﻿386 
  ME. 
  H. 
  WOODS 
  ON 
  THE 
  MOLLUSCA 
  [Aug. 
  1 
  897, 
  

  

  Remarks. 
  — 
  This 
  species 
  is 
  fairly 
  common 
  in 
  the 
  Chalk 
  Rock. 
  In 
  

   Westphalia 
  it 
  is 
  a 
  characteristic 
  fossil 
  of 
  the 
  Beussianum-zoiie, 
  and 
  

   in 
  Saxony 
  of 
  the 
  Scaphites-be&s. 
  Sowerby's 
  types 
  are 
  preserved 
  in 
  

   the 
  British 
  Museum, 
  but 
  I 
  have 
  not 
  been 
  able 
  to 
  trace 
  the 
  specimens 
  

   figured 
  by 
  Mantell. 
  Judging 
  from 
  the 
  figure 
  alone, 
  I 
  think 
  it 
  

   probable 
  that 
  Mantell's 
  Plagiostoma 
  hrightonensis 
  will 
  prove 
  to 
  be 
  

   identical 
  with 
  this 
  species. 
  Sjp. 
  spinosus 
  shows 
  considerable 
  varia- 
  

   tion 
  in 
  proportions 
  and 
  in 
  the 
  number 
  of 
  spines, 
  but 
  there 
  is 
  

   a 
  perfect 
  gradation 
  between 
  the 
  different 
  forms. 
  

  

  In 
  various 
  Memoirs 
  of 
  the 
  Geological 
  Survey 
  of 
  England 
  (for 
  ex- 
  

   ample, 
  nos. 
  15, 
  17, 
  21, 
  25, 
  & 
  31 
  of 
  the 
  bibliographic 
  list 
  given 
  on 
  p. 
  71 
  

   of 
  Part 
  I. 
  of 
  this 
  paper) 
  this 
  species 
  has 
  been 
  referred 
  to 
  the 
  genus 
  

   Lima, 
  but 
  I 
  do 
  not 
  know 
  on 
  what 
  grounds, 
  since 
  it 
  differs 
  from 
  

   Lima 
  in 
  the 
  inequality 
  of 
  the 
  valves, 
  the 
  absence 
  of 
  the 
  area 
  on 
  the 
  

   right 
  valve, 
  and 
  the 
  character 
  of 
  the 
  ornamentation. 
  As 
  long 
  ago 
  

   as 
  1828 
  it 
  was 
  clearly 
  shown 
  by 
  Deshayes 
  * 
  that 
  this 
  and 
  other 
  

   Cretaceous 
  species 
  on 
  which 
  the 
  genera 
  Diancliora, 
  Sowerby, 
  2 
  

   Podopsis, 
  Lamarck, 
  3 
  and 
  Pacliytos, 
  Def 
  ranee, 
  4 
  were 
  founded, 
  differ 
  

   from 
  Spondylus 
  only 
  in 
  the 
  fact 
  that 
  certain 
  parts 
  of 
  the 
  shell 
  

   have 
  been 
  removed 
  by 
  water 
  in 
  solution. 
  Deshayes 
  states 
  that 
  the 
  

   shell 
  in 
  existing 
  species 
  is 
  formed 
  of 
  an 
  outer 
  thin 
  layer 
  and 
  an 
  

   inner 
  thick 
  layer, 
  and 
  that 
  the 
  former 
  is 
  absent 
  on 
  the 
  area. 
  The 
  

   inner 
  layer 
  has 
  been 
  removed 
  from 
  the 
  Chalk 
  specimens, 
  and 
  this 
  

   accounts 
  for 
  the 
  thinness 
  of 
  their 
  shells, 
  and 
  the 
  absence 
  of 
  the 
  

   muscular 
  impression 
  and 
  area. 
  The 
  greater 
  solubility 
  of 
  the 
  inner 
  

   layer 
  is 
  due 
  to 
  the 
  fact 
  that 
  it 
  consists 
  of 
  aragonite, 
  5 
  whereas 
  the 
  

   outer 
  layer 
  is 
  calcite. 
  The 
  triangular 
  opening 
  under 
  the 
  umbo, 
  

   which 
  results 
  from 
  the 
  removal 
  of 
  the 
  area, 
  was 
  regarded 
  by 
  

