﻿INVEBTEBEATES. 
  335 
  

  

  ORDER 
  AGELACRINOIDEA. 
  

  

  FAMILY 
  AGELACRIN1D./E. 
  

  

  Echinodiscus, 
  n. 
  gen. 
  

  

  (Ety, 
  echinus, 
  eea 
  urchin: 
  diskos, 
  a 
  round 
  plate.) 
  

  

  Body 
  discoid, 
  depressed 
  convex, 
  the 
  smaller 
  plates 
  being 
  found 
  in 
  

   the 
  border 
  and 
  ambulacra 
  adjacent 
  thereto, 
  and 
  the 
  larger 
  plates 
  

   occupying 
  the 
  central 
  part 
  of 
  the 
  interradial 
  spaces, 
  none 
  of 
  them 
  

   imbricating. 
  The 
  border 
  consists 
  of 
  narrow 
  plates 
  elongated 
  in 
  the 
  

   direction 
  of 
  the 
  circumference, 
  and 
  passing 
  around 
  upon 
  the 
  under 
  

   side, 
  forming 
  a 
  cylindrical 
  nonsessile 
  rim. 
  Ambulacra 
  five, 
  con- 
  

   nected 
  near 
  the 
  central 
  area, 
  but 
  not 
  arising 
  from 
  a 
  common 
  point., 
  

   consisting 
  of 
  narrow, 
  curving 
  angular 
  arches, 
  sharply 
  defined 
  on 
  

   either 
  side, 
  and 
  composed 
  of 
  numerous 
  interlocking 
  or 
  dove-tailing 
  

   plates. 
  Mouth 
  or 
  ovarian 
  pyramid 
  central 
  or 
  subcentral. 
  

  

  This 
  genus 
  is 
  distinguished 
  from 
  Agelacrinus, 
  with 
  which 
  it 
  seems 
  

   to 
  be 
  most 
  nearly 
  related, 
  by 
  many 
  important 
  characters. 
  First, 
  

   it 
  is 
  not 
  composed 
  of 
  imbricating 
  plates. 
  Second, 
  the 
  manner 
  of 
  

   attachment 
  to 
  other 
  objects 
  was 
  different, 
  as 
  in 
  this 
  genus 
  the 
  mar- 
  

   gin 
  was 
  free. 
  Third, 
  the 
  ambulacra 
  do 
  not 
  arise 
  from 
  a 
  pyramidal 
  

   elevation 
  or 
  common 
  point, 
  but, 
  one 
  may 
  be 
  said 
  to 
  arise 
  on 
  one 
  

   side 
  of 
  the 
  mouth, 
  and 
  to 
  connect, 
  by 
  an 
  ambulacral 
  series 
  of 
  plates, 
  

   with 
  the 
  points, 
  at 
  which 
  two 
  arise, 
  on 
  either 
  side 
  of 
  it, 
  or 
  that 
  two 
  

   series 
  bifurcate, 
  if 
  it 
  is 
  considered 
  that 
  three 
  series 
  originate 
  on 
  one 
  

   side 
  of 
  the 
  mouth, 
  in 
  the 
  first 
  instance. 
  Fourth, 
  the 
  mouth 
  or 
  ovarian 
  

   pyramid 
  is 
  subcentral, 
  while 
  in 
  Agelacrinus 
  it 
  is 
  submarginal. 
  This 
  

   elevation 
  would 
  seem 
  to 
  be 
  homologous 
  with 
  the 
  mouth 
  in 
  the 
  echi- 
  

   noids, 
  for 
  below 
  it, 
  within 
  the 
  visceral 
  cavity, 
  there 
  occur 
  several 
  

   pieces, 
  which 
  were 
  evidently 
  connected 
  with 
  the 
  digestive 
  functions, 
  

   and 
  therefore 
  homologous 
  with 
  the 
  jaws 
  in 
  the 
  latter 
  order. 
  

  

  