﻿152 
  MR. 
  P. 
  A. 
  BATHER 
  ON 
  TRIGONOCRESTUS 
  FROM 
  

  

  one 
  completely 
  preserved 
  (P1.YI. 
  fig. 
  1): 
  the 
  two 
  together 
  form 
  

   one 
  spine, 
  to 
  which, 
  however, 
  the 
  process 
  bow 
  under 
  consideration 
  

   contributes 
  but 
  a 
  small 
  portion. 
  In 
  the 
  case 
  of 
  the 
  right-hand 
  

   larger 
  radial, 
  the 
  process 
  is 
  connected 
  with 
  the 
  process 
  of 
  the 
  left- 
  

   hand 
  larger 
  radial 
  mentioned 
  above 
  as 
  being 
  broken 
  off 
  halfway 
  

   (PI. 
  VI. 
  fig. 
  1) 
  : 
  in 
  this 
  instance 
  the 
  two 
  processes 
  appear 
  to 
  have 
  

   been 
  of 
  almost 
  the 
  same 
  size 
  ; 
  this 
  is 
  consistent 
  with 
  the 
  greater 
  

   size 
  in 
  other 
  respects 
  of 
  the 
  right-hand 
  larger 
  radial. 
  In 
  the 
  case 
  

   of 
  the 
  left-hand 
  smaller 
  radial 
  the 
  spine 
  of 
  which 
  it 
  formed 
  part 
  is 
  

   broken 
  right 
  off 
  (PI. 
  VI. 
  fig. 
  2) 
  ; 
  but, 
  to 
  judge 
  from 
  the 
  suture, 
  

   this 
  process 
  must 
  have 
  formed 
  nearly 
  half 
  the 
  spine, 
  in 
  thickness 
  if 
  

   not 
  in 
  length. 
  In 
  the 
  case 
  of 
  the 
  right 
  of 
  the 
  two 
  smaller 
  radials 
  

   this 
  process 
  is 
  minute 
  (PI. 
  VI. 
  fig. 
  2), 
  and 
  it 
  combines 
  with 
  the 
  

   equally 
  minute 
  process 
  of 
  its 
  neighbour 
  to 
  form 
  a 
  tiny 
  ridge, 
  which 
  

   runs 
  at 
  right 
  angles 
  to 
  the 
  circumference 
  up 
  to 
  the 
  rim 
  round 
  the 
  

   calycal 
  cavity 
  now 
  to 
  be 
  described. 
  

  

  Between 
  their 
  respective 
  processes 
  the 
  radials 
  gently 
  bend 
  in- 
  

   wards 
  and 
  upwards 
  towards 
  the 
  Calycal 
  Cavity. 
  The 
  ventral 
  aper- 
  

   ture 
  of 
  this 
  cavity 
  is 
  circular 
  (PI. 
  VI. 
  fig. 
  3) 
  ; 
  it 
  is 
  rather 
  less 
  

   than 
  1 
  mm. 
  in 
  diameter, 
  and 
  therefore 
  smaller 
  than 
  the 
  basal 
  aper- 
  

   ture. 
  It 
  is 
  bounded 
  by 
  a 
  slightly 
  elevated 
  rim, 
  which 
  connects 
  the 
  

   left 
  process 
  with 
  the 
  right 
  process 
  of 
  each 
  radial. 
  There 
  are 
  no 
  

   remains 
  of 
  arms, 
  and 
  I 
  can 
  distinguish 
  no 
  regular 
  markings 
  on 
  the 
  

   inbent 
  surfaces 
  of 
  the 
  radials 
  between 
  the 
  spines, 
  and 
  no 
  openings 
  

   for 
  canals 
  (cf. 
  PI. 
  VI. 
  fig. 
  5). 
  

  

  Specimen 
  B. 
  — 
  This 
  resembles 
  A 
  in 
  every 
  essential 
  character 
  ; 
  I 
  

   shall 
  therefore 
  merely 
  indicate 
  the 
  points 
  of 
  difference. 
  The 
  Basals 
  

   have 
  been 
  destroyed, 
  but 
  traces 
  are 
  left 
  of 
  that 
  which 
  in 
  A 
  was 
  de- 
  

   scribed 
  as 
  the 
  largest 
  basal. 
  The 
  basals 
  appear 
  to 
  have 
  passed 
  up 
  

   for 
  a 
  short 
  distance 
  on 
  the 
  inner 
  side 
  of 
  the 
  radials, 
  so 
  that 
  the 
  

   Radials 
  rested 
  in 
  a 
  groove 
  on 
  the 
  upper 
  surface 
  of 
  the 
  basals. 
  The 
  

   union 
  between 
  the 
  basal 
  ring 
  and 
  the 
  radials, 
  in 
  harmony 
  with 
  the 
  

   tendency 
  to 
  anchylosis 
  shown 
  throughout 
  the 
  calyx, 
  was 
  apparently 
  

   close 
  ; 
  hence 
  this 
  specimen 
  shows 
  no 
  clear 
  articular 
  or 
  sutural 
  sur- 
  

   face 
  at 
  the 
  aboral 
  end 
  of 
  the 
  radials. 
  The 
  Interradial 
  Sutures 
  are 
  

   not 
  so 
  clear 
  as 
  in 
  A, 
  but 
  the 
  grooves 
  show 
  their 
  former 
  position 
  : 
  

   the 
  groove 
  between 
  the 
  two 
  smaller 
  radials 
  is, 
  however, 
  not 
  nearly 
  

   so 
  evident 
  as 
  in 
  A. 
  The 
  left 
  of 
  the 
  two 
  smaller 
  radials 
  has, 
  at 
  

   three 
  quarters 
  of 
  its 
  height, 
  on 
  the 
  left 
  side, 
  a 
  relatively 
  large 
  pro- 
  

   tuberance 
  ; 
  this 
  has 
  deflected 
  the 
  process 
  in 
  the 
  left 
  upper 
  corner 
  

   towards 
  the 
  left, 
  and 
  it 
  therefore 
  constitutes 
  a 
  larger 
  portion 
  of 
  the 
  

   spine 
  at 
  this 
  angle 
  than 
  is 
  usually 
  the 
  case 
  : 
  the 
  right 
  upper 
  process 
  

   of 
  the 
  next 
  radial 
  on 
  the 
  left 
  is 
  correspondingly 
  atrophied. 
  The 
  

   Ornament 
  of 
  the 
  radials 
  is 
  well 
  preserved, 
  and 
  its 
  essentially 
  gra- 
  

   nular 
  nature 
  is 
  more 
  obvious 
  than 
  in 
  A 
  (PI. 
  VI. 
  fig. 
  6). 
  The 
  

   three 
  larger 
  Spines 
  are 
  fairly 
  well 
  preserved, 
  but 
  the 
  end 
  of 
  each 
  is 
  

   broken 
  (PI. 
  VI. 
  fig. 
  5) 
  ; 
  the 
  small 
  spine 
  is 
  a 
  fine, 
  well-marked 
  

   ridge. 
  The 
  region 
  of 
  each 
  radial 
  lying 
  between 
  its 
  processes 
  is 
  not 
  

   bent 
  inwards 
  at 
  quite 
  so 
  sharp 
  an 
  angle 
  as 
  in 
  A 
  ; 
  here, 
  too, 
  I 
  can 
  

  

  