﻿1864.] 
  DUNCAN 
  — 
  WEST 
  INDIAN 
  COEALS. 
  359 
  

  

  difficult. 
  Many 
  of 
  the 
  species 
  of 
  the 
  genus 
  Astrcea 
  (Heliastrcea, 
  

   Edwards 
  and 
  Haime), 
  described 
  in 
  the 
  first 
  part 
  of 
  this 
  communi- 
  

   cation*, 
  can 
  only 
  be 
  determined 
  after 
  a 
  careful 
  examination 
  of 
  

   their 
  mineralization, 
  so 
  altered 
  are 
  the 
  structures 
  in 
  their 
  relative 
  

   appearance, 
  colour, 
  and 
  preservation. 
  The 
  difficulty 
  is 
  to 
  decide 
  

   which 
  parts, 
  in 
  a 
  calcareous, 
  calcareo-siliceous, 
  or 
  siliceous 
  corallum, 
  

   represent 
  the 
  former 
  hard 
  parts, 
  and 
  which 
  the 
  interspaces 
  ; 
  what 
  

   relation 
  casts 
  may 
  bear 
  to 
  the 
  original 
  forms 
  ; 
  and, 
  amidst 
  opaque 
  or 
  

   very 
  transparent, 
  opalescent 
  or 
  highly-coloured 
  silica, 
  how 
  much 
  

   of 
  the 
  loss 
  of 
  the 
  finer 
  details 
  must 
  be 
  considered 
  the 
  result 
  of 
  mi- 
  

   neralization. 
  Both 
  Dr. 
  Nugent 
  and 
  Mr. 
  Parkinson 
  attributed 
  their 
  

   failure 
  in 
  the 
  classification 
  and 
  description 
  of 
  the 
  siliceous 
  corals 
  to 
  

   the 
  great 
  variety 
  of 
  their 
  forms 
  of 
  mineralization. 
  

  

  The 
  corals 
  of 
  every 
  age, 
  from 
  the 
  Silurian 
  to 
  the 
  Quaternary, 
  

   present 
  great 
  alterations 
  in 
  the 
  appearance 
  of 
  the 
  sclerenchyma, 
  

   and 
  are 
  therefore 
  not 
  always 
  readily 
  classified 
  and 
  distinguished 
  ; 
  

   the 
  Oolitic 
  species 
  are 
  particularly 
  affected 
  by 
  mineralization 
  ; 
  but 
  it 
  

   is 
  certainly 
  true 
  that, 
  as 
  a 
  whole, 
  the 
  West 
  Indian 
  Miocene 
  fossil 
  

   corals 
  are 
  the 
  most 
  recondite 
  in 
  their 
  fossilization. 
  

  

  The 
  corals 
  of 
  the 
  Chert 
  and 
  Marl 
  of 
  Antigua 
  are 
  readily 
  distin- 
  

   guished 
  both 
  from 
  each 
  other 
  and 
  from 
  the 
  fossils 
  of 
  the 
  other 
  

   islands 
  : 
  occasionally 
  a 
  siliceous 
  coral 
  is 
  found 
  in 
  the 
  Nivaje 
  shale 
  

   and 
  in 
  the 
  beds 
  of 
  the 
  smaller 
  islands 
  ; 
  but 
  the 
  majority 
  of 
  the 
  

   Jamaican 
  and 
  San-Domingan 
  specimens 
  are 
  either 
  a 
  little 
  harder 
  

   than 
  during 
  life, 
  and 
  calcareous, 
  or 
  have 
  been 
  more 
  or 
  less 
  

   altered 
  during 
  a 
  process 
  of 
  silicification, 
  which 
  has 
  been 
  rarely 
  

   completed. 
  It 
  is 
  obvious 
  that 
  the 
  form 
  of 
  mineralization 
  cannot 
  be 
  

   received 
  as 
  an 
  evidence 
  of 
  the 
  age 
  of 
  the 
  strata 
  containing 
  the 
  corals 
  ; 
  

   for 
  the 
  Marl 
  of 
  Antigua 
  is 
  not 
  older 
  than 
  the 
  Jamaican 
  or 
  Nivaje 
  

   deposits, 
  yet 
  its 
  corals 
  are 
  more 
  intensely 
  silicified, 
  and, 
  moreover, 
  

   the 
  condition 
  of 
  specimens 
  taken 
  from 
  these 
  last 
  is 
  often 
  the 
  same 
  

   as 
  of 
  those 
  which 
  are 
  to 
  be 
  obtained 
  from 
  the 
  dense 
  limestone 
  of 
  

   recent 
  reefs. 
  Nevertheless 
  perfect 
  silicification 
  is 
  a 
  proof 
  of 
  a 
  Mid- 
  

   tertiary 
  age 
  in 
  West 
  Indian 
  fossil 
  corals. 
  

  

  The 
  anatomy 
  of 
  the 
  stony 
  corals 
  is 
  very 
  simple, 
  and 
  the 
  only 
  

   difficulty 
  which 
  can 
  arise 
  in 
  reference 
  to 
  the 
  various 
  forms 
  of 
  mine- 
  

   ralization 
  is 
  the 
  relation 
  which 
  the 
  interspaces, 
  or 
  the 
  parts 
  in- 
  

   cluded 
  by 
  the 
  sclere 
  ichyma, 
  bear 
  to 
  it. 
  It 
  must 
  be 
  remembered 
  

   that 
  the 
  hard 
  parts 
  are 
  more 
  or 
  less 
  traversed 
  by 
  fine 
  pores 
  during 
  life, 
  

   and 
  in 
  one 
  grea': 
  family 
  this 
  porosity 
  is 
  in 
  excess; 
  and 
  that 
  the 
  

   lower 
  parts 
  of 
  simple 
  corals 
  are 
  frequently 
  filled 
  up 
  by 
  a 
  dense 
  tissue 
  

   before 
  death, 
  just 
  as 
  the 
  intermural 
  spaces 
  of 
  compound 
  forms 
  are 
  

   in 
  some 
  species. 
  The 
  sclerenchyma 
  consists 
  of 
  the 
  Wall, 
  Costae, 
  

   Exotheca, 
  Ep'theca, 
  Septa, 
  Pali, 
  Columella, 
  and 
  Endotheca 
  ; 
  the 
  Exo- 
  

   theca 
  comprehends 
  the 
  costal 
  dissepiments 
  and 
  the 
  Ccenenchyma 
  ; 
  

   and 
  the 
  E-dotheca, 
  the 
  septal 
  dissepiments, 
  synapticulse, 
  and 
  tabula?. 
  

   The 
  interspaces, 
  during 
  life, 
  contain 
  the 
  soft 
  tissues, 
  and 
  admit 
  the 
  

   isea-water 
  ; 
  but, 
  as 
  growth 
  progresses, 
  many 
  of 
  them 
  remote 
  from 
  

  

  * 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xix. 
  pp. 
  412 
  et 
  seq. 
  

  

  