﻿198 
  MISS 
  G. 
  L. 
  ELLES 
  ON 
  THE 
  SUBGENERA 
  [May 
  1 
  897, 
  

  

  case 
  it 
  scarcely 
  reaches 
  the 
  level 
  of 
  the 
  base 
  of 
  theca 
  3 
  2 
  . 
  The 
  

   earliest 
  thecae 
  are 
  strongly 
  curved, 
  as 
  in 
  P. 
  -palmeus, 
  var. 
  latus, 
  and 
  

   in 
  all 
  details 
  of 
  development 
  are 
  precisely 
  similar. 
  The 
  earliest 
  

   thecae 
  are 
  rather 
  shorter 
  than 
  those 
  subsequently 
  developed. 
  

  

  Character 
  of 
  Mature 
  Rhabdosoma. 
  — 
  The 
  length 
  of 
  the 
  mature 
  

   rhabdosoma 
  is 
  generally 
  between 
  J 
  and 
  1 
  inch. 
  The 
  form 
  varies 
  

   within 
  certain 
  limits 
  in 
  the 
  relative 
  amounts 
  of 
  what 
  may 
  be 
  

   termed 
  the 
  ' 
  ovate' 
  or 
  the 
  ' 
  elongate 
  ' 
  parts. 
  

  

  As 
  in 
  P. 
  palmeus, 
  var. 
  latus 
  (Barr.), 
  the 
  maximum 
  width 
  is 
  attained 
  

   almost 
  at 
  once, 
  usually 
  opposite 
  the 
  aperture 
  of 
  the 
  second 
  theca 
  of 
  

   the 
  primordial 
  series. 
  The 
  width 
  attained 
  is 
  ^ 
  inch, 
  the 
  same 
  as 
  in 
  

   var. 
  latus. 
  Thus, 
  in 
  all 
  the 
  early 
  stages, 
  it 
  is 
  very 
  hard 
  to 
  distinguish 
  

   P. 
  p. 
  var. 
  latus 
  from 
  P. 
  p. 
  var. 
  ovato-elongatus. 
  The 
  maximum 
  width 
  

   may 
  be 
  maintained 
  for 
  a 
  short 
  distance 
  if 
  the 
  ' 
  ovate 
  ' 
  part 
  is 
  long, 
  

   or 
  else 
  a 
  diminution 
  in 
  width 
  takes 
  place 
  directly. 
  

  

  The 
  decrease 
  is 
  sometimes 
  gradual, 
  sometimes 
  rather 
  abrupt. 
  

   This 
  decreased 
  width 
  is 
  then 
  maintained 
  up 
  to 
  the 
  distal 
  end. 
  

  

  The 
  thecse 
  are 
  alternate 
  ; 
  they 
  are 
  -^ 
  inch 
  long, 
  and 
  four 
  times 
  as 
  

   long 
  as 
  wide 
  in 
  the 
  ' 
  ovate 
  ' 
  part, 
  but 
  rather 
  less 
  near 
  the 
  distal 
  

   end. 
  The 
  thecse 
  are 
  widest 
  at 
  the 
  apertures, 
  and 
  frequently 
  show 
  

   growth-lines. 
  The 
  apertures 
  near 
  the 
  proximal 
  end 
  are 
  almost 
  

   parallel 
  with 
  the 
  long 
  axis 
  of 
  the 
  rhabdosoma, 
  but 
  become 
  more 
  

   oblique 
  towards 
  the 
  distal 
  extremity. 
  Near 
  the 
  proximal 
  end 
  the 
  

   thecae 
  are 
  inclined 
  at 
  45°, 
  and 
  describe 
  a 
  decided 
  curve 
  as 
  they 
  

   grow 
  outward, 
  but 
  the 
  angle 
  of 
  inclination 
  diminishes 
  to 
  20° 
  near 
  

   the 
  distal 
  end, 
  and 
  the 
  thecae 
  throughout 
  the 
  ' 
  elongate 
  ' 
  part 
  are 
  

   almost 
  straight. 
  Hence 
  the 
  form 
  is 
  attained 
  by 
  an 
  alteration 
  in 
  the 
  

   amount 
  of 
  curvature 
  and 
  inclination, 
  accompanied 
  by 
  a 
  slight 
  

   diminution 
  in 
  the 
  length 
  of 
  the 
  thecae. 
  

  

  There 
  seems 
  to 
  have 
  been 
  a 
  continuous 
  septum 
  (see 
  PI. 
  XI 
  Y. 
  

   fig. 
  14). 
  

  

  General 
  Characters. 
  —The 
  form 
  of 
  the 
  mature 
  rhabdosoma 
  is 
  

   sufficient 
  to 
  distinguish 
  it 
  from 
  all 
  other 
  varieties 
  of 
  P. 
  palmeus 
  

   (Barr.). 
  

  

  Horizon. 
  — 
  The 
  species 
  has 
  a 
  long 
  range. 
  It 
  appears 
  early 
  in 
  the 
  

   zone 
  of 
  M. 
  gregarius 
  (Lapw.), 
  and 
  survives 
  into 
  the 
  Lower 
  Gala- 
  

   Tarannon 
  beds. 
  

  

  It 
  is 
  common 
  in 
  the 
  Rastrites-8h&les 
  of 
  Tommarp, 
  Bollerup, 
  and 
  

   Klubudden, 
  and 
  has 
  been 
  recorded 
  from 
  Zelkovice, 
  Berown, 
  etc., 
  in 
  

   Bohemia. 
  

  

  British 
  Localities. 
  — 
  Moffat 
  District 
  : 
  Dobb's 
  Linn, 
  Garple 
  Linn, 
  

   etc. 
  Lake 
  District 
  : 
  Skelgill, 
  Mealy 
  Gill. 
  Wales 
  : 
  Conway 
  

   (Tarannon 
  Shales). 
  

  

  