﻿310 
  E. 
  ETHERIDGE 
  ON 
  THE 
  RELATIONSHIP 
  BETWEEN 
  

  

  on 
  the 
  lower 
  edge. 
  Mr. 
  Young 
  also 
  observes 
  that 
  one 
  of 
  the 
  other 
  two 
  

   sides 
  is 
  provided 
  with 
  a 
  small 
  groove, 
  into 
  which 
  would 
  fit 
  the 
  edge 
  of 
  

   a 
  succeeding 
  plate, 
  and 
  so 
  probably 
  prevent 
  too 
  great 
  a 
  laxity 
  of 
  test. 
  

  

  If, 
  for 
  a 
  moment, 
  we 
  now 
  examine 
  the 
  state 
  of 
  matters 
  as 
  regards 
  

   Palachinus, 
  we 
  shall 
  find, 
  judging 
  from 
  M'Coy's 
  beautiful 
  figures 
  in 
  

   his 
  ' 
  Synopsis 
  of 
  the 
  Carboniferous 
  Fossils 
  of 
  Ireland' 
  (pi. 
  24), 
  that 
  

   the 
  test 
  in 
  this 
  genus 
  had 
  no 
  overlapping 
  plates, 
  but 
  was 
  rigid, 
  after 
  

   the 
  normal 
  character 
  of 
  the 
  Echini*. 
  

  

  Sufficiently 
  good 
  specimens 
  of 
  Archceocidaris 
  have 
  unfortunately 
  

   not 
  been 
  obtained 
  to 
  show 
  a 
  series 
  of 
  these 
  overlapping 
  plates 
  in 
  

   juxtaposition; 
  but 
  I 
  think 
  a 
  careful 
  examination 
  of 
  well-preserved 
  

   interambulacral 
  plates, 
  with 
  their 
  peculiar 
  bevelled 
  edges, 
  cannot 
  

   but 
  prove 
  that 
  these 
  edges 
  could 
  have 
  served 
  no 
  other 
  purpose. 
  

   Confirmatory 
  evidence 
  of 
  this 
  is 
  to 
  be 
  met 
  with 
  in 
  a 
  closely 
  allied 
  

   genus, 
  Lepidechinus, 
  which 
  is 
  described 
  by 
  Hall 
  as 
  having 
  the 
  

   " 
  plates 
  of 
  the 
  interambulacral 
  series 
  imbricating 
  from 
  the 
  dorsal 
  

   side, 
  and 
  the 
  lower 
  edges 
  of 
  each 
  range 
  overlapping 
  those 
  below 
  ; 
  

   while 
  the 
  plates 
  of 
  the 
  ambulacral 
  series 
  are 
  imbricating 
  in 
  the 
  op- 
  

   posite 
  direction 
  "f, 
  a 
  complete 
  repetition 
  of 
  the 
  arrangement 
  seen 
  in 
  

   Calveria. 
  [Note. 
  — 
  In 
  the 
  3rd 
  vol. 
  ' 
  Illinois 
  Geol. 
  Report, 
  p. 
  552, 
  

   Messrs. 
  Meek 
  & 
  "Wbrthen 
  remark 
  that 
  Prof. 
  Hall 
  has 
  since 
  stated 
  

   that 
  his 
  original 
  description 
  of 
  L. 
  imbricatus, 
  the 
  type 
  of 
  the 
  genus, 
  

   was 
  not 
  absolutely 
  correct, 
  but 
  that 
  the 
  imbrication 
  of 
  all 
  the 
  plates 
  

   is 
  exactly 
  the 
  reverse 
  of 
  what 
  he 
  had 
  supposed. 
  He 
  suggests 
  that 
  

   he 
  may 
  have 
  been 
  mistaken 
  in 
  regard 
  to 
  which 
  was 
  the 
  dorsal 
  and 
  

   which 
  the 
  ventral 
  side.] 
  

  

  From 
  a 
  consideration 
  of 
  these 
  facts, 
  and 
  admitting 
  that 
  if 
  the 
  

   existence 
  of 
  overlapping 
  plates 
  be 
  accepted 
  in 
  Archceocidaris, 
  whilst 
  

   the 
  presence 
  of 
  simply 
  abutting 
  plates 
  is 
  recognized 
  in 
  PcdcecMnus, 
  

   we 
  have 
  in 
  M'Coy's 
  Perischoechinidce 
  two 
  types 
  of 
  structure 
  — 
  one, 
  

   Archceocidaris, 
  with 
  overlapping 
  plates, 
  allied 
  to 
  Thomson's 
  Echino- 
  

   ihuriclce, 
  the 
  other, 
  PcdcecMnus, 
  with 
  abutting 
  plates, 
  allied 
  to 
  the 
  

   ordinary 
  forms 
  of 
  Echini, 
  but 
  both 
  still 
  possessing 
  their 
  great 
  

   distinctive 
  character, 
  viz. 
  the 
  increase 
  in 
  the 
  number 
  of 
  interambu- 
  

   lacral 
  plates, 
  which 
  at 
  once 
  presents 
  a 
  clear 
  line 
  of 
  demarcation 
  be- 
  

   tween 
  them 
  and 
  all 
  recent 
  types. 
  

  

  Perischodomus, 
  Melonites, 
  Eocidaris, 
  and 
  Oligojjorus 
  have 
  not 
  been 
  

   shown 
  to 
  possess 
  overlapping 
  plates, 
  although, 
  as 
  will 
  be 
  presently 
  

   pointed 
  out, 
  from 
  their 
  resemblance 
  to 
  one 
  of 
  the 
  two 
  preceding, 
  

   it 
  is 
  probable 
  that 
  some 
  of 
  them 
  at 
  least 
  will 
  be 
  found 
  to 
  do 
  so. 
  

  

  I 
  shall 
  now 
  proceed 
  to 
  note 
  the 
  more 
  intimate 
  test-structure 
  of 
  the 
  

   genera 
  constituting 
  the 
  Perischoechinidce, 
  and 
  showhow 
  they 
  agree 
  with 
  

   this 
  family 
  on 
  the 
  one 
  hand, 
  and 
  with 
  the 
  Echinothuridce 
  on 
  the 
  other, 
  

   simply 
  premising 
  that 
  they 
  all 
  possess 
  the 
  distinctive 
  character 
  of 
  

   the 
  former, 
  viz. 
  the 
  great 
  increase 
  in 
  the 
  number 
  of 
  the 
  interambu- 
  

   lacral 
  plates. 
  

  

  * 
  Mr. 
  W. 
  H. 
  Baily 
  has 
  also 
  given 
  good 
  figures 
  of 
  Palaschinus, 
  which 
  show 
  

   the 
  arrangement 
  of 
  the 
  interambulacral 
  plates. 
  See 
  Trans. 
  Eoy. 
  Geol. 
  Soc. 
  

   Ireland, 
  vol. 
  i. 
  

  

  f 
  Descr. 
  New 
  Sp. 
  Crinoidea, 
  Prelim. 
  Notice, 
  Albany, 
  1861, 
  p. 
  18. 
  

  

  