﻿496 
  Mr. 
  J. 
  W. 
  Gregory 
  on 
  Zeuglopleurus. 
  

  

  Ambulacra 
  narrow, 
  straight, 
  slightly 
  raised. 
  Each 
  plate 
  

   bears 
  a 
  single 
  prominent 
  tubercle, 
  from 
  the 
  boss 
  of 
  which 
  

   radiates 
  a 
  series 
  of 
  costulate 
  ridges, 
  usually 
  seven 
  in 
  number, 
  

   some 
  of 
  which 
  unite 
  with 
  the 
  ridges 
  of 
  the 
  adjoining 
  plates. 
  

   Between 
  the 
  ridges 
  are 
  series 
  of 
  miliary 
  granules 
  and 
  some- 
  

   times 
  a 
  very 
  small 
  secondary 
  tubercle. 
  

  

  The 
  pairs 
  of 
  pores 
  are 
  in 
  primaries 
  for 
  the 
  first 
  few 
  youngest 
  

   plates 
  ; 
  these 
  are 
  succeeded 
  actinally 
  by 
  compound 
  plates 
  

   formed 
  of 
  two 
  primaries 
  and 
  are 
  usually 
  separated 
  by 
  simple 
  

   primaries; 
  compound 
  plates 
  of 
  triple 
  primaries 
  occur 
  more 
  

   rarely. 
  

  

  Interambulacra 
  of 
  vertical 
  series 
  of 
  about 
  ten 
  or 
  eleven 
  

   plates, 
  ornamented 
  by 
  tubercles, 
  miliary 
  granules, 
  and 
  costu- 
  

   late 
  ridges. 
  The 
  primary 
  tubercles 
  form 
  two 
  rows, 
  one 
  on 
  

   each 
  side 
  of 
  each 
  interradius, 
  and 
  resemble 
  those 
  of 
  the 
  ambu- 
  

   lacra, 
  which 
  they 
  slightly 
  exceed 
  in 
  size. 
  Near 
  the 
  ambitus 
  

   each 
  plate 
  bears 
  a 
  secondary 
  tubercle. 
  The 
  bosses 
  of 
  the 
  

   primary 
  tubercles 
  are 
  connected 
  by 
  costulate 
  ridges, 
  which 
  

   intersect 
  the 
  scrobicular 
  areas 
  ; 
  the 
  vertical 
  costse 
  are 
  the 
  

   most 
  prominent 
  and 
  form 
  a 
  vertical 
  ridge, 
  the 
  two 
  aboral 
  

   costas 
  of 
  one 
  plate 
  uniting 
  with 
  the 
  adoral 
  single 
  costa 
  of 
  the 
  

   plate 
  above. 
  The 
  miliary 
  granulation 
  is 
  prominent 
  and 
  

   crowded. 
  

  

  The 
  horizontal 
  sutures 
  are 
  furrowed, 
  the 
  furrow 
  most 
  

   affecting 
  the 
  adoral 
  side 
  of 
  the 
  two 
  plates, 
  so 
  that 
  the 
  aboral 
  

   projects 
  above 
  the 
  former. 
  

  

  Peristome 
  small, 
  circular 
  or 
  very 
  slightly 
  oval, 
  with 
  small 
  

   branchial 
  slits 
  ; 
  the 
  margin 
  of 
  the 
  ambulacra 
  occupies 
  less 
  of 
  

   the 
  circle 
  than 
  that 
  of 
  the 
  interambulacra. 
  

  

  Dimensions. 
  

  

  millim. 
  

  

  Diameter, 
  anteroposterior 
  11 
  

  

  „ 
  transverse 
  105 
  

  

  Height 
  6 
  

  

  Diameter 
  of 
  apical 
  disk 
  4 
  

  

  „ 
  „ 
  anus 
  2 
  

  

  Ratio 
  of 
  ambulacra 
  to 
  interambulacra 
  5 
  : 
  8. 
  

  

  Distribution. 
  Mid 
  Chalk 
  of 
  Charlton, 
  in 
  Kent, 
  and 
  Chalk- 
  

   Marl 
  of 
  Glynde, 
  in 
  Sussex. 
  (British 
  Museum.) 
  

  

  2. 
  Zeuglopleurus 
  pusillus 
  (Roem.). 
  

  

  1S40. 
  Echinopsis 
  pusilltf, 
  Rcem., 
  Versteinerungen 
  des 
  norddeutscben 
  

   Kreidegebirge, 
  p. 
  30, 
  pi. 
  vi. 
  fig. 
  10. 
  

  

  