﻿Vol. 
  53-] 
  

  

  ECHINOCYSTIS 
  AND 
  PAL2E0DISCUS. 
  

  

  125 
  

  

  Pig. 
  1. 
  — 
  Ambulacral 
  

   -plates 
  of 
  Echinocystis. 
  

  

  Museum 
  specimens, 
  supplemented 
  by 
  Thomson's 
  figures 
  for 
  the 
  

   shape 
  of 
  the 
  test 
  : 
  — 
  

  

  Description. 
  — 
  Form 
  spheroidal, 
  either 
  prolate 
  or 
  oblate. 
  The 
  test 
  

   is 
  thin 
  and 
  flexible. 
  The 
  peristome 
  is 
  central 
  on 
  the 
  lower 
  surface. 
  

   The 
  anus 
  is 
  protected 
  by 
  a 
  group 
  of 
  large 
  plates 
  ; 
  it 
  is 
  not 
  opposite 
  

   the 
  mouth, 
  but 
  opens 
  in 
  the 
  posterior 
  inter 
  ambulacrum. 
  

  

  Ambulacra 
  regular 
  in 
  width, 
  tapering 
  gradually 
  to 
  the 
  apex. 
  

   Each 
  ambulacrum 
  consists 
  of 
  four 
  series 
  

   of 
  plates. 
  The 
  plates 
  are 
  small, 
  simple 
  

   demi-plates. 
  The 
  pore-pairs 
  are 
  biserial 
  

  

  (%• 
  i). 
  

  

  Interambulacra 
  of 
  numerous 
  ir- 
  

   regular, 
  angular 
  plates. 
  In 
  the 
  widest 
  

   parts 
  of 
  the 
  interambulacra 
  they 
  are 
  about 
  

   ten 
  in 
  width, 
  decreasing 
  in 
  number 
  above 
  

   and 
  below. 
  The 
  plates 
  vary 
  from 
  trian- 
  

   gular 
  to 
  hexagonal. 
  Most 
  of 
  the 
  interam- 
  

   bulacra]. 
  plates 
  bear 
  small 
  granules, 
  which 
  

   support 
  short, 
  sharp, 
  movable 
  spines. 
  

   Small 
  miliary 
  granules 
  also 
  occur. 
  

  

  M 
  ad 
  rep 
  o 
  rite 
  large, 
  circular 
  or 
  polygonal; 
  granular 
  in 
  appear 
  -- 
  

   ance, 
  being 
  perhaps 
  perforated 
  by 
  many 
  fine 
  pores. 
  It 
  is 
  situated 
  

   in 
  the 
  posterior 
  interambulacrum, 
  a 
  short 
  

   distance 
  from 
  the 
  meeting-point 
  of 
  the 
  five 
  

   ambulacra. 
  

  

  Apical 
  plates 
  absent. 
  

   Jaws 
  large 
  and 
  powerful. 
  The 
  masti- 
  

   catory 
  apparatus 
  is 
  described 
  by 
  Thomson 
  

  

  as 
  ' 
  a 
  five-sided 
  pyramid 
  composed 
  

  

  of 
  pairs 
  of 
  strong, 
  hollow, 
  wedge-shaped 
  

   jaws.' 
  The 
  apparatus 
  consists 
  of 
  five 
  

   pyramids, 
  which 
  in 
  general 
  character 
  re- 
  

   semble 
  those 
  of 
  the 
  Cidaridse, 
  although 
  

   there 
  does 
  not 
  appear 
  to 
  have 
  been 
  either 
  

   rotula 
  or 
  brace. 
  The 
  symphysis 
  between 
  

   the 
  two 
  half-pyramids 
  is 
  long, 
  and 
  the 
  

   epiphyses 
  are 
  short. 
  Seen 
  from 
  the 
  back 
  

   (fig. 
  2), 
  a 
  deep 
  triangular 
  depression 
  ap- 
  

   pears 
  to 
  occupy 
  most 
  of 
  the 
  surface 
  of 
  

   each 
  half. 
  At 
  the 
  free 
  angle 
  of 
  each 
  

   half-pyramid 
  there 
  are 
  three 
  depressions, 
  

   separated 
  by 
  branches 
  from 
  the 
  marginal 
  

   ridge 
  ; 
  these 
  depressions 
  and 
  ridges 
  were 
  

   probably 
  used 
  for 
  muscular 
  attachments. 
  

   The 
  distal 
  apex 
  of 
  the 
  pyramid 
  is 
  very 
  sharp 
  

   (fig. 
  3) 
  : 
  according 
  to 
  Thomson 
  there 
  were 
  

   strong 
  spines 
  situated 
  at 
  this 
  point, 
  a 
  state- 
  

   ment 
  which 
  I 
  have 
  been 
  unable 
  to 
  verify. 
  

  

  Distribution* 
  — 
  Leintwardine 
  Flags, 
  Lower 
  Ludlow, 
  near 
  Leiut-- 
  

   wardine, 
  Shropshire. 
  There 
  are 
  two 
  species 
  — 
  E. 
  uva 
  and 
  E. 
  pomum, 
  

   but 
  the 
  former 
  may 
  be 
  only 
  a 
  young 
  stage 
  of 
  the 
  latter. 
  

  

  Fig. 
  2. 
  — 
  Pyramid 
  from 
  

   masticatory 
  apparatus 
  

   of 
  Echinocystis, 
  seen 
  

   from 
  behind. 
  

  

  Fig. 
  3. 
  — 
  Distal 
  end 
  of 
  

   pyramid 
  of 
  Echino- 
  

   cystis 
  from 
  the 
  side. 
  

  

  