﻿210 
  H. 
  S. 
  Williams 
  — 
  Silurian-Devonian 
  boundary 
  

  

  labelled 
  on 
  plate 
  5 
  certainly 
  appears 
  as 
  if 
  nearly 
  flat. 
  Even 
  

   were 
  this 
  not 
  the 
  case 
  (which 
  an 
  examination 
  of 
  the 
  type 
  speci- 
  

   mens 
  would 
  demonstrate), 
  the 
  fact 
  that 
  we 
  have 
  in 
  the 
  Gaspe 
  

   series 
  both 
  fully 
  gibbous 
  and 
  flat 
  forms, 
  which 
  closely 
  resemble 
  

   each 
  other 
  in 
  other 
  respects, 
  gives 
  reason 
  for 
  supposing 
  that 
  

   the 
  British 
  form 
  is 
  at 
  least 
  represented 
  by 
  these 
  Gaspe 
  and 
  

   Elaine 
  species. 
  

  

  In 
  Hall's 
  description 
  of 
  the 
  Arisaig 
  specimens 
  of 
  Chonetes 
  

   nova-scotica, 
  it 
  is 
  stated, 
  "A 
  stronger 
  and 
  more 
  elevated 
  stria 
  

   often 
  marks 
  the 
  median 
  line 
  from 
  beak 
  to 
  base 
  of 
  the 
  ventral 
  

   valve."* 
  This 
  feature 
  is 
  seen 
  on 
  several 
  of 
  the 
  specimens 
  of 
  

   the 
  Chapman 
  sandstone, 
  and, 
  curiously, 
  it 
  is 
  not 
  always 
  the 
  

   central 
  plication. 
  In 
  one 
  case 
  several 
  plications, 
  a 
  little 
  out 
  of 
  

   the 
  center, 
  are 
  thus 
  enlarged. 
  Such 
  a 
  trick 
  of 
  variation 
  would 
  

   seem, 
  distinctly, 
  to 
  indicate 
  close 
  phylogenetic 
  relationship. 
  

  

  In 
  the 
  Oriskany 
  of 
  Albany 
  and 
  Schoharie 
  Counties, 
  in 
  New 
  

   York, 
  a 
  form 
  (6 
  y 
  . 
  complanatus) 
  quite 
  as 
  large 
  as 
  Billings' 
  C. 
  

   canadensis, 
  is 
  described 
  by 
  Hall. 
  The 
  larger 
  forms 
  are 
  all 
  

   from 
  the 
  Gaspe, 
  Maine 
  and 
  Oriskany 
  faunas 
  at 
  the 
  summit 
  of 
  

   the 
  Silurian. 
  Leptwna 
  lata, 
  which 
  is 
  figured 
  as 
  larger 
  than 
  

   the 
  ordinary 
  types 
  of 
  Chonetes, 
  is 
  said 
  to 
  be 
  " 
  one 
  of 
  the 
  most 
  

   characteristic 
  shells 
  of 
  the 
  TJjjper 
  Ludlow 
  " 
  (Sil. 
  Syst., 
  p. 
  603). 
  

  

  The 
  Bellerophons 
  furnish 
  a 
  second 
  set 
  of 
  diagnostic 
  species. 
  

   In 
  the 
  original 
  list 
  of 
  the 
  Tilestone 
  fauna, 
  four 
  species 
  of 
  

   Bellerophon 
  are 
  named, 
  viz 
  : 
  

  

  Bellerophon 
  carinatus. 
  

   B. 
  striatus. 
  

  

  B. 
  trilobatus. 
  

  

  B. 
  globatus. 
  

  

  Bellerophon 
  trilobatus 
  is 
  said 
  to 
  be 
  a 
  characteristic 
  form 
  of 
  

   the 
  Tilestone 
  (Sil. 
  Syst., 
  p. 
  141). 
  This 
  species 
  is 
  represented 
  

   in 
  both 
  the 
  Chapman 
  sandstone 
  and 
  formation 
  D 
  of 
  Arisaig. 
  

   In 
  " 
  Siluria," 
  it 
  is 
  stated 
  that 
  B. 
  expansus, 
  " 
  B. 
  Murchisonce, 
  

   B. 
  carinatus, 
  and 
  B. 
  trilobatus, 
  generally 
  of 
  small 
  size, 
  are 
  

   most 
  abundant 
  everywhere 
  in 
  the 
  upper 
  beds 
  of 
  the 
  Ludlow 
  

   rock" 
  (p. 
  231). 
  Bellerophon 
  carinatus 
  and 
  B. 
  trilobatus 
  are 
  

   both 
  represented 
  in 
  the 
  Chapman 
  ; 
  and 
  Honeyman 
  reports 
  all 
  

   four 
  species 
  from 
  the 
  Upper 
  Arisaig, 
  Zone 
  D 
  (Q. 
  J. 
  G. 
  S., 
  vol. 
  

   xx, 
  p. 
  343). 
  

  

  A 
  third 
  diagnostic 
  form 
  is 
  called 
  " 
  Agnostus 
  tuberculatus 
  

   (Battus 
  tuberculatus 
  Kloden) 
  " 
  by 
  Sowerby 
  in 
  1839 
  (Silurian 
  

   System, 
  p. 
  604, 
  plate 
  3, 
  fig. 
  17). 
  This 
  is 
  Beyrichia 
  tuberculata 
  

   Kloden. 
  Several 
  specimens 
  of 
  this 
  species, 
  or 
  a 
  very 
  closely 
  

   related 
  form, 
  are 
  recognized 
  in 
  the 
  Chapman 
  fauna. 
  Hall 
  

  

  * 
  Descriptions 
  of 
  New 
  Species 
  of 
  Fossils 
  from 
  the 
  Silurian 
  Rocks 
  of 
  Nova 
  Scotia, 
  

   by 
  James 
  Hall. 
  Can. 
  Nat. 
  and 
  Geo!., 
  vol. 
  v, 
  p. 
  145. 
  

  

  