﻿THE 
  ECHIN0THTTRIDJ3 
  AND 
  THE 
  PEEISCHOECHINID-35. 
  315 
  

  

  plates 
  of 
  the 
  apical 
  disk 
  : 
  in 
  Palceehinus 
  the 
  ocular 
  plates 
  are 
  doubly 
  

   perforated, 
  the 
  genital 
  plates 
  triply 
  so 
  ; 
  in 
  Melonites 
  the 
  perforations 
  

   appear 
  to 
  vary 
  in 
  different 
  individuals. 
  

  

  In 
  conclusion, 
  it 
  will 
  be 
  obvious 
  that 
  the 
  diagnosis 
  of 
  the 
  Peris- 
  

   choechinidce 
  must 
  be 
  so 
  far 
  modified 
  as 
  to 
  include 
  the 
  characters 
  of 
  

   the 
  overlapping 
  plates 
  of 
  Lepidechinus 
  and 
  Archceocidaris, 
  and 
  the 
  

   increase 
  in 
  the 
  number 
  of 
  the 
  rows 
  of 
  plates 
  in 
  the 
  ambulacra 
  of 
  

   Melonites 
  and 
  Oligoporus. 
  

  

  It 
  has 
  also 
  been 
  pointed 
  out 
  that 
  Mr. 
  Baily 
  has 
  discovered 
  primary 
  

   tubercles 
  in 
  Palceehinus 
  ; 
  how 
  far 
  this 
  will 
  affect 
  M'Coy's 
  division 
  

   of 
  the 
  Perischoechinidce 
  into 
  two 
  families, 
  based 
  on 
  the 
  presence 
  or 
  

   absence 
  of 
  primary 
  tubercles, 
  will 
  very 
  much 
  depend 
  on 
  the 
  cha- 
  

   racters 
  of 
  these 
  structures 
  in 
  some 
  of 
  the 
  other 
  genera 
  mentioned. 
  

  

  [Since 
  these 
  remarks 
  were 
  written 
  I 
  have 
  seen 
  the 
  description 
  of 
  

   another 
  genus 
  of 
  this 
  group, 
  Lepidesthes, 
  Meek 
  & 
  Worthen, 
  in 
  the 
  

   3rd 
  vol. 
  of 
  their 
  admirable 
  ' 
  Illinois 
  Survey 
  Report,' 
  p. 
  522. 
  Her© 
  

   the 
  interambulacra 
  were 
  composed 
  of 
  plates 
  imbricating 
  from 
  below 
  

   upwards 
  and 
  outwards 
  from 
  the 
  middle 
  ; 
  the 
  ambulacra 
  were 
  com- 
  

   posed 
  of 
  ten 
  rows 
  of 
  small 
  pieces 
  imbricating 
  from 
  above 
  downwards, 
  

   with 
  two 
  nearly 
  central 
  pores 
  to 
  each 
  piece. 
  The 
  whole 
  surface 
  

   appears 
  to 
  have 
  been 
  ornamented 
  with 
  small 
  granules, 
  probably 
  for 
  

   the 
  articulation 
  of 
  small 
  spines. 
  Lepidesthes, 
  in 
  the 
  mode 
  of 
  imbri- 
  

   cation 
  of 
  its 
  plates, 
  agrees 
  with 
  Lepidechinus, 
  Hall, 
  as 
  now 
  under- 
  

   stood, 
  but 
  differs 
  in 
  the 
  relative 
  breadth 
  of 
  the 
  ambulacra 
  and 
  inter- 
  

   ambulacra. 
  In 
  the 
  presence 
  of 
  ten 
  rows 
  of 
  plates 
  in 
  the 
  former, 
  it 
  

   agrees 
  with 
  Melonites, 
  but 
  differs 
  in 
  their 
  imbricating 
  character. 
  

   Messrs. 
  Meek 
  and 
  Worthen 
  remark 
  that 
  this 
  peculiar 
  imbrication 
  of 
  

   the 
  plates 
  will 
  probably 
  " 
  be 
  found 
  to 
  be 
  of 
  even 
  more 
  than 
  generic 
  

   importance."] 
  

  

  EXPLANATION 
  OF 
  PLATE 
  XXIV. 
  

  

  Pig. 
  1. 
  Archesocidaris, 
  interambulacral 
  plates, 
  from 
  drawings 
  kindly 
  supplied 
  

   by 
  Mr. 
  J. 
  Young, 
  F.G.S. 
  : 
  a, 
  bevelled 
  edges. 
  

  

  2. 
  Palachinus, 
  portion 
  of 
  an 
  ambulacrum, 
  and 
  the 
  bordering 
  interambu- 
  

  

  lacral 
  plates, 
  enlarged. 
  (From 
  Prof. 
  M'Coy's 
  figures, 
  'Synop. 
  Garb. 
  

   Fbss. 
  Ireland,' 
  pi. 
  24.) 
  

  

  3. 
  Palcechinus, 
  apical 
  disk, 
  enlarged 
  : 
  a, 
  genital 
  plates, 
  with 
  triple 
  perfora- 
  

  

  tions; 
  b, 
  ocular 
  plates, 
  with 
  dual 
  perforations. 
  (From 
  Mr. 
  Baily's 
  

   figure 
  in 
  a 
  paper 
  read 
  before 
  Hoy. 
  Geol. 
  Soc. 
  Ireland, 
  March 
  9, 
  1864.) 
  

  

  4. 
  Oligoporus, 
  portion 
  of 
  an 
  ambulacrum 
  and 
  bordering 
  interambulacral 
  

  

  plates. 
  (After 
  Meek 
  & 
  Worthen's 
  figure, 
  ' 
  Illinois 
  Geol. 
  Kep.' 
  ii. 
  p. 
  248, 
  

   fig. 
  28.) 
  

  

  5. 
  Melonites, 
  similar 
  figure 
  to 
  last. 
  (L. 
  c. 
  fig. 
  27.) 
  

  

  6. 
  , 
  apical 
  disk 
  : 
  a, 
  genital 
  plates, 
  with 
  pores 
  ; 
  b, 
  ocular 
  plates, 
  with- 
  

   out 
  pores. 
  (L. 
  c. 
  p. 
  228, 
  fig. 
  22.) 
  

  

  7. 
  Oral 
  disk 
  and 
  teeth 
  of 
  a 
  recent 
  Cidaris 
  : 
  a, 
  the 
  five 
  ambulacral 
  segments, 
  

  

  with 
  notched 
  and 
  perforated 
  plates. 
  (From 
  Mr. 
  S. 
  P. 
  Woodward's 
  

   figure, 
  ' 
  Geologist,' 
  1863, 
  pi. 
  18.) 
  

  

  Discussion. 
  

  

  Mr. 
  Ethekidge 
  described 
  Calveria 
  as 
  resembling 
  an 
  elastic 
  ball 
  

   rather 
  than 
  an 
  ordinary 
  Sea-urchiu, 
  its 
  calcareous 
  plates 
  being 
  held 
  

  

  