﻿26 
  T. 
  M'KENNY 
  HUGHES 
  ON 
  THE 
  GEOLOGY 
  OE 
  ANGLESEY. 
  

  

  referred 
  it 
  to 
  0. 
  Carausii, 
  an 
  Arenig 
  or 
  Tremadoc 
  fossil 
  ; 
  but 
  as 
  I 
  

   think 
  I 
  can 
  show 
  that 
  0. 
  Carausii 
  and 
  0. 
  HicJcsii 
  are 
  the 
  same 
  species, 
  

   under 
  the 
  latter 
  name 
  we 
  have 
  it 
  recorded 
  from 
  the 
  Menevian. 
  

  

  These 
  species 
  both 
  belong 
  to 
  the 
  O.-Actonice 
  group, 
  differing 
  from 
  

   the 
  0. 
  -call 
  i 
  gramma 
  group 
  in 
  the 
  relative 
  shallowness 
  of 
  the 
  dorsal 
  

   valve, 
  being 
  almost 
  plano-convex, 
  and 
  from 
  the 
  O.-flabellulum 
  type 
  

   in 
  having 
  the 
  ventral 
  valve 
  the 
  more 
  convex. 
  It 
  will 
  be 
  observed 
  

   from 
  the 
  descriptions 
  in 
  Davidson 
  that 
  the 
  two 
  species 
  agree 
  in 
  every 
  

   particular 
  till 
  we 
  come 
  to 
  the 
  absolute 
  size, 
  the 
  relative 
  length 
  and 
  

   width, 
  and 
  the 
  number 
  and 
  arrangement 
  of 
  the 
  ribs. 
  Prom 
  the 
  

   specimens 
  exhibited 
  it 
  will 
  be 
  seen 
  that 
  the 
  relation 
  of 
  length 
  to 
  

   width 
  is 
  not 
  constant, 
  that 
  the 
  size 
  is 
  a 
  question 
  of 
  age, 
  and 
  also 
  

   that, 
  in 
  respect 
  of 
  the 
  number 
  and 
  arrangement 
  of 
  ribs, 
  every 
  inter- 
  

   mediate 
  form 
  between 
  0. 
  HicJcsii 
  and 
  0. 
  Carausii 
  is 
  represented 
  in 
  

   the 
  specimens 
  procured 
  from 
  one 
  and 
  the 
  same 
  bed 
  in 
  Anglesey 
  ; 
  

   therefore 
  we 
  must 
  put 
  this 
  down 
  as 
  an 
  early 
  form 
  of 
  Orthis 
  follow- 
  

   ing 
  the 
  receding 
  shore 
  in 
  Cambrian 
  times 
  through 
  the 
  long 
  ages 
  

   that 
  elapsed 
  between 
  the 
  deposition 
  of 
  the 
  rocks 
  known 
  as 
  Menevian 
  

   and 
  that 
  of 
  the 
  beds 
  called 
  Arenig. 
  

  

  The 
  Neseuretus 
  previously 
  recorded 
  (Q,. 
  J. 
  G. 
  S. 
  vol. 
  xxxvi., 
  May 
  

   1880, 
  p. 
  238) 
  is 
  probably 
  from 
  a 
  slightly 
  higher 
  horizon, 
  about 
  

   2 
  miles 
  to 
  the 
  S.W., 
  where 
  the 
  Orthis 
  also 
  is 
  abundant. 
  

  

  In 
  some 
  ash-like 
  beds 
  at 
  the 
  base 
  of 
  the 
  black 
  shales 
  east 
  of 
  

   Mynyddmawr, 
  near 
  Llanerchymedd, 
  a 
  curious 
  fossil 
  was 
  found 
  by 
  

   Mr. 
  Teall. 
  It 
  looks 
  like 
  a 
  Conularia, 
  around 
  which 
  there 
  was 
  some- 
  

   thing 
  of 
  a 
  nodular 
  form 
  which 
  has 
  perished, 
  leaving 
  only 
  a 
  large 
  

   cavity 
  showing 
  no 
  markings 
  on 
  the 
  outside. 
  The 
  quadrangular 
  

   central 
  portion 
  is 
  furrowed 
  longitudinally 
  and, 
  especially 
  near 
  the 
  

   thicker 
  end, 
  shows 
  a 
  cross 
  platy 
  arrangement. 
  This 
  last 
  feature 
  is 
  

   probably, 
  as 
  it 
  stands, 
  due 
  to 
  mineralization, 
  but, 
  it 
  may 
  be, 
  following 
  

   organic 
  structure. 
  The 
  form 
  of 
  the 
  whole 
  fossil 
  certainly 
  suggests 
  

   an 
  organic 
  origin*. 
  

  

  In 
  a 
  quarry 
  by 
  Caemawr, 
  1^ 
  mile 
  north 
  of 
  Llanerchymedd 
  (see 
  

   fig. 
  3), 
  there 
  is 
  a 
  black 
  shale 
  in 
  which 
  Graptolites 
  are 
  tolerably 
  

   abundant 
  along 
  certain 
  bands. 
  

  

  The 
  forms 
  which 
  Dr. 
  Roberts 
  and 
  I 
  found 
  are 
  all 
  referred 
  by 
  Pro- 
  

   fessor 
  Lapworth 
  to 
  

  

  Diclymograptus 
  Murchisoni, 
  Beck. 
  

  

  In 
  the 
  black 
  slates 
  tangled 
  among 
  the 
  dykes 
  on 
  the 
  N.E. 
  coast, 
  

   S.E. 
  of 
  Llysdulas, 
  we 
  found 
  a 
  few 
  Graptolites, 
  which 
  are, 
  according 
  

   to 
  Professor 
  Lapworth, 
  

  

  1. 
  Climacograptus 
  coelatus, 
  Lapw. 
  

  

  2. 
  C. 
  bicornis?, 
  Hall. 
  

  

  3. 
  C. 
  — 
  ?, 
  sp. 
  nov. 
  

  

  The 
  last 
  two 
  are 
  not 
  sufficiently 
  well 
  preserved 
  to 
  allow 
  of 
  certain 
  

   determination 
  ; 
  and 
  Professor 
  Lapworth 
  says 
  that 
  No. 
  2 
  may 
  be 
  

   C. 
  coelatus, 
  and 
  No. 
  3 
  may 
  be 
  only 
  a 
  stem 
  of 
  Dicranograptus, 
  sp. 
  

  

  It 
  is 
  worth 
  observing 
  that 
  whereas 
  dykes 
  usually 
  interfere 
  with 
  

   the 
  fossil- 
  collector, 
  in 
  Anglesey 
  they 
  often 
  help 
  ; 
  for 
  the 
  black 
  slates 
  

   * 
  See 
  r>. 
  22, 
  footnote. 
  

  

  