﻿Anatomy 
  0/ 
  Paleeechinus 
  (Scouler), 
  M'Coy. 
  197 
  

  

  proct. 
  It 
  is 
  very 
  interesting 
  to 
  find 
  this 
  variability 
  of 
  the 
  

   entrance 
  of 
  the 
  radial 
  plates, 
  which 
  is 
  not 
  uncommon 
  in 
  

   Mesozoic 
  and 
  recent 
  species 
  of 
  Echinoidea, 
  exemplified 
  in 
  the 
  

   Palreechinoidea. 
  

  

  It 
  was 
  recorded 
  by 
  a 
  late 
  distinguished 
  palaeontologist, 
  de 
  

   Koninck, 
  that 
  the 
  Palceeckini 
  were 
  without 
  radial 
  plates 
  to 
  

   their 
  dorso-central 
  systems 
  (Geol. 
  Mag. 
  vol. 
  vii. 
  p. 
  259, 
  

   pi. 
  vii. 
  fig. 
  1). 
  It 
  appears 
  now 
  evident 
  that 
  the 
  specimens 
  

   studied 
  by 
  that 
  able 
  palaeontologist 
  had 
  been 
  subject 
  to 
  crush 
  

   and 
  irregular 
  pressure, 
  so 
  that 
  the 
  radial 
  plates 
  were 
  either 
  

   pushed 
  into 
  the 
  test 
  or 
  pressed 
  away. 
  In 
  one 
  of 
  the 
  speci- 
  

   mens 
  of 
  Pala>echinus 
  in 
  the 
  British 
  Museum 
  one 
  half 
  of 
  an 
  

   ambulacrum 
  was 
  thus 
  disposed 
  of, 
  so 
  that 
  an 
  interradium 
  

   is 
  placed 
  next 
  to 
  the 
  median 
  suture 
  of 
  an 
  ambulacrum. 
  There 
  

   is 
  a 
  specimen 
  of 
  a 
  Palcvechinus 
  in 
  the 
  Woodwardian 
  Museum 
  

   showing 
  small 
  radial 
  plates 
  partly 
  crushed 
  inwards. 
  

  

  The 
  apical 
  system 
  of 
  the 
  largest 
  specimen 
  of 
  Palcvechinus 
  

   sjjhcertcus 
  in 
  the 
  British 
  Museum 
  has 
  great 
  resemblance 
  to 
  

   the 
  system 
  of 
  P. 
  elegans 
  described 
  and 
  drawn 
  by 
  Baily, 
  

   Geol. 
  Mag. 
  vol. 
  ii. 
  p. 
  44, 
  and 
  reproduced 
  by 
  R. 
  Etheridge, 
  

   Jun., 
  in 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  1874, 
  vol. 
  xxx. 
  pi. 
  xxiv. 
  

   In 
  fact 
  the 
  differences 
  are 
  the 
  apparent 
  flatness 
  of 
  the 
  system 
  

   in 
  Baily's 
  figure 
  and 
  the 
  presence 
  of 
  the 
  perforations, 
  which 
  

   are 
  obliterated 
  in 
  the 
  British-Museum 
  specimens. 
  

  

  It 
  is 
  evident 
  that 
  the 
  generic 
  diagnosis 
  of 
  Palceechinus 
  

   must 
  refer 
  to 
  the 
  presence 
  of 
  five 
  radial 
  plates, 
  which 
  may 
  or 
  

   may 
  not 
  be 
  perfectly 
  intercalated 
  between 
  the 
  corresponding 
  

   basal 
  plates. 
  The 
  specific 
  diagnosis 
  of 
  P. 
  sphcrricus 
  must 
  be 
  

   altered 
  from 
  that 
  of 
  de 
  Koninck 
  and 
  Loven 
  (' 
  Etudes,' 
  p. 
  41), 
  

   for 
  the 
  absence 
  of 
  radial 
  plates 
  is 
  accidental. 
  

  

  The 
  small, 
  thick, 
  irregularly 
  shaped 
  anal 
  plates 
  of 
  P. 
  spthce- 
  

   ricus 
  in 
  no 
  way 
  resemble 
  those 
  of 
  the 
  Saleniida?, 
  nor 
  in 
  fact 
  

   do 
  those 
  of 
  P. 
  elegans, 
  as 
  figured 
  by 
  Baily 
  ; 
  there 
  is 
  no 
  

   appearance 
  of 
  a 
  u 
  sur-anal." 
  

  

  The 
  Anatomy 
  of 
  (lie 
  Ambulacral 
  Plates. 
  

  

  The 
  specimens 
  of 
  several 
  species 
  of 
  Palceechinus 
  in 
  the 
  

   Museum 
  of 
  Practical 
  Geology, 
  in 
  the 
  Woodwardian 
  Museum, 
  

   and 
  in 
  the 
  British 
  Museum 
  show 
  the 
  construction 
  of 
  the 
  

   ambulacra 
  perfectly. 
  The 
  following 
  observations 
  were 
  made. 
  

  

  I. 
  There 
  are 
  two 
  vertical 
  rows 
  of 
  pairs 
  of 
  pores 
  throughout 
  

   each 
  side 
  of 
  an 
  ambulacrum 
  ; 
  the 
  pores 
  of 
  the 
  pairs 
  are 
  nearly 
  

   or 
  quite 
  horizontal, 
  but 
  may 
  be 
  oblique, 
  are 
  separated 
  by 
  a 
  

   thin 
  convex 
  septum 
  placed 
  vertically 
  or 
  nearly 
  so, 
  and 
  there 
  is 
  

   no 
  peripodium 
  ; 
  the 
  adoral 
  pore 
  of 
  a 
  pair 
  is 
  not 
  near 
  the 
  adoral 
  

  

  