﻿PROM 
  THE 
  LOWER 
  SILURIAN 
  OF 
  NORTH 
  AMERICA. 
  511 
  

  

  nodulated 
  masses, 
  which 
  agree 
  in 
  other 
  respects 
  with 
  the 
  free 
  

   forms, 
  but 
  which 
  are 
  attached 
  parasitically 
  to 
  foreign 
  bodies, 
  and 
  

   which 
  therefore 
  do 
  not 
  possess 
  a 
  concave 
  under 
  surface. 
  I 
  am, 
  

   however, 
  unable 
  to 
  decide 
  whether 
  this 
  of 
  itself 
  should 
  be 
  re- 
  

   garded 
  as 
  a 
  character 
  of 
  specific 
  value, 
  though 
  it 
  would 
  seem 
  not 
  

   to 
  be 
  so. 
  

  

  Locality 
  and 
  Formation. 
  Common, 
  and 
  very 
  variable 
  in 
  form, 
  in 
  

   the 
  Trenton 
  Limestone 
  of 
  Belleville, 
  Peterborough, 
  and 
  other 
  locali- 
  

   ties 
  in 
  Ontario. 
  Abundant 
  in 
  the 
  Hudson-River 
  group 
  of 
  Ontario. 
  

   Less 
  frequent 
  in 
  the 
  Cincinnati 
  group 
  of 
  Ohio. 
  

  

  16. 
  Ch^etetes 
  riiscoiDEus, 
  James. 
  PI. 
  XXX. 
  figs. 
  4-4 
  d. 
  

  

  Chostetes 
  discoideus, 
  James, 
  Catalogue 
  of 
  Possils 
  of 
  the 
  Cincinnati 
  

   group, 
  1871 
  (named, 
  but 
  not 
  figured 
  or 
  described). 
  

  

  Corallum 
  free, 
  discoid, 
  plano-convex, 
  sharp-edged, 
  from 
  5 
  to 
  8 
  

   lines 
  in 
  diameter, 
  and 
  about 
  1 
  line 
  in 
  greatest 
  thickness. 
  Under 
  

   surface 
  concave, 
  covered 
  with 
  a 
  very 
  thin, 
  smooth, 
  and 
  not 
  regu- 
  

   larly 
  striated 
  epitheca, 
  which 
  usually 
  exhibits 
  two 
  or 
  three 
  con- 
  

   centric 
  wrinkles. 
  In 
  general, 
  the 
  epitheca 
  is 
  so 
  delicate 
  as 
  to 
  reveal 
  

   clearly 
  through 
  its 
  substance 
  the 
  bases 
  of 
  the 
  superjacent 
  corallites. 
  

   Upper 
  surface 
  gently 
  convex, 
  not 
  exhibiting 
  any 
  tubercles 
  or 
  

   elevations 
  of 
  any 
  kind. 
  Corallites 
  subequal 
  ; 
  calices 
  with 
  mode- 
  

   rately 
  thin 
  walls, 
  polygonal, 
  from 
  eight 
  to 
  ten 
  in 
  the 
  space 
  of 
  one 
  

   line. 
  Xo 
  groups 
  of 
  larger-sized 
  corallites, 
  nor 
  any 
  very 
  minute 
  

   intermediate 
  tubuli. 
  

  

  I 
  do 
  not 
  feel 
  altogether 
  certain 
  that 
  this 
  form 
  is 
  distinct 
  from 
  the 
  

   young 
  of 
  Chostetes 
  petropolitanus. 
  It 
  is, 
  however, 
  a 
  common 
  form, 
  

   and 
  is 
  very 
  constant 
  in 
  its 
  dimensions. 
  Apart 
  from 
  its 
  discoidal 
  

   plano-convex 
  form, 
  it 
  is 
  distinguished 
  by 
  its 
  great 
  tenuity 
  (compara- 
  

   tively 
  speaking), 
  the 
  sharp 
  thin 
  edges 
  of 
  the 
  disk, 
  the 
  absence 
  of 
  

   surface-tuberosities 
  or 
  groups 
  of 
  large-sized 
  corallites, 
  and 
  the 
  

   extreme 
  thinness 
  of 
  the 
  epitheca, 
  which 
  is 
  transparent 
  and 
  is 
  not 
  

   regularly 
  striated 
  concentrically. 
  But 
  for 
  one 
  character, 
  I 
  should 
  

   have 
  been 
  disposed 
  to 
  have 
  placed 
  this 
  species 
  under 
  Chostetes 
  (Nebu- 
  

   lipora) 
  lens, 
  M'Coy 
  ; 
  and 
  that 
  is 
  the 
  absence 
  in 
  our 
  examples 
  of 
  any 
  

   groups 
  of 
  large-sized 
  corallites, 
  whereas 
  their 
  presence 
  is 
  a 
  marked 
  

   feature 
  in 
  the 
  latter. 
  The 
  underside 
  of 
  G. 
  discoideus 
  resembles 
  

   Lichenalia 
  calycula, 
  James 
  ; 
  but 
  it 
  may 
  be 
  distinguished 
  by 
  the 
  

   absence 
  of 
  radiating 
  striae, 
  and 
  by 
  other 
  characters 
  as 
  well. 
  The 
  

   above 
  description 
  of 
  the 
  species 
  is 
  drawn 
  from 
  type 
  specimens 
  for- 
  

   warded 
  to 
  me 
  by 
  Mr. 
  U. 
  P. 
  James. 
  

  

  Locality 
  and 
  Formation. 
  Cincinnati 
  group, 
  near 
  Cincinnati, 
  Ohio. 
  

  

  IV. 
  Incrttsting 
  Species. 
  

  

  In 
  this 
  group 
  the 
  corallum 
  is 
  parasitic, 
  and 
  forms 
  a 
  thin 
  crust 
  

   composed 
  of 
  a 
  single 
  layer 
  of 
  short 
  corallites, 
  which 
  are 
  attached 
  by 
  

   their 
  bases 
  to 
  some 
  foreign 
  object, 
  such 
  as 
  the 
  shell 
  of 
  a 
  Brachiopod, 
  

   the 
  epitheca 
  of 
  a 
  coral, 
  or 
  the 
  column 
  of 
  a 
  Crinoid. 
  

  

  