﻿90 
  Mr. 
  W. 
  K. 
  Fisher 
  on 
  t 
  

  

  wo 
  

  

  are 
  intermediate 
  in 
  many 
  respects 
  between 
  Ludwig's 
  two 
  

   families. 
  They 
  resemble 
  Luidiaster 
  (Oheirasteridse) 
  in 
  the 
  

   structure 
  of 
  the 
  abactinal 
  spinopaxillaj, 
  heavily 
  armed 
  mar- 
  

   ginals, 
  and 
  adambulacral 
  plates, 
  but 
  possess 
  also 
  the 
  

   diagnostic 
  feature 
  of 
  the 
  other 
  family 
  — 
  the 
  odd 
  interradial 
  

   marginal 
  plates. 
  For 
  this 
  reason 
  I 
  am 
  inclined 
  to 
  maintain 
  

   the 
  family 
  Benthopectinidse 
  in 
  the 
  extended 
  sense, 
  and 
  

   without 
  subfamilies. 
  

  

  Ludwig 
  recognizes 
  two 
  genera 
  with 
  odd 
  interradial 
  mar- 
  

   ginal 
  plates 
  : 
  Pararchaster, 
  having 
  the 
  abactinal 
  plates 
  with 
  

   several 
  spinelets, 
  and 
  Benthopecten, 
  with 
  only 
  one 
  spine 
  or 
  

   spinelet 
  to 
  a 
  plate, 
  rarely 
  two 
  or 
  three. 
  No 
  other 
  characters 
  

   are 
  designated. 
  The 
  two 
  groups 
  are 
  extremely 
  close, 
  and 
  

   often 
  difficulty 
  is 
  encountered 
  in 
  deciding 
  to 
  which 
  genus 
  a 
  

   given 
  form 
  belongs. 
  This 
  is 
  true 
  of 
  the 
  variable 
  B. 
  acantho- 
  

   notus, 
  Fisher, 
  which 
  is 
  Benthopecten 
  on 
  the 
  rays, 
  and 
  

   Pararchaster 
  over 
  most 
  of 
  the 
  disk. 
  My 
  B. 
  claviger 
  would 
  

   fall 
  in 
  Pararchaster 
  and 
  B. 
  mutabilis 
  probably 
  in 
  Bentho- 
  

   pecten, 
  although 
  many 
  plates 
  have 
  several 
  spinelets 
  (1 
  to 
  7). 
  

   Unfortunately 
  the 
  type 
  of 
  Pararchaster 
  is 
  not 
  so 
  well 
  

   differentiated 
  from 
  Benthopecten 
  as 
  some 
  other 
  species 
  of 
  the 
  

   group 
  — 
  e. 
  g., 
  P. 
  spinosissimus. 
  I 
  have 
  used 
  the 
  name 
  

   Benthopecten 
  in 
  the 
  subjoined 
  synopsis 
  in 
  the 
  extended 
  sense, 
  

   equivalent 
  to 
  Sladen's 
  (not 
  Lud 
  wig's) 
  Pararchester. 
  This 
  

   key 
  is 
  intended 
  to 
  complement 
  the 
  second 
  portion 
  of 
  Ludwig's 
  

   synopsis 
  {" 
  Notomyota," 
  p. 
  442). 
  

  

  An 
  unpaired 
  (odd) 
  marginal 
  plate 
  present 
  in 
  

  

  some 
  or 
  all 
  of 
  the 
  interradii. 
  Dorsal 
  

  

  muscle-bands 
  not 
  attached 
  by 
  a 
  tendon 
  to 
  

  

  a 
  proximal 
  ambulacral 
  ossicle. 
  

   a. 
  Abactinal 
  plates 
  of 
  papular 
  areas 
  low, 
  tabulate, 
  

  

  and 
  strongly 
  stellate, 
  the 
  larger 
  bearing 
  a 
  

  

  conspicuous 
  central 
  spine 
  surrounded 
  by 
  a 
  

  

  circle 
  of 
  more 
  or 
  less 
  elongate 
  spinelets 
  

  

  (fig. 
  5, 
  p. 
  92) 
  ; 
  superomarginals 
  with 
  2 
  or 
  3, 
  

  

  inferomarginals 
  with 
  2 
  to 
  5 
  long 
  bristling 
  

  

  spines 
  (fig. 
  3) 
  ; 
  odd 
  interradial 
  marginal 
  not 
  

  

  especially 
  prominent 
  ; 
  usually 
  absent 
  in 
  

  

  some 
  of 
  the 
  interradii, 
  never 
  in 
  all, 
  and 
  

  

  sometimes 
  present 
  in 
  only 
  one. 
  

   b. 
  Papulae 
  extending 
  one-third 
  to 
  three-fifths 
  

   length 
  of 
  ray 
  ; 
  mouth-plates 
  with 
  nume- 
  

   rous 
  (9 
  to 
  16) 
  marginal 
  spines 
  disposed 
  

   in 
  three 
  independent 
  series 
  on 
  each 
  pair 
  

   of 
  mouth-plates; 
  the 
  median 
  teeth 
  the 
  

   longest, 
  and 
  less 
  numerous 
  than 
  either 
  

   furrow 
  series 
  (fig. 
  1) 
  ; 
  dorsal 
  muscle- 
  

   bands 
  not 
  very 
  strong 
  or 
  prominent, 
  

   often 
  weak 
  Nearchaster, 
  gen. 
  nov. 
  

  

  