﻿art 
  9 
  UNUSUAL 
  FORMS 
  OF 
  FOSSIL 
  CKINOIDS 
  SPRINGER 
  11 
  

  

  closely 
  apposed; 
  either 
  regularly 
  paired 
  on 
  alternate 
  columnals, 
  

   or 
  regularly 
  alternating, 
  one 
  from 
  the 
  broad 
  end 
  of 
  successive 
  

   columnals. 
  Crown 
  usually 
  concealed 
  by 
  the 
  coil; 
  its 
  detailed 
  struc- 
  

   ture 
  unknown. 
  

  

  Among 
  the 
  collections 
  made 
  for 
  me 
  by 
  Professor 
  Pate 
  in 
  the 
  

   Brownsport 
  formation 
  of 
  Decatur 
  county, 
  Tennessee, 
  are 
  upwards 
  of 
  

   rifty 
  specimens 
  belonging 
  to 
  this 
  genus. 
  With 
  a 
  solitary 
  exception 
  

   they 
  all 
  have 
  alternating 
  columnals 
  according 
  to 
  one 
  of 
  two 
  plans: 
  

   either 
  1, 
  long, 
  hourglass-shaped 
  ossicles 
  as 
  seen 
  from 
  the 
  inner 
  side, 
  

   with 
  a 
  cirrus 
  at 
  each 
  end 
  and 
  a 
  shorter, 
  lenticular, 
  non-cirriferous 
  

   ossicle 
  interposed; 
  or 
  2, 
  uniform 
  wedge-shaped 
  ossicles, 
  with 
  a 
  

   cirrus 
  springing 
  only 
  from 
  the 
  broad 
  end 
  of 
  each. 
  The 
  cirri 
  are 
  in 
  

   close 
  contact, 
  short, 
  tapering 
  rapidly, 
  when 
  intact 
  meeting 
  at 
  the 
  

   center 
  of 
  the 
  coil, 
  or 
  slightly 
  overlapping 
  at 
  the 
  smaller 
  ends 
  — 
  thus 
  

   filling 
  the 
  entire 
  visible 
  space 
  with 
  a 
  conspicious 
  convergent 
  struc- 
  

   ture. 
  Seen 
  from 
  either 
  side, 
  there 
  is 
  one 
  cirrus 
  for 
  every 
  two 
  

   columnals. 
  The 
  difference 
  between 
  this 
  form 
  and 
  M. 
  convolutus 
  is 
  

   readily 
  apparent. 
  The 
  latter 
  has 
  twice 
  as 
  many 
  cirri, 
  which 
  are 
  

   relatively 
  only 
  half 
  as 
  wide, 
  as 
  the 
  former. 
  In 
  the 
  exception 
  above 
  

   mentioned 
  the 
  specimen 
  differs 
  so 
  decisively 
  in 
  structure 
  that 
  I 
  have 
  

   separated 
  it 
  as 
  M. 
  brevis. 
  

  

  These 
  specimens 
  fall 
  into 
  two 
  categories: 
  1, 
  with 
  a 
  short 
  stem, 
  

   closely 
  coiled, 
  the 
  distal 
  end 
  tapering 
  while 
  still 
  in 
  contact 
  with 
  the 
  

   coil, 
  thus 
  indicating 
  that 
  it 
  did 
  not 
  extend 
  much 
  farther, 
  and 
  in 
  

   fact, 
  when 
  not 
  broken 
  off, 
  narrowing 
  to 
  a 
  point 
  ; 
  2, 
  with 
  stem 
  much 
  

   elongated, 
  extending 
  by 
  broad 
  curves 
  beyond 
  the 
  small 
  proximal 
  coil, 
  

   without 
  noticeable 
  dimunition 
  in 
  width 
  to 
  near 
  the 
  distal 
  end 
  — 
  - 
  

   sometimes 
  becoming 
  almost 
  straight. 
  This 
  difference 
  is 
  not 
  due 
  to 
  

   age, 
  for 
  both 
  large 
  and 
  small 
  individuals 
  have 
  an 
  open 
  coil, 
  while 
  

   those 
  that 
  are 
  closely 
  coiled 
  are 
  sometimes 
  quite 
  robust, 
  although 
  

   generally 
  smaller 
  than 
  the 
  former. 
  

  

  These 
  forms 
  constitute 
  two 
  well 
  marked 
  subdivisions 
  of 
  the 
  type 
  

   under 
  consideration, 
  each 
  numerously 
  represented 
  among 
  the 
  mate- 
  

   rial 
  from 
  the 
  Decatur 
  County 
  area. 
  In 
  a 
  few 
  specimens 
  where 
  the 
  

   stem 
  is 
  broken 
  off 
  close 
  to 
  the 
  proximal 
  region 
  of 
  the 
  coil 
  the 
  identi- 
  

   fication 
  is 
  uncertain, 
  but 
  in 
  most 
  cases 
  one 
  can 
  determine 
  from 
  the 
  

   condition 
  at 
  the 
  point 
  of 
  fracture 
  whether 
  the 
  stem 
  is 
  beginning 
  to 
  

   taper 
  distalward 
  or 
  not; 
  and 
  the 
  two 
  structures 
  impart 
  a 
  certain 
  

   superficial 
  aspect 
  by 
  which, 
  when 
  once 
  understood, 
  the 
  forms 
  are 
  

   readily 
  recognized. 
  In 
  specimens 
  which 
  are 
  nearly 
  complete 
  the 
  

   difference 
  is 
  apparent 
  at 
  a 
  glance. 
  

  

  Each 
  of 
  these 
  divisions 
  includes 
  specimens 
  with 
  both 
  types 
  of 
  

   columnals 
  as 
  above 
  described, 
  which 
  I 
  have 
  been 
  unable 
  to 
  correlate 
  

   with 
  any 
  other 
  character 
  for 
  a 
  further 
  and 
  desirable 
  subdivision. 
  

   Bather, 
  when 
  describing 
  his 
  //. 
  cmimonis, 
  recognized 
  two 
  varieties 
  

  

  