﻿INVERTEBRATES. 
  351 
  

  

  divided 
  longitudinally 
  in 
  the 
  same 
  direction 
  as 
  the 
  basals. 
  The 
  inner 
  

   leaf 
  is 
  at 
  its 
  border 
  as 
  distinctly 
  marked 
  as 
  the 
  outer 
  one 
  ; 
  it 
  extends 
  

   not 
  far 
  beyond 
  the 
  column, 
  but 
  is 
  considerably 
  wider, 
  and 
  very 
  much 
  

   higher 
  than 
  the 
  stem 
  joints, 
  which 
  are 
  so 
  extremely 
  short 
  that 
  there 
  

   are 
  eight 
  joints 
  to 
  a 
  length 
  of 
  half 
  a 
  line. 
  The 
  structure 
  is 
  such 
  

   that 
  there 
  can 
  be 
  but 
  little 
  doubt 
  that 
  the 
  inner 
  leaf, 
  although 
  dif- 
  

   fering 
  considerably 
  in 
  its 
  dimensions 
  from 
  the 
  succeeding 
  stem- 
  

   joints, 
  is 
  actually 
  nothing 
  else 
  but 
  the 
  proximal 
  joint, 
  which 
  had 
  

   become 
  enlarged 
  and 
  gradually 
  anchylosed 
  with 
  the 
  basals. 
  It 
  is 
  

   quite 
  evident 
  that 
  the 
  outer 
  clover-leaf 
  had 
  a 
  similar 
  origin. 
  At 
  

   the 
  outer 
  leaf 
  the 
  sides 
  of 
  the 
  anchylosed 
  joint 
  were 
  more 
  extended, 
  

   and 
  became 
  almost 
  entirely 
  absorbed 
  into 
  the 
  basal 
  plates, 
  leaving 
  

   only 
  external 
  marks, 
  while 
  at 
  the 
  inner 
  ones 
  the 
  column 
  structure 
  

   can 
  yet 
  be 
  recognized, 
  although 
  the 
  joint 
  has 
  here 
  actually 
  become 
  

   a 
  part 
  of 
  the 
  basal 
  plates. 
  This 
  is 
  the 
  only 
  specimen 
  in 
  which 
  I 
  

   have 
  observed 
  a 
  second 
  series 
  of 
  plate-like 
  marks, 
  and 
  I 
  do 
  not 
  know 
  

   whether 
  this 
  instance 
  represents 
  an 
  abnormal 
  case 
  or 
  is 
  owing 
  to 
  

   the 
  extreme 
  size 
  of 
  the 
  specimen; 
  at 
  any 
  rate 
  it 
  gives 
  an 
  explana- 
  

   tion 
  how 
  similar 
  marks 
  were 
  produced. 
  

  

  In 
  corroboration 
  of 
  these 
  views 
  I 
  will 
  further 
  direct 
  attention 
  to 
  

   Pentremites 
  abbreviatus, 
  Hambach, 
  a 
  very 
  depressed 
  species, 
  in 
  

   which 
  the 
  basals, 
  even 
  in 
  young 
  specimens, 
  have 
  an 
  almost 
  

   horizontal 
  position, 
  and 
  in 
  which 
  as 
  might 
  be 
  expected, 
  there 
  is 
  

   no 
  conical 
  projection 
  at 
  the 
  end 
  of 
  the 
  basals. 
  In 
  this 
  speci- 
  

   men, 
  the 
  leaf-like 
  marks 
  are 
  very 
  large 
  and 
  conspicuous, 
  they 
  

   form 
  a 
  somewhat 
  triangular, 
  clover-leaf 
  shaped 
  disc, 
  with 
  rounded 
  

   extremities 
  directed 
  to 
  the 
  basal 
  sutures. 
  The 
  sutures 
  are 
  always 
  

   slightly 
  depressed, 
  and 
  the 
  parts 
  covering 
  these 
  depressions, 
  

   stand 
  out 
  so 
  prominent, 
  that 
  it 
  appears 
  almost 
  as 
  if 
  in 
  this 
  

   case, 
  exceptionally, 
  the 
  anchylosis 
  had 
  not 
  been 
  completed, 
  and 
  

   in 
  fact 
  I 
  found 
  two 
  specimens 
  in 
  which 
  the 
  upper 
  structure 
  appa- 
  

   rently 
  had 
  partly 
  fallen 
  out, 
  leaving 
  in 
  its 
  place, 
  at 
  the 
  surface 
  

   of 
  the 
  basal 
  plates, 
  a 
  clover-leaf-like 
  impression. 
  The 
  place 
  to 
  

   which 
  the 
  column 
  was 
  attached 
  is 
  generally 
  well 
  marked, 
  and 
  in 
  

   one 
  specimen 
  I 
  found 
  remains 
  of 
  the 
  proximal 
  segment 
  which 
  is 
  

   exceedingly 
  thin 
  and 
  delicate, 
  and 
  like 
  the 
  anchylosed 
  joint 
  trian- 
  

   gular 
  in 
  form, 
  but 
  the 
  points 
  of 
  the 
  angles 
  directed 
  toward 
  the 
  

   sides 
  of 
  the 
  upper 
  one. 
  

  

  I 
  think 
  this 
  fully 
  sufficient 
  to 
  prove 
  that 
  the 
  so-called 
  supple- 
  

   mentary 
  basals 
  in 
  Pentremites 
  consist 
  of 
  a 
  columnar 
  joint, 
  anchyl- 
  

   osed 
  with 
  the 
  basals 
  and 
  more 
  or 
  less 
  completely 
  absorbed 
  into 
  the 
  

  

  