﻿136 
  COLLECTIONS 
  FBOM 
  MELANESIA. 
  

  

  4. 
  OpMopinax 
  stellatus. 
  (Plate 
  VIII. 
  fig. 
  C.) 
  

   Pectinura 
  stellata, 
  Lyman, 
  Trd. 
  List, 
  p. 
  3. 
  

  

  Disk 
  somewhat 
  pentagonal, 
  not 
  pnfFed, 
  with 
  a 
  central 
  rosette 
  of 
  

   plates 
  varying 
  a 
  good 
  deal 
  in 
  the 
  extent 
  to 
  which 
  it 
  is 
  well 
  defined, 
  

   and 
  occupying 
  about 
  half 
  the 
  upper 
  surface 
  ; 
  in 
  some 
  cases 
  a 
  central 
  

   plate 
  and 
  two 
  not 
  very 
  regular 
  circlets 
  of 
  variously 
  sized 
  plates 
  of 
  

   irregular 
  form 
  can 
  be 
  made 
  out. 
  The 
  interradial 
  series 
  of 
  plates, 
  

   which 
  extend 
  from 
  the 
  rosette 
  to 
  the 
  edge 
  of 
  the 
  disk, 
  are 
  stouter 
  

   and 
  more 
  prominent 
  than 
  the 
  radial 
  series 
  ; 
  they 
  are 
  composed 
  of 
  

   three 
  or 
  four 
  plates 
  varying 
  in 
  size 
  and 
  form, 
  and 
  sometimes 
  paired 
  

   at 
  the 
  margin 
  of 
  the 
  disk 
  ; 
  the 
  larger 
  median 
  have 
  smaller 
  plates 
  on 
  

   either 
  side. 
  The 
  radial 
  series 
  may, 
  for 
  its 
  undivided 
  portion, 
  be 
  

   formed 
  by 
  a 
  single 
  plate, 
  but, 
  as 
  a 
  more 
  general 
  rule, 
  there 
  are 
  

   three 
  ; 
  there 
  are 
  no 
  smaller 
  marginal 
  plates. 
  The 
  bifurcated 
  band 
  

   which 
  embraces 
  the 
  base 
  of 
  the 
  arm 
  has 
  usually 
  three 
  pieces 
  on 
  

   either 
  half. 
  Just 
  below 
  the 
  margin 
  of 
  the 
  disk 
  there 
  is 
  a 
  prominent 
  

   plate, 
  which 
  is 
  placed 
  interradially 
  ; 
  in 
  shape 
  this 
  is 
  irregularly 
  

   cordiform 
  ; 
  above, 
  on 
  either 
  side, 
  it 
  is 
  bounded 
  by 
  a 
  compressed 
  or 
  

   diamond-shaped 
  plate 
  ; 
  between 
  it 
  and 
  the 
  mouth-plate 
  there 
  is 
  a 
  

   pair 
  of 
  smaller 
  plates 
  and 
  several 
  still 
  smaller 
  plates 
  intervening 
  ; 
  

   on 
  the 
  side 
  there 
  is 
  an 
  elongated 
  plate, 
  with 
  two 
  smaller 
  ones, 
  lying 
  

   along 
  the 
  edge 
  of 
  the 
  genital 
  slit. 
  The 
  radial 
  shields 
  are 
  large 
  and 
  

   triangular, 
  and 
  fill 
  up 
  all 
  the 
  space 
  on 
  the 
  disk 
  between 
  the 
  radial 
  

   and 
  interradial 
  plates 
  which 
  is 
  not 
  occupied 
  by 
  the 
  rosette. 
  

  

  The 
  inner 
  mouth-shields 
  have 
  a 
  straight 
  outer 
  edge, 
  the 
  sides 
  are 
  

   at 
  first 
  straight, 
  and 
  then 
  bending 
  inwards 
  meet 
  one 
  another 
  at 
  a 
  

   more 
  or 
  less 
  acute 
  angle 
  ; 
  they 
  are 
  longer 
  than 
  broad 
  ; 
  the 
  space 
  

   between 
  them 
  and 
  the 
  edges 
  of 
  the 
  mouth 
  is 
  rather 
  loosely 
  granular 
  ; 
  

   the 
  outer 
  mouth-shields 
  are 
  about 
  as 
  broad 
  as 
  long, 
  and 
  have 
  a 
  

   slightly 
  convex 
  outer 
  edge 
  ; 
  the 
  umbo 
  is 
  well 
  marked, 
  and 
  that 
  

   shield 
  is 
  not 
  divided 
  ; 
  the 
  side 
  mouth-shields 
  are 
  well 
  developed, 
  

   and 
  are 
  separated 
  from 
  the 
  genital 
  slits 
  by 
  a 
  small 
  granulated 
  patch. 
  

   Seven 
  mouth-papillte, 
  the 
  three 
  innermost 
  small, 
  the 
  innermost 
  

   smallest 
  ; 
  the 
  next 
  three 
  large, 
  increasing 
  in 
  size 
  from 
  within 
  out- 
  

   wards 
  ; 
  the 
  outermost 
  again 
  small. 
  

  

  The 
  arms 
  begin 
  to 
  narrow 
  immediately 
  after 
  leaving 
  the 
  disk, 
  

   and 
  become 
  very 
  delicate 
  at 
  their 
  free 
  ends 
  ; 
  in 
  a 
  well-developed 
  

   specimen, 
  in 
  which 
  the 
  disk 
  measures 
  17 
  millim. 
  in 
  diameter, 
  they 
  

   are 
  70 
  millim. 
  long, 
  5 
  millim. 
  wide, 
  and 
  4-5 
  millim. 
  high 
  at 
  their 
  

   insertion 
  into 
  the 
  disk. 
  Upper 
  surface 
  carinated 
  ; 
  upper 
  arm-plates 
  

   about 
  three 
  times 
  as 
  wide 
  as 
  they 
  are 
  long, 
  but, 
  owing 
  to 
  the 
  invasion 
  

   of 
  the 
  side 
  arm-plates, 
  the 
  aboral 
  is 
  longer 
  than 
  the 
  adoral 
  edge 
  ; 
  

   both 
  these 
  edges 
  are 
  straight. 
  

  

  There 
  is 
  a 
  pair 
  of 
  pores 
  between 
  the 
  first 
  and 
  second 
  lower 
  arm- 
  

   plates 
  ; 
  the 
  lower, 
  like 
  the 
  upper, 
  arm-plates 
  have 
  their 
  aboral 
  longer 
  

   than 
  their 
  adoral 
  edges, 
  and 
  this, 
  as 
  on 
  the 
  upper 
  surface, 
  is 
  due 
  to 
  

   the 
  encroachments 
  of 
  the 
  side 
  arm-plates, 
  and 
  is 
  more 
  strongly 
  

   marked 
  near 
  to, 
  than 
  far 
  from, 
  the 
  disk. 
  None 
  of 
  the 
  edges 
  of 
  these 
  

  

  