  by 
  lateral 
  gemma- 
  

   tion, 
  so 
  that 
  a 
  fracture 
  exposes 
  the 
  outer 
  walls 
  of 
  the 
  tubes. 
  Mural 
  

   pores 
  of 
  course 
  are 
  absent, 
  as 
  in 
  the 
  whole 
  family 
  of 
  the 
  Chcetetince. 
  

   Lonsdale's 
  definition, 
  as 
  above 
  given, 
  is 
  accepted 
  in 
  all 
  its 
  essential 
  

   details 
  by 
  M'Coy 
  (Brit. 
  Pal. 
  Foss. 
  p. 
  24), 
  who 
  describes 
  several 
  

   species 
  under 
  this 
  head. 
  Milne-Edwards 
  and 
  Haime, 
  in 
  the 
  Intro- 
  

   duction 
  to 
  the 
  ' 
  British 
  Fossil 
  Corals 
  ' 
  (p. 
  lxi), 
  accept 
  the 
  genus 
  

   Stenopora, 
  defining 
  it 
  simply 
  as 
  " 
  very 
  similar 
  to 
  Chcetetes, 
  but 
  

   having 
  small 
  styliform 
  processes 
  at 
  the 
  angles 
  of 
  the 
  calices." 
  They 
  

   thus 
  do 
  not 
  notice 
  the 
  characters 
  relied 
  upon 
  by 
  Lonsdale 
  and 
  

   M'Coy 
  as 
  separating 
  Stenopora 
  and 
  Chcetetes, 
  whilst 
  they 
  introduce 
  

   a 
  feature 
  not 
  mentioned 
  by 
  either 
  of 
  these 
  observers. 
  In 
  other 
  

   words, 
  they 
  break 
  up 
  Lonsdale's 
  genus 
  into 
  two 
  portions, 
  one 
  of 
  

   which, 
  typified 
  by 
  S. 
  spinigera, 
  Lonsd., 
  they 
  retain 
  under 
  Stenopora 
  ; 
  

   whilst 
  the 
  other, 
  comprising 
  all 
  the 
  species 
  enumerated 
  by 
  M'Coy, 
  

   King, 
  Geinitz, 
  and 
  Howse, 
  they 
  place 
  under 
  Chcetetes 
  and 
  Favosites. 
  

   The 
  type 
  species, 
  S. 
  spinigera, 
  I 
  have 
  never 
  seen 
  ; 
  but 
  it 
  appears 
  to 
  

   me 
  that 
  the 
  characters 
  relied 
  upon 
  by 
  Lonsdale 
  and 
  M'Coy 
  as 
  sepa- 
  

   rating 
  Stenopora 
  and 
  Chcetetes 
  are 
  clearly 
  insufficient 
  for 
  this 
  purpose, 
  

   and 
  that 
  Milne-Edwards 
  and 
  Haime 
  are 
  therefore 
  fully 
  justified 
  in 
  

   the 
  course 
  they 
  adopted. 
  As 
  far, 
  at 
  any 
  rate, 
  as 
  our 
  American 
  

  

  