﻿12 
  PROCEEDINGS 
  OF 
  THE 
  NATIONAL 
  MUSEUM 
  vol.67 
  

  

  based 
  upon 
  these 
  characters, 
  which 
  he 
  called 
  bijugici?TUS, 
  with 
  the 
  

   hourglass-shaped, 
  and 
  alternicirrus, 
  with 
  the 
  wedge-shaped, 
  ossicles. 
  

   He 
  did 
  not 
  think 
  it 
  advisable 
  to 
  separate 
  them 
  as 
  species, 
  because 
  he 
  

   found 
  the 
  structures 
  somewhat 
  intermingled 
  in 
  his 
  principal 
  type 
  

   specimen, 
  and 
  also 
  in 
  view 
  of 
  the 
  practical 
  difficulty 
  of 
  distinguishing 
  

   them 
  owing 
  to 
  the 
  fact 
  that 
  the 
  stem 
  in 
  both 
  varieties, 
  as 
  seen 
  from 
  

   the 
  outer 
  curve, 
  and 
  also 
  from 
  the 
  side, 
  presents 
  the 
  same 
  appearance, 
  

   it 
  being 
  usually 
  only 
  on 
  the 
  inner 
  curve 
  that 
  any 
  difference 
  is 
  appar- 
  

   ent. 
  I 
  have 
  found 
  the 
  same 
  difficulty, 
  and 
  among 
  the 
  numerous 
  

   specimens 
  now 
  in 
  hand 
  there 
  are 
  several, 
  otherwise 
  well 
  preserved, 
  

   which 
  I 
  should 
  be 
  unable 
  to 
  identify 
  upon 
  this 
  character 
  alone. 
  The 
  

   intermingling 
  of 
  the 
  two 
  varieties 
  is 
  well 
  shown 
  on 
  plates 
  2 
  and 
  3, 
  

   and 
  in 
  one 
  fragment 
  from 
  St. 
  Paul 
  (pi. 
  3, 
  fig. 
  12) 
  both 
  are 
  seen 
  to 
  

   exist 
  in 
  the 
  same 
  specimen. 
  

  

  Out 
  of 
  22 
  specimens 
  of 
  this 
  species 
  in 
  which 
  the 
  form 
  of 
  the 
  col- 
  

   umnals 
  can 
  be 
  readily 
  observed, 
  13 
  are 
  of 
  the 
  variety 
  bijugicirrus 
  

   and 
  9 
  of 
  variety 
  alternicirrus. 
  And 
  among 
  21 
  specimens 
  of 
  the 
  

   species 
  with 
  the 
  loose 
  coil, 
  M. 
  extensus, 
  7 
  are 
  of 
  variety 
  hijugicirrus, 
  

   and 
  14 
  of 
  variety 
  alternicirrus. 
  Thus 
  while 
  the 
  two 
  characters 
  are 
  

   so 
  intermingled 
  as 
  to 
  preclude 
  the 
  basing 
  of 
  species 
  upon 
  them, 
  yet 
  

   on 
  the 
  strength 
  of 
  preponderance 
  in 
  numbers 
  the 
  evidence 
  of 
  these 
  

   varieties 
  may 
  be 
  considered 
  as 
  confirmatory 
  of 
  the 
  separation 
  of 
  the 
  

   two 
  species 
  which 
  we 
  have 
  made 
  upon 
  other 
  grounds. 
  

  

  I 
  have 
  referred 
  the 
  close 
  coiled 
  form 
  to 
  M. 
  (H.) 
  a??imonis. 
  on 
  the 
  

   strength 
  not 
  only 
  of 
  Bather's 
  statement 
  in 
  the 
  specific 
  diagnosis, 
  but 
  

   of 
  the 
  measurements 
  which 
  he 
  gives 
  on 
  page 
  50, 
  showing 
  the 
  diminu- 
  

   tion 
  in 
  width 
  of 
  the 
  stem 
  in 
  a 
  distal 
  direction, 
  and 
  of 
  his 
  figure 
  56 
  

   on 
  plate 
  2, 
  which 
  bears 
  a 
  striking 
  resemblance 
  to 
  many 
  of 
  my 
  speci- 
  

   mens. 
  For 
  the 
  form 
  with 
  the 
  loose, 
  extended 
  coil 
  I 
  am 
  proposing 
  

   the 
  new 
  species, 
  M. 
  extensus. 
  From 
  the 
  evidence 
  of 
  specimens 
  which 
  

   I 
  have 
  from 
  Dudley, 
  I 
  judge 
  that 
  the 
  two 
  types 
  with 
  the 
  close 
  and 
  

   open 
  coil 
  exist 
  among 
  the 
  English 
  forms 
  also. 
  

  

  The 
  recognition 
  of 
  this 
  Swedish 
  and 
  English 
  species 
  in 
  the 
  Amer- 
  

   ican 
  rocks 
  of 
  equivalent 
  age 
  adds 
  another 
  fact 
  to 
  the 
  evidence 
  of 
  the 
  

   close 
  relationship 
  between 
  the 
  Silurian 
  faunas 
  of 
  the 
  two 
  regions. 
  

  

  As 
  a 
  rule 
  the 
  specimens 
  are 
  tightly 
  inrolled, 
  so 
  that 
  the 
  cirri 
  are 
  

   usually 
  better 
  preserved 
  than 
  in 
  the 
  open 
  coiled 
  form. 
  They 
  have 
  

   a 
  very 
  compact, 
  robust, 
  and 
  well-rounded 
  appearance. 
  The 
  stem 
  

   swells 
  from 
  the 
  proximal 
  region 
  to 
  a 
  considerable 
  width 
  (often 
  wider 
  

   than 
  in 
  open 
  coiled 
  specimens 
  of 
  much 
  greater 
  length) 
  at 
  about 
  mid- 
  

   way 
  of 
  the 
  exposed 
  part, 
  and 
  from 
  there 
  diminishes 
  to 
  a 
  narrow 
  point 
  

   at 
  the 
  distal 
  end, 
  which 
  is 
  just 
  beyond 
  the 
  last 
  contact 
  with 
  the 
  pre- 
  

   ceeding 
  coil 
  (pi. 
  2, 
  fig. 
  3a). 
  Usually 
  this 
  narrow 
  terminal 
  is 
  broken 
  

   off, 
  but 
  it 
  is 
  present 
  in 
  several 
  specimens 
  (pi. 
  2, 
  various 
  figures) 
  ; 
  

  

  