﻿^°1' 
  53*] 
  FOSSILS 
  FROM 
  THE 
  MORTE 
  SLATES, 
  ETC. 
  453 
  

  

  as 
  the 
  width 
  of 
  the 
  shell. 
  Hinge-area 
  narrow, 
  slightly 
  triangular, 
  

   with 
  fine 
  crenulations 
  on 
  the 
  proximal 
  margin. 
  Margins 
  curving 
  

   slightly 
  on 
  the 
  sides 
  and 
  more 
  rapidly 
  in 
  front. 
  Surface 
  covered 
  

   with 
  between 
  150 
  and 
  200 
  fine 
  straight 
  elevated 
  striae, 
  divaricating 
  

   at 
  rather 
  regular 
  intervals, 
  separated 
  by 
  somewhat 
  wider 
  furrows, 
  

   and 
  crossed 
  by 
  a 
  few 
  strong 
  growth-lines, 
  and 
  by 
  very 
  numerous 
  

   fine 
  threads, 
  which 
  on 
  portions 
  of 
  the 
  specimens 
  give 
  rise 
  to 
  minute 
  

   radially-arranged 
  slits 
  (possibly 
  due 
  to 
  these 
  threads 
  becoming 
  

   foliaceous 
  on 
  the 
  striae). 
  

  

  Remarks. 
  Some 
  of 
  the 
  specimens 
  are 
  magnificent, 
  but 
  unfortu- 
  

   nately 
  their 
  critical 
  characters 
  are 
  much 
  obscured. 
  It 
  maybe 
  noted 
  

   (1) 
  that 
  in 
  one 
  or 
  two 
  specimens 
  there 
  seems 
  the 
  appearance 
  of 
  a 
  

   slight 
  median 
  fold, 
  which, 
  however, 
  is 
  probably 
  due 
  only 
  to 
  

   accidental 
  causes 
  ; 
  (2) 
  that 
  the 
  ribs 
  are 
  extremely 
  regular, 
  though 
  

   occasionally 
  a 
  stronger 
  one 
  occurs, 
  and 
  there 
  are 
  no 
  signs 
  of 
  a 
  minor 
  

   intermediate 
  series; 
  (3) 
  that 
  on 
  the 
  wings 
  where 
  longitudinal 
  

   strain 
  has 
  occurred 
  the 
  transverse 
  marks 
  are 
  represented 
  by 
  

   radiating 
  scalariform 
  rows 
  of 
  exceedingly 
  regular 
  slits, 
  abruptly 
  

   ended; 
  (4) 
  that 
  while 
  in 
  all 
  the 
  specimens 
  the 
  hinge 
  is 
  very 
  

   defective, 
  one 
  seems 
  to 
  have 
  crenulations 
  which 
  indicate 
  that 
  it 
  is 
  a 
  

   Btropheodonta, 
  and 
  another 
  shows 
  a 
  definitely 
  triangular 
  though 
  very 
  

   narrow 
  area, 
  while 
  one 
  or 
  two 
  others 
  show 
  short 
  umbonal 
  grooves 
  

   on 
  crural 
  ridges 
  set 
  at 
  right 
  angles 
  to 
  each 
  other, 
  and 
  extremely 
  

   indistinct 
  signs 
  of 
  a 
  rather 
  large 
  muscular 
  area 
  ; 
  and 
  (5) 
  that 
  while 
  

   nearly 
  every 
  specimen 
  has 
  lost 
  its 
  true 
  shape, 
  a 
  small 
  one, 
  which 
  

   seems 
  to 
  have 
  retained 
  it, 
  is 
  nearly 
  square. 
  

  

  Several 
  equally 
  gigantic 
  forms 
  occur 
  in 
  the 
  Lower 
  Devonian 
  

   of 
  other 
  localities. 
  St. 
  gigas 
  (M'Coy), 
  1 
  from 
  Looe, 
  is 
  at 
  once 
  

   distinguished 
  by 
  its 
  numerous 
  minute 
  minor 
  ribs 
  between 
  the 
  

   major 
  series. 
  M'Coy 
  describes 
  the 
  ribs 
  of 
  that 
  species 
  as 
  separated 
  

   by 
  punctured 
  lines, 
  while 
  Davidson 
  mentions 
  that 
  in 
  Streptorliynchus 
  

   umbraculum 
  (which 
  is 
  similarly 
  distinguished 
  by 
  its 
  minor 
  ribs) 
  

   ' 
  the 
  ribs 
  are 
  crossed 
  by 
  scale-like 
  striae, 
  while 
  the 
  interspaces 
  are 
  

   crossed 
  by 
  finer 
  and 
  more 
  numerous 
  lines 
  of 
  growth.' 
  

  

  Strophomena 
  Steini, 
  Kayser, 
  2 
  equals 
  ours 
  in 
  size 
  and 
  apparently 
  

   in 
  shape, 
  but 
  its 
  ribs 
  seem 
  fewer, 
  wider 
  apart, 
  and 
  more 
  arching 
  

   laterally 
  (the 
  latter 
  character 
  being 
  apparently 
  regarded 
  by 
  Kayser 
  

   as 
  of 
  specific 
  value). 
  

  

  On 
  the 
  other 
  nand, 
  I 
  find 
  nothing 
  to 
  separate 
  our 
  shells 
  from 
  

   JSt. 
  explanata 
  (Sowerby), 
  3 
  which 
  is 
  also 
  figured 
  by 
  Kayser 
  i 
  from 
  

  

  1 
  1852. 
  M'Coy, 
  ' 
  Brit. 
  Pal. 
  Foss.' 
  p. 
  386 
  & 
  pi. 
  ii 
  a. 
  fig. 
  7. 
  

  

  2 
  1889. 
  Kayser, 
  Abb. 
  k. 
  Preuss. 
  geol. 
  Landesanst., 
  n. 
  s., 
  pt. 
  i. 
  p. 
  103 
  & 
  

   pi. 
  xii. 
  fig. 
  1. 
  * 
  

  

  3 
  1842. 
  Sowerby, 
  Trans. 
  Geol. 
  Soc, 
  ser. 
  2, 
  vol. 
  vi. 
  pt. 
  ii. 
  p. 
  409 
  & 
  pi. 
  xxxviii. 
  

   fig. 
  15. 
  

  

  4 
  1889. 
  Kayser, 
  Abh. 
  k. 
  Preuss. 
  geol. 
  Landesanst., 
  n. 
  s., 
  pt. 
  i. 
  p. 
  102 
  & 
  

   pis. 
  xxi. 
  figs. 
  1-3, 
  xxii. 
  fig. 
  1. 
  He 
  says: 
  'St. 
  explanata 
  is 
  known 
  to 
  me 
  from 
  all 
  

   the 
  beds 
  of 
  the 
  Rhenish 
  Lower 
  Devonian 
  and 
  is 
  nowhere 
  particularly 
  

  

  