   Sowerby 
  as 
  having 
  served 
  for 
  the 
  passage 
  of 
  the 
  byssus 
  ; 
  but, 
  since 
  

   these 
  forms 
  were 
  attached 
  by 
  the 
  left 
  valve, 
  an 
  organ 
  of 
  fixation 
  

   would 
  have 
  been 
  useless. 
  In 
  no 
  case 
  have 
  I 
  seen 
  the 
  teeth 
  of 
  

   Sp. 
  spinosus, 
  but 
  certain 
  casts, 
  which 
  probably 
  belong 
  to 
  this 
  species, 
  

   give 
  indications 
  of 
  them. 
  Since 
  the 
  area 
  is 
  missing, 
  the 
  teeth 
  

   would 
  obviously 
  not 
  be 
  found 
  on 
  the 
  left 
  valve 
  ; 
  a 
  specimen 
  of 
  the 
  

   right 
  valve 
  in 
  the 
  Wiltshire 
  Collection 
  (Woodwardian 
  Museum) 
  

   shows 
  distinctly 
  the 
  interior 
  of 
  the 
  shell 
  and 
  the 
  hinge-line, 
  but 
  

   there 
  are 
  no 
  teeth 
  visible 
  ; 
  another 
  specimen 
  which 
  I 
  cleaned 
  out 
  

   agrees 
  with 
  this 
  one. 
  The 
  absence 
  of 
  teeth 
  is 
  explained 
  by 
  the 
  

   circumstance 
  that 
  they 
  consist 
  entirely 
  of 
  aragonite 
  : 
  a 
  section 
  of 
  

   Sp. 
  aurantms, 
  Lamk., 
  which 
  I 
  have 
  had 
  prepared 
  shows 
  this 
  clearly. 
  

   The 
  section 
  is 
  cut 
  perpendicular 
  to 
  the 
  hinge-line, 
  and 
  passes 
  through 
  

   a 
  tooth, 
  the 
  area, 
  and 
  the 
  umbo. 
  The 
  thin 
  outer 
  layer 
  referred 
  to 
  

   by 
  Deshayes 
  consists 
  of 
  fine 
  prisms 
  arranged 
  obliquely 
  and 
  rather 
  

   irregularly 
  ; 
  it 
  does 
  not 
  pass 
  beyond 
  the 
  umbo. 
  The 
  inner 
  layer 
  

  

  1 
  Ann. 
  Sci. 
  Nat. 
  vol. 
  xv. 
  (1828) 
  p. 
  427; 
  also 
  ' 
  Coq. 
  Foss. 
  des 
  Envir.de 
  

   Paris,' 
  vol. 
  i. 
  (1830) 
  p. 
  318. 
  

  

  2 
  ' 
  Min. 
  Conch.' 
  vol. 
  i. 
  (1815) 
  p. 
  183. 
  

  

  s 
  ' 
  Hist, 
  nat. 
  des 
  Animaux 
  sans 
  Vert.' 
  vol. 
  vi. 
  pt. 
  i. 
  (1819) 
  p. 
  194. 
  

  

  4 
  ' 
  Diet. 
  Sci. 
  Nat.' 
  vol. 
  xxxvii. 
  (1825) 
  p. 
  207- 
  

  

  5 
  Sorby, 
  Pres. 
  Addr., 
  Quart. 
  Journ. 
  Geol. 
  Soc. 
  vol. 
  xxxv. 
  (1879) 
  Proc. 
  p. 
  60. 
  

  

